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 TIROS-N Series 
 Direct Readout Services 
 Users Guide 
 
 Washington, D.C. 
 March 1982 
 
 U.S. DEPARTMENT OF COMMERCE 
 
 National Oceanic and Atmospheric Administration 
 
 National Earth Satellite Service 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 LYRASIS Members and Sloan Foundation 
 
 http://archive.org/details/tirosnseriesdireOObarn 
 

 
 TIROS-N Series 
 Direct Readout Services 
 Users Guide 
 
 Prepared by 
 
 James C. Barnes 
 Michael D. Smallwood 
 
 Environmental Research and Technology, Inc. 
 Concord, Massachusetts 
 
 for 
 
 National Oceanic and Atmospheric Administration 
 National Earth Satellite Service 
 
 Washington, D.C. 
 March 1 982 
 
 U.S. DEPARTMENT OF COMMERCE 
 
 Malcolm Baldrige, Secretary 
 
 National Oceanic and Atmospheric Administration 
 
 John V. Byrne, Administrator 
 
 National Earth Satellite Service 
 
 John H. Mc Elroy, Assistant Administrator for Satellites 
 
 0.5.DeposUoryCopy 
 
FOREWORD 
 
 This report was prepared for the United States Department of 
 Commerce/National Oceanic and Atmospheric Administration (NOAA) by 
 Environmental Research § Technology, Inc., 696 Virginia Rd., Concord, 
 MA 01742, under Contract No. 03-8-A01-78-4312. 
 
 The authors wish to acknowledge the assistance of Mr. Robert W. 
 Popham of NOAA/National Earth Satellite Service (NESS), who was the 
 Contract Monitor. We are also grateful to Alden Electronics of 
 Westboro, Massachusetts, and Leonessa Engineering of Bedford, 
 Massachusetts, for their contributions of TIROS-N image products. 
 
 For further information relating to this User's Guide or other 
 aspects of the TIROS-N series satellites, please contact: 
 
 Coordinator, Direct Readout Services 
 United States Department of Commerce 
 National Oceanic § Atmospheric Administration 
 National Earth Satellite Service 
 Washington, D.C. 20233 
 
TABLE OF CONTENTS 
 
 Page 
 
 FOREWORD iii 
 
 LIST OF ILLUSTRATIONS v 
 
 LIST OF TABLES vi 
 
 REFERENCES vii 
 
 GLOSSARY ix 
 
 1. INTRODUCTION TO THE TIROS-N SYSTEM 1-1 
 
 1.1 History of the United States Meteorological 
 
 Satellite Program 1-1 
 
 1.2 The Tiros-N Concept 1t2 
 
 1.3 TIROS-N Spacecraft Characteristics 1-4 
 
 2. DESCRIPTION OF THE TIROS-N SENSOR PACKAGE 2-1 
 
 2.1 Advanced Very High Resolution Radiometer 2-1 
 
 2.2 Other TIROS-N Sensors 2-2 
 
 3. ORBITAL CONSIDERATIONS 3-1 
 
 3. 1 General Aspects of Orbits and Their Effect 3-1 
 
 3.2 Selection of Orbit 3-4 
 
 3.3 Equator Crossing Time 3-6 
 
 3.4 Effects of Precession of Orbital Plane 3-8 
 
 3.5 Spacecraft Track Over the Earth 3-9 
 
 4. REAL-TIME DATA SYSTEMS FOR LOCAL USERS 4-1 
 
 4. 1 APT System 4-1 
 
 4.2 HRPT System 4-1 
 
 4.3 Direct Sounder Broadcasts (DSB) 4-2 
 
 4.4 Data Collection and Location Systems 4-3 
 
 5. TRACKING PROCEDURES FOR DIRECTIONAL ANTENNAS USED TO 
 
 ACQUIRE DATA FROM REAL-TIME TRANSMISSIONS SYSTEM SENSORS 5-1 
 
 5.1 Introduction 5-1 
 
 5.2 APT Predict (TBUS) Bulletin 5-1 
 
 5.3 Plotting Board 5-5 
 
 5.4 Tracking Diagram 5-8 
 
 5.5 Clock 5-9 
 
 5.6 Tracking Procedures 5-13 
 
 5.7 Alternate Sources and Forms of Satellite 
 
 Prediction Position Information 5-17 
 
 in 
 
TABLE OF CONTENTS (Continued) 
 
 Page 
 
 6. GEOGRAPHIC REFERENCING TECHNIQUES 6-1 
 
 6. 1 Grid Format 6-1 
 
 6.2 Grid Labels 6-1 
 
 6.3 Fitting Grids to APT Images (Overlaying Technique) 6-3 
 
 6.4 Fitting Grids to Images (Projection Technique) 6-4 
 
 7. FORMAT OF DIRECT READOUT DATA 7-1 
 
 7.1 APT Data 7-1 
 
 7.2 HRPT Data 7-8 
 
 7.3 Enhancement of Digitized AVHRR Data 7-15 
 REFERENCES R-l 
 
 APPENDIX A 
 APPENDIX B 
 APPENDIX C 
 APPENDIX D 
 
 APT PREDICT (TBUS) BULLETIN A-l 
 
 PLOTTING BOARD PREPARATION B-l 
 
 TRACKING EXERCISE C-l 
 
 GRIDDING EXERCISE D-l 
 
 IV 
 
LIST OF ILLUSTRATIONS 
 
 Figure Page 
 
 1-1 TIROS-N Spacecraft Characteristics 1-5 
 
 3-1 Diagram for Computing Area Viewed by Satellite 3-1 
 
 3-2 TIROS-N/NOAA Equator Crossing Times 3-7 
 
 3-3 Diagram of Elliptical Orbit 3-10 
 
 3-4 Diagram of Change in Perigee and Apogee 3-11 
 
 5-1 Schematic Representation of Information Conveyed 
 
 in TBUS-1 and TBUS-2 Bulletins 5-3 
 
 5-2 Plotting Board — Northern Hemisphere 5-6 
 
 5-3 Plotting Board — Southern Hemisphere 5-7 
 
 5-4 Diagram for Computing Spacecraft Elevation 
 
 Angle 5-8 
 
 5-5 Tracking Diagram 5-10 
 
 6-1 Reproduction of TIROS-N Grid for an 840-km 
 
 Orbital Height 6-2 
 
 7-1 Example of Unenhanced APT Data 7-2 
 
 7-2a 7-4 
 
 and ?b Examples of Enhanced APT Images 7-5 
 
 7-3 Examples of TIROS-N Channels 1 and 2 7-6 
 
 7-4 Examples of TIROS-N Channels 3 and 4 7-7 
 
 7-5a 7-9 
 
 and 5b Examples of Daytime HRPT Images 7-10 
 
 7-6a-d Examples of AVHRR Channels Received in LAC Mode 7-11 
 
LIST OF TABLES 
 
 Table Page 
 
 2-1 TIROS-N AVHRR Channel Characteristics 2-3 
 
 3-1 Relationship of Orbital Period to Spacecraft 
 
 Altitude 3-3 
 
 3-2 Nominal Orbital Parameter for TIROS-N Series 
 
 and NOAA Series Satellites 3-5 
 
 5-1 Great Circle Arc Length at Zero-Degree Elevation 5-11 
 
 5-2 Elevation Angle as a Function of Great Circle 
 
 Arc Length and Altitude 5-12 
 
 VI 
 
LIST OF ACRONYMS AND ABBREVIATIONS 
 
 AFTN 
 
 AGC 
 
 APT 
 
 ARRL 
 
 ASECNA 
 
 AVHRR 
 
 AZ/EL 
 
 CDA 
 
 CEMES 
 
 co 2 
 
 CNES 
 DCLS 
 
 DCS 
 ESM 
 ESSA 
 
 FANAS 
 
 GAC 
 
 GMT 
 
 GOES 
 
 GTS 
 
 HE PAD 
 
 HIRS/2 
 
 HRPT 
 
 IMP 
 
 IOC 
 
 IR 
 
 ITOS 
 
 keV 
 
 LAC 
 
 LST 
 
 MEPED 
 
 Aeronautical Fixed Telecommunications Network 
 
 Automatic Gain Control 
 
 Automatic Picture Transmission 
 
 American Radio Relay League 
 
 Agency pour la Seceirite de la Navigation Ericanne 
 
 Advanced Very High Resolution Radiometer 
 
 Azimuth/Elevation 
 
 Command and Data Acquisition 
 
 Centre d* Etudes de la Meteorologie Spatiale 
 
 carbon dioxide 
 
 Centre National d' Etudes Spatiales 
 
 Data Collection and Location System (also referred to as 
 DCS - Data Collection System) 
 
 see DCLS 
 
 Equipment Support Module 
 
 Environmental Science Services Administration 
 also Environmental Survey Satellite 
 
 Forecast for Ascending Node for Automatic Satellites 
 
 Global Area Coverage 
 
 Greenwich Mean Time 
 
 Geostationary Operational Environmental Satellite 
 
 Global Telecommunications Service 
 
 High Energy Proton and Alpha Detector 
 
 High Resolution Infrared Radiation Sounder 
 
 High Resolution Picture Transmission 
 
 Instrument Mounting Platform 
 
 Index of Cooperation 
 
 Infrared 
 
 Improved TIROS Operational Satellite 
 
 kiloelectron volts 
 
 Local Area Coverage 
 
 Local Solar Time 
 
 Medium Energy Proton and Electron Detector 
 
 vn 
 
MEV - megaelectron volts 
 
 MH - megahertz 
 
 MIRP - Manipulated Information Rate Processor 
 
 MSU - Microwave Sounding Unit 
 
 NASA - National Aeronautics and Space Administration 
 
 NESS - National Earth Satellite Service 
 
 NOAA - National Oceanic and Atmospheric Administration 
 
 0_ - ozone 
 
 RSS - Reaction Support Structure 
 
 SEM - Space Environment Monitor 
 
 SR - Scanning Radiometer 
 
 SST - Sea Surface Temperature 
 
 SSU - Stratospheric Sounding Unit 
 
 TED - Total Energy Detector 
 
 TIP - TIROS Information Processor 
 
 TIROS - Television Infrared Observation Satellite 
 
 TOS - TIROS Operational Satellite 
 
 TOVS - TIROS Operational Vertical Sounder 
 
 VHF - Very High Frequency 
 
 VHRR - Very High Resolution Radiometer 
 
 VIS - Visible 
 
 VISSR - Visible/Infrared Spin Scan Radiometer 
 
 VTPR - Vertical Temperature Profile Radiometer 
 
 WEFAX - Weather Facsimile 
 
 VI 11 
 
GLOSSARY 
 
 ANOMALISTIC PERIOD OF SATELLITE: The time elapsing between successive 
 passages of a satellite through the perigee. 
 
 APOGEE: The point in its orbit at which the satellite is farthest 
 from the center of the earth. 
 
 APSIS: The points in an elliptic orbit at which the radius vectors 
 reach a maximum or minimum (e.g., perigee and apogee of a 
 satellite orbit). The line joining these two points is called 
 the line of apsides. 
 
 ARGUMENT OF PERIGEE: The geocentric angle of the perigee measured 
 
 in the orbital plane from its ascending node in the direction of 
 motion. 
 
 ARGUMENT OF SATELLITE: The geocentric angle of a satellite measured 
 in the orbital plane from the ascending node in the direction of 
 motion. 
 
 ARGUMENT OF SATELLITE AT MINIMUM (MAXIMUM) NADIR ANGLE: (See Argument 
 of Satellite and Nadir Angle). 
 
 ASCENDING NODE (AN): The point at the equator at which the satellite 
 in its orbital motion crosses from the southern to the northern 
 hemisphere. Terrestrial Ascending Node (TAN) is given in degrees 
 longitude. Celestial Ascending Node (CAN) is given on right 
 ascension hours, minutes, etc. 
 
 ASCENDING NODE TIME: The time when the satellite passes the ascending 
 node. 
 
 ATTITUDE (SATELLITE): The position of the axis of a satellite with 
 respect to (a) its orbital plane, (b) the earth's surface, or 
 (c) any fixed set of coordinates. 
 
 AZIMUTH (a): A horizontal direction expresses in degrees measured 
 
 clockwise from an adopted reference direction, usually true north. 
 
 BULGE OF THE EARTH (R E -R p ): The difference between the equatorial 
 and the polar radii cf the earth. 
 
 CELESTIAL: The prefix to designate lines or points which have been 
 projected onto the celestial sphere by means of the radials 
 through the center of the earth. 
 
 CELESTIAL EQUATOR: The great circle along which the plane of the 
 earth's equator intersects the celestial sphere. 
 
 IX 
 
CELESTIAL SPHERE: An imaginary sphere of infinite radius with its 
 
 center located at the observer or at the center of the earth. In 
 satellite meteorology the center of the celestial sphere is at 
 the center of the earth. Lines or points are projected onto the 
 celestial sphere using radials through the center of the earth. 
 
 CELESTIAL SUBSATELLITE POINT: A point on the celestial sphere the 
 
 declination of which is identical to the geocentric latitude of 
 TSP and the right ascension of that of TSP. (See SUBPOINT TRACK.) 
 
 DATA ACQUISITION STATION (CDA): A ground station at which various 
 
 functions to control satellite operations and to obtain data from 
 the satellite are performed* The CDA transmits programming 
 signals to the satellite, and commands transmission of data to 
 the ground. 
 
 DECLINATION (6): The angular distance of an object north (+) or south 
 (-) from the celestial equator measured along the hour circle 
 passing through the object. 
 
 DEGRADATION: The lessening of picture image quality because of 
 
 "noise," or any optical, electronic, or mechanical distortions in 
 the image-forming system. 
 
 DESCENDING NODE (DN): The southbound equator crossing of the 
 
 satellite; given in degrees longitude, date, and time for any 
 given orbit or pass. 
 
 DIRECT ORBIT: The orbit with inclination between 0° and 90° 
 
 measured counterclockwise from the equator. Same as the prograde 
 orbit. 
 
 DISTORTION: An apparent warping and twisting of a picture image 
 received from a satellite. This distortion has two causes: 
 electronic and optical. "Electronic Distortion" is caused by 
 imperfections in the circuitry, the tape recorder, the vidicon 
 tube structure, the transmission system, or the signal 
 characteristics. "Optical Distortion" is caused by the 
 characteristics of the lens and optical alignment. 
 
 ECLIPTIC: The great circle in which the plane of the earth's orbit 
 intersects the celestial sphere. 
 
 EQUATORIAL ORBIT: Orbit with zero degree inclination. 
 
 GAMMA: Gamma is the angle at the satellite between the' satellite 
 spin-axis and the satellite-sun line. 
 
 HEADING LINE: The instantaneous projection of the spacecraft velocity 
 vector on the earth's surface. 
 
 IMAGE PRINCIPAL LINE: (See Principal Line). 
 
 INCLINATION (OF SATELLITE ORBIT): (See Orbit Inclination). 
 
LATITUDE-LONGITUDE GRID: A latitude-longitude grid is a form of 
 
 perspective grid in which the quadrilaterals are latitudes and 
 longitudes. 
 
 LOCAL VERTICAL: A line perpendicular to the surface of the earth. 
 
 NADIR ANGLE: The angle measured at the satellite between a specific 
 axis or ray and the local vertical. 
 
 NODE: The points at the equator at which the satellite in its orbital 
 motion crosses the equator. The line connecting the ascending 
 and the descending nodes is called the line of nodes. 
 
 NODAL PERIOD: The time elapsing between successive passages of the 
 satellite through successive ascending nodes. 
 
 NODAL INCREMENT: Degrees of longitude between successive ascending 
 nodes. 
 
 NOISE: A voltage received through an antenna system or within the 
 
 ground amplifier and other electronics that does not correspond 
 to any intended signal. 
 
 OBJECT SPACE ANGLE: The angle between two rays in space that intersect 
 at the front nodal point of a camera lens. 
 
 OPTICAL AXIS: A straight line passing through the front and rear nodal 
 points of a camera lens. The direction on the axis is considered 
 positive from the rear to the front nodal points. 
 
 OPTICAL AND ELECTRONIC DISTORTIONS: (See Distortion.) 
 
 ORBIT: The path which a celestial object follows in its motion through 
 space, relative to some selected point. 
 
 ORBIT INCLINATION: The angle between the plane of the satellite 
 orbit and the earth's equatorial plane. Inclination of 
 retrograde orbit is expressed by 180 degrees minus the prograde 
 inclination. 
 
 ORBIT NUMBER: In satellite meteorology orbit number refers to a 
 particular circuit beginning at the satellite ascending node. 
 The orbit number from launch to the first ascending node is 
 designated zero, thereafter the number increases by one at each 
 ascending node. 
 
 ORBITAL PLANE: The plane, or two dimensional space which contains the 
 path of an orbiting satellite. 
 
 ORBIT, POLAR: An orbit which passes directly over both the geographic 
 poles of the earth. 
 
 ORBIT, SUN- SYNCHRONOUS: (See Sun-Synchronous Orbit.) 
 
 XI 
 
PERIGEE: The point in its orbit at which the satellite is closest to 
 the center of the earth. 
 
 PERIGEE RATE: Rate of change of the argument of perigee. It is 
 usually measured in degrees per day. 
 
 PRECESSION RATE ( ): The fixed space angular motion of the orbital 
 line of nodes; positive to the East, negative to the West. 
 Precession rate for a sun-synchronous orbit is -0.986 degrees per 
 day. 
 
 PITCH: Angular deviation of the camera axis from the vertical along 
 the orbital plane at the time of picture taking. 
 
 PRINCIPAL DISTANCE: The distance measured along the optical axis 
 
 either from the nodal point or the interior perspective center to 
 the image or target plane. 
 
 PRINCIPAL LINE: A line of intersection between the principal plane 
 and the image plane (Image Principal Line), or principal plane 
 and the earth. 
 
 PRINCIPAL PLANE: The plane which includes the optical axis of a 
 
 camera and the local vertical through the front nodal-point of a 
 satellite camera lens. 
 
 PRINCIPAL POINT: The point of intersection of the optical axis of the 
 camera with the image plane, or with the earth. 
 
 PROGRADE ORBIT: (See Direct Orbit.) 
 
 R/0: Abbreviation for readout orbit. 
 
 RADIOMETER: A scanning sensor that operates continuously, each scan 
 line sweeping across the earth from horizon-to-horizon along a 
 path normal to the direction of travel of the spacecraft. 
 
 RASTER: The pattern followed by the electron-beam exploring-element 
 
 scanning the screen of a television transmitter or receiver. The 
 scanning pattern is, theoretically, a series of straight parallel 
 lines. 
 
 RASTER LINE: One scan line of a TV (Vidicon) system. 
 
 READ-OUT STATION: (See Data Acquisition Station.) 
 
 RESOLUTION: The ability of a film, a lens, a combination of both, or 
 a vidicon system to render barely distinguishable a standard 
 pattern of black and white lines. When the resolution is said to 
 be 10 lines per millimeter, it means that the pattern whose 
 line-plus-space width is 0.1 mm is barely resolved, the finer 
 patterns are not resolved, and the coarser patterns are more 
 clearly resolved. 
 
 XII 
 
RETROGRADE ORBIT: The orbit with inclination between 0° and 90° 
 measured clockwise from the equator when viewed from the 
 north. 
 
 RIGHT ASCENSION: The arc measured eastward along the celestial 
 equator from the Vernal Equinox to the great circle passing 
 through the celestial poles and the object projected onto the 
 celestial sphere. This angle is frequently given in hours and 
 minutes--24 hours equivalent to 360 degrees. 
 
 ROLL: Angular deviation of the satellite axis from the plane 
 tangent to the orbital plane. 
 
 SCANLINE: One sweep of a radiometer across the earth from horizon- 
 to-horizon. 
 
 SU3P0INT TRACK: Locus of subsatellite points on the earth (TSP), 
 on an imaqe, or on a celestial sphere. 
 
 SUBSATELLITE POINT: Intersections of the local vertical passing 
 through the satellite with the earth's surface, with the image 
 plane and with the celestial sphere. 
 
 SUBSOLAR POINT (SS): Intersection of the local vertical through 
 the sun with the surface of the earth. 
 
 SUN-SYNCHRONO'JS ORBIT: Nominally a retrograde, quasi-polar orbit 
 such that the satellite crosses the equator on the ascending 
 node always at the same local (solar) time. 
 
 SYNCHRONOUS SATELLITE: A satellite in a west-to-east orbit of the 
 earth at an altitude of 22,300 statute miles. At this 
 altitude it circles the axis of the earth once in 24 hours. 
 Thus its speed in orbit is synchronous with the earth's 
 rotation. 
 
 TERRESTRIAL: The term used to designate a line or a point on the 
 earth's surface. 
 
 TIME PAST ASCENDING NODE: The amount of time for a body in orbit 
 to advance from the last ascending node to an arbitrary 
 position. 
 
 TIME PAST PERIGEE: The amount of time for a body in orbit to 
 advance from the last perigee to an arbitrary position. 
 
 TRACK: A line connecting successive positions of a moving point. 
 
 VERNAL EQUINOX: (Also called the Right Ascension of Aries.) The 
 point of intersection of the celestial equator with the 
 ecliptic, at the point where the sun crosses from the south 
 to the north side of the equator. 
 
 xm 
 
1. INTRODUCTION TO THE TIROS-N SYSTEM 
 1.1 History of the United States Meteorological Satellite Program 
 
 The National Earth Satellite Service (NESS) of the 
 National Oceanic and Atmospheric Administration (NOAA) operates a 
 network of polar and geostationary satellites that provide data used 
 to meet some of NOAA's responsibilities. These satellite data 
 products are used for meteorological prediction and warning, 
 oceanographic and hydrologic services, and space environment 
 monitoring and prediction. The two satellite systems (polar and 
 geostationary) are complementary, each having been designed to meet 
 mission requirements for which it is uniquely qualified. For example, 
 continuous monitoring of storm areas in the temperate and tropical 
 latitudes is primarily a geostationary satellite mission, whereas data 
 collection at higher latitudes is an objective of a polar orbiting 
 satellite. 
 
 The launch of TIROS-N from the Western Test Range at Lompoc, 
 California, on 13 October 1978 introduced the third generation 
 operational, polar orbiting, environmental satellite system. The new 
 TIROS-N system has evolved from the world's first meteorological 
 satellite, TIROS-1, which was launched into orbit on 1 April 1960 from 
 Cape Canaveral, Florida. The early TIROS experimental spacecraft 
 series (TIROS 1-10) was followed by the TIROS Operational Satellite 
 (TOS) series (ESSA 1-9). This series was introduced on 3 February 
 1966 with the launch of ESSA-1. These "wheel-type M TOS satellites 
 were designed to fill the need for a relatively low cost, fully 
 operational weather satellite system to provide worldwide photographic 
 coverage of the earth and its atmosphere. In effect, this satellite 
 "rolled" along its orbit on its rim as a wheel would roll along the 
 earth's surface. 
 
 The Improved TIROS Operational Satellite, ITOS-1, was launched on 
 23 January 1970 and ushered in the second generation of operational 
 polar orbiting satellites. It dramatically exceeded the capabilities 
 of the first generation TOS system. The last of the ITOS series 
 spacecraft, NOAA-5, was launched on 29 July 1976. 
 
 1-1 
 
As the potential advantages of observations from space were 
 recognized, evolving requirements brought about changes in the 
 satellite instrumentation. The Automatic Picture Transmission (APT) 
 service, tested experimentally on TIROS-8 in 1963 and on Nimbus-1 in 
 1964, became a continuing service with the launch of ESSA-2 in 1966. 
 Vidicon cameras, the primary source of data for both the central 
 processing and the APT service, were replaced by radiometers on 
 NOAA-2, launched in 1972. At the same time, the High Resolution 
 Picture Transmission (HRPT) service was begun, making data from the 
 Very High Resolution Radiometer (VHRR) instrument available to local 
 readout stations with the proper receiving equipment. The NOAA-2 
 spacecraft also included a Vertical Temperature Profile Radiometer 
 (VTPR) instrument as a part of the payload; this instrument, based on 
 technology tested on research satellites, was the first operational 
 vertical sounder to be flown. 
 
 The instrumentation for TIROS-N has been selected based on the 
 NOAA satellite program philosophy of meeting operational requirements 
 for products with instruments whose potential has been proven in 
 space. Predecessor instruments have flown experimentally on both 
 Nimbus and ITOS satellites. These instruments have been redesigned to 
 meet both scientific and technical requirements of the mission; the 
 goal of the redesign has been to improve the reliability of the 
 instrument and the quality of the data without changing the previously 
 proven concepts. 
 
 1.2 The TIROS-N Concept 
 
 The spacecraft of the TIROS-N series incorporate new 
 environmental instruments that are major technological advances over 
 those on board the ITOS series spacecraft they have replaced. New 
 capabilities offered by the TIROS-N series include: improved day and 
 night cloud cover observations on a local and global scale, improved 
 observations of vertical temperature and water vapor profiles on a 
 global scale, and a high capacity data collection, relay, and platform 
 location system. 
 
 1-2 
 
In the TIROS-N operations concept, two spacecraft are always in 
 operation. One spacecraft passes over in the morning between 0600 and 
 1000 local time; the other spacecraft passes over in the afternoon, 
 about 1500 local time. The initial spacecraft of the series, 
 designated TIROS-N, was placed into the afternoon orbit; N0AA-6, the 
 second spacecraft of the series, was placed in the 0730 local 
 southbound orbit on 27 June 1979. With the exception of TIROS-N, all 
 satellites of the series will be designated by letter before launch, 
 but will be numbered consecutively after launch. 
 
 Four pairs of spacecraft are planned for the series through 1985, 
 launched roughly at the rate of one pair every two years after the 
 launches of TIROS-N and N0AA-6. NOAA-G, the last satellite planned 
 for this series, is currently scheduled for launch in May 1984 and 
 thus will ensure polar satellite services into 1986. Initial planning 
 has begun for the fourth generation of polar spacecraft to be ready by 
 1985 to continue the uninterrupted acquisition of data from the 
 earth's atmosphere, from its surface, and from within its near space. 
 
 The TIROS-N series of satellites is a cooperative effort of the 
 United States, the United Kingdom, and France. The National 
 Aeronautics and Space Administration (NASA) funded the development and 
 launch of the initial satellite of the series (TIROS-N); subsequent 
 satellites will be procured and launched by NASA using NOAA funds. 
 The operational ground facilities, including the Command and Data 
 Acquisition (CDA) stations, the Satellite Control Center, and the data 
 processing facilities (with the exception of the Data Collection 
 System (DCS)* processing facility) are funded and operated by NOAA. 
 The United Kingdom, through its Meteorological Office, Ministry of 
 Defense, is providing a Stratospheric Sounding Unit (SSU), one of 
 three sounding instruments for each satellite. The Centre National 
 d'Etudes Spatiales (CNES) of France is providing the DCLS instrument 
 for each satellite and provides the facilities necessary to process 
 and make available to users the data obtained from this system. The 
 
 *The DCS is also referred to as the Data Collection and Location 
 System (DCLS) in some documents. 
 
 1-3 
 
Centre d' Etudes de la Meteorologie Spatiale (CEMES) of France provides 
 ground facilities for receipt of sounder data during the blind orbit 
 periods. 
 
 1.3 TIROS-N Spacecraft Characteristics 
 
 The TIROS N spacecraft, as shown in Figure 1-1, is a five-sided 
 box-like structure that is 3.71 meters long and 1.88 meters in 
 diameter. Four of the side faces are equal in size and accommodate 
 thermal control louvers. The fifth side is wider than the other four 
 and accommodates the earth-facing communications antennae and some of 
 the earth-viewing sensors. The spacecraft weighs a total of 
 1,409 kilograms (kg) at launch, including expendables. 
 
 At the end of the central body, known as the Equipment Support 
 Module (ESM), is the Reaction Support Structure (RSS), which includes 
 the last stage launch injection motor, an attitude control propulsion 
 
 system, and a boom-mounted solar cell array. The solar array is 11.6 
 
 2 
 square meters (m ) and is motor driven to rotate once per orbit to 
 
 enable it to face the sun continuously during the daylight portions of 
 
 the orbit. 
 
 At the other end of the ESM is the highly stable Instrument 
 Mounting Platform (IMP) on which are mounted the attitude control 
 sensors and the instruments whose scan directions must be very 
 accurately controlled. With the exception of the Space Environment 
 Monitor (SEM), all instruments face the earth when the satellite is in 
 its mission mode. The earth-oriented platform is controlled to within 
 0.2° of the local geographic reference for all axes. 
 
 A most important spacecraft subsystem is the data handling 
 subsystem, which consists of the following components: 
 
 1) TIROS Information Processor (TIP), 
 
 2) Manipulated Information Rate Processor (MIRP), and 
 
 3) digital tape recorders. 
 
 1-4 
 
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 4J 
 
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 CD 
 
 J- 
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 1-5 
 
All data available for transmission to the ground are processed by 
 some or all of these components. The TIP formats all low bit-rate 
 instruments and housekeeping telemetry data and controls the data 
 outputs. The MIRP processes the high bit-rate data from the Advanced 
 Very High Resolution Radiometer (AVHRR) to provide separate outputs 
 for APT and HRPT transmissions in real time, recorded Global Area 
 Coverage (GAC) of reduced resolution data for central processing, and 
 recorded Local Area Coverage (LAC) of high resolution data also for 
 central processing. 
 
 1-6 
 
2. DESCRIPTION OF THE TIROS-N SENSOR PACKAGE 
 
 The four primary TIROS-N spacecraft instrument systems are listed 
 below: 
 
 • Advanced Very High Resolution Radiometer (AVHRR) 
 
 • TIROS Operational Vertical Sounder (TOVS) 
 
 • Data Collection System (DCS) 
 
 • Space Environment Monitor (SEM) 
 
 2.1 Advanced Very High Resolution Radiometer . 
 
 The AVHRR provides data for real-time transmission to both APT 
 and HRPT users and for storage on the spacecraft tape recorders for 
 later playback. Thus, the AVHRR instrument continues and improves the 
 ITOS/NOAA satellite services related to stored and direct readout of 
 radiometric data for day and night cloud mapping, sea surface 
 temperature mapping, and other oceanographic and hydrologic 
 applications. The data from the AVHRR instrument are available from 
 the satellite in four operational modes: 
 
 1) APT (Automatic Picture Transmission): direct readout to 
 worldwide ground stations of the APT visible and infrared 
 data (4 km resolution); panoramic distortion is removed. 
 
 2) HRPT (High Resolution Picture Transmission): direct readout 
 to worldwide ground stations of the HRPT data for all 
 spectral channels (1.1 km resolution). 
 
 3) GAC (Global Area Coverage): global on-board recording of 
 4 km resolution data from all spectral channels for 
 commanded readout for processing in the NOAA central 
 computer facility at Suitland, Maryland. 
 
 4) LAC (Local Area Coverage): on-board recording of data from 
 selected portions of each orbit at 1.1 km resolution of all 
 spectral channels for central processing. 
 
 2-1 
 
The AVHRR for TIROS-N and the four follow-on spacecraft is sensitive 
 in four spectral regions (see Table 2-1). A future change in the 
 instrument design will add a fifth channel in the 12 micrometer ( m) 
 region. The resulting five-channel instruments are thus planned for 
 flight on later spacecraft in the series. The four-channel radiometer 
 is designated the AVHRR/1 instrument; the later, five channel 
 radiometer is the AVHRR/2 instrument. 
 
 AVHRR Channels 1 and 2 can be used to discern clouds, land-water 
 boundaries, and snow and ice extent; when data from the two channels 
 are compared, an indication of ice/snow melt inception is provided. 
 The data from Channel 4 (infrared (IR) window) can be used to measure 
 cloud distribution day and night and to determine the temperature of 
 the radiating surface. Channels 3 and 4 can be used to determine the 
 sea surface temperature (SST). By using these two data sets, it is 
 possible to remove an ambiguity introduced when clouds fill a portion 
 of the field-of-view. Examples of imagery from the AVHRR instrument 
 are given in Section 7. 
 
 2.2 Other TIROS-N Sensors 
 
 2.2.1 TIROS Operational Vertical Sounder 
 
 Data from the TOVS, which is a three-instrument system, are 
 available locally as a part of the HRPT transmission and on the 
 spacecraft beacon transmission. The three TOVS instruments are as 
 follows: 
 
 • The High Resolution Infrared Radiation Sounder (HIRS/2) — a 
 20-channel instrument making measurements primarily in the 
 IR region of the spectrum. The instrument is designed to 
 provide data that will permit calculating (1) temperature 
 profile from the surface to 10 millibars (mb), (2) water 
 vapor content in three layers in the atmosphere, and 
 (3) total ozone (0_) content. The design is based on the 
 HIRS instrument flown on the Nimbus satellite. 
 
 2-2 
 
TABLE 2-1 
 TIROS-N AVHRR CHANNEL CHARACTERISTICS 
 
 Resolution at 
 Channel * Subpoint (km) Wavelength (ym) Primary Use 
 
 1 1 0.55-0.90 Daytime cloud and surface 
 
 mapping 
 
 2 1 0.725 - 1.10 Surface water delineation 
 
 3 1 3.55-3.93 Sea surface temperature 
 
 (SST), nighttime cloud 
 mapping 
 
 4 1 10.5-11.5 SST, day/night cloud 
 
 mapping 
 
 5 1 11.5 - 12.5 SST 
 
 *Channel 1 wavelength will be 0.58 to 0.68 pm for all instruments after 
 the TIROS-N flight model. Channel 4 wavelength will be 10.3 to 
 11.3 pm for all AVHRR/2 instruments. Channel 5 will not be on early 
 AVHRR/ 1 flights but will be added with the AVHRR/2 instruments to 
 enhance further SST measurements in the tropics. The AVHRR/2 instru- 
 ment will be flown on all later spacecraft of the series; users will 
 be notified when this change takes place. 
 
 2-3 
 
• The Stratospheric Sounding Unit (SSU) — using a selective 
 absorption technique to make measurements in three 
 channels. The spectral characteristics of each channel are 
 determined by the pressure in a carbon dioxide (CO ) gas 
 cell in the optical path. The amount of CO in the cells 
 determines the height of the weighting function peaks in the 
 atmosphere. The primary objective of this instrument is to 
 obtain data from which stratospheric (25 to 50 km) 
 temperature profiles can be obtained. 
 
 • The Microwave Sounding Unit (MSU) — a 4-channel Dicke 
 radiometer, making passive measurements in the 
 5.5-millimeter (mm) oxygen band. The microwave data will 
 permit computations to be made in the presence of clouds * 
 since measurements in this region are generally unaffected 
 by nonprecipitating water droplets. 
 
 2.2.2 The Data Collection and Location System 
 
 The DCS is a random access system that acquires data from fixed 
 and free-floating terrestrial and atmospheric platforms. Platforms 
 can be located by ground processing the Doppler measurements of 
 carrier frequencies. Data collected from each platform include 
 identification, as well as environmental, measurements. These data 
 are also included in the HRPT and beacon transmissions. 
 
 2.2.3 The Space Environment Monitor 
 
 The SEM data are also included in the HRPT and beacon 
 transmissions. The SEM consists of three separate instruments and a 
 data processing unit. 
 
 • The total energy detector (TED) measures a broad range of 
 energetic particles from 0.3 kiloelectron volts (keV) to 
 20 keV in 11 bands. 
 
 2-4 
 
The medium energy proton and electron detector (MEPED) 
 senses protons, electrons, and ions with energies from 
 30 keV to several tens of megaelectron volts (MEV). 
 The high energy proton and alpha detector (HEPAD) senses 
 protons and alphas from a few hundred MEV up through 
 relativistic particles above 840 MEV. 
 
 2-5 
 
3. ORBITAL CONSIDERATIONS 
 
 3.1 General Aspects of Orbits and Their Effects 
 
 When plans are made to place a meteorological satellite in orbit, 
 three factors must be considered to decide how best to meet the 
 mission requirement of daily global coverage: the area of view of 
 each sensor, the orbital period and its relationship to the frequency 
 and spatial continuity of observations, and the resolution of detail 
 within the field of view of each sensor. 
 
 3.1.1 Coverage 
 
 The TIROS-N AVHKR is a line-scanning radiometer, similar to the 
 scanning radiometer (.SR;, which has provided data from the ITOS series 
 of satellites to APT users. Unlike the earlier APT camera, which 
 takes a picture and transmits it by slowly scanning a vidicon tube, a 
 scanning sensor operates continuously, each scan line sweeping across 
 the earth from horizon to horizon along a path normal to the direction 
 of travel of the spacecraft. 
 
 The geometry involved in computing the area that can be seen by a 
 spacecraft sensor is shown in Figure 3-1. For a line scan system, 
 angle W is limited to the area at right angles to the satellite 
 heading line. The area viewed by a single scan line is essentially 
 perpendicular to the direction in which the spacecraft is moving and 
 has a component in the direction of movement equal to the line width 
 on the earth. Swath length is equal to the distance traveled by the 
 spacecraft jver the earth in the time given. 
 
 W is the angle of view 
 c fe 
 
 <j> is the geocentric angle to intersection 
 
 of W on earth 
 c 
 
 R is the radius of earth 
 
 h is the spacecraft altitude 
 
 d is the distance on earth from subpoint to 
 picture point for viewing angle W 
 
 Figure 3-1 Diagram for Computing Area Viewed by Satellite 
 
 3-1 
 
When a line-scan instrument is pointed normal to the earth's 
 surface and scans outward to an angle W , the overall area viewed 
 (swath width) depends on spacecraft height alone; the higher the 
 satellite, the larger the area viewed. Using Equations 1 through 3, 
 the area viewed can be computed. 
 
 (R+h) . „ M n 
 
 sin $ = — - — sin W (1; 
 
 R c 
 
 <t> 
 
 c 
 
 = 180 - (W + 0) (2) 
 
 d = <t> x 111 kilometers (km) (3) 
 
 c 
 
 For example, at a satellite altitude of 1,387 km and a maximum viewing 
 angle of 45 (W = 45 ), the area viewed is approximately 1,604 km 
 from center to side; at a 740 km altitude it is approximately 786 km. 
 For the TIROS-N series, the swath width is approximately 2,700 km 
 (full side to side swath width). 
 
 3.1.2 Orbital Period 
 
 The orbits of earth satellites, whether natural or manmade, are 
 controlled by the same physical laws. A satellite in orbit follows a 
 path that is the result of balanced forces; the acceleration of 
 gravity on the satellite is exactly balanced by the outward component 
 of the tangential linear velocity. The requirement for the existence 
 of this balance permits" the computation of the orbital speed and, 
 hence, the orbital period of a satellite for any given height. The 
 orbit period T for a near circular orbit is: 
 
 T = 84.4^1 + | V /2 minutes, (4) 
 
 where h is the altitude of the satellite and R is the radius of the 
 earth. Thus, as h increases, T increases, as shown in Table 3-1. 
 
 3-2 
 
TABLE 3-1 
 
 RELATIONSHIP OF ORBITAL PERIOD 
 
 TO SPACECRAFT ALTITUDE 
 
 Altitude Orbit/Period 
 
 h (km) h (NM) T (minutes) 
 
 371 
 
 200 
 
 854 
 
 460 
 
 1,112 
 
 600 
 
 1,390 
 
 750 
 
 1,462 
 
 790 
 
 91.9 
 102.0 
 107.4 
 113.5 
 115.1 
 
 35,790 19,312 1,440 (Synchronous 
 
 altitude) 
 
 Because the orbit plane is essentially "fixed" in space for short 
 time periods and the earth rotates within the orbit, the orbit period 
 (T) determines the longitudinal distance between successive equatorial 
 crossings. Thus, for a satellite orbit of 100 minutes, this 
 longitudinal distance is 25 degrees (one degree per four minutes). 
 For example, if equator crossing "n" takes place at 75 degrees W, the 
 next crossing (n + 1), 100 minutes later, will be at 100 degrees W. 
 This fact provides the basis of orbital position computations used in 
 tracking and gridding procedures discussed later. 
 
 The altitude of the satellite thus determines the time difference 
 and the longitudinal distance (<j> F ) between successive equatorial 
 crossings. If the data acquired on successive orbits are to be 
 contiguous (just touch) at the equator, the swath width measured in 
 geocentric angles A - (a at edge of swath) must equal the 
 longitudinal distance between successive equator crossings. If 
 contiguity is not possible at the equator ($ < <j> F ), the lowest 
 latitude (Xo) at which contiguity occurs is where Xo equals cos 
 
 e^E^* Data swaths that just touch at the equator overlap by 
 about 30% at 45 degrees of latitude. 
 
 3-3 
 
3.1.3 Resolution 
 
 For a given spacecraft altitude, the resolution of a line-scan 
 device, such as the AVHRR, is a function of the instantaneous 
 field-of-view of the sensor (spot size on the earth). As the sensor 
 scans outward from the satellite subpoint, the spot size on the 
 surface of the earth, of course, increases in size. The TIROS-N AVHRR 
 has a field-of-view of 1.3 milliradians ; at this field-of-view, the 
 subpoint resolution is 1.1 km and the resolution at the edge of the 
 data swath is approximately 4 km. 
 
 3.2 Selection of Orbit 
 
 A near polar, sun-synchronous orbit has been chosen for 
 meteorological satellites with real-time transmission systems because 
 it offers the best geometry for routine coverage on a regular basis. 
 A sun-synchronous orbit maintains the satellite in a relatively 
 constant relationship to the sun so that the ascending node 
 (northbound equator crossing) remains at a constant solar time, thus 
 permitting receipt of data at approximately the same local time each 
 day. The choice of time of day is based on meteorological and 
 spacecraft operating considerations; no change is possible once the 
 spacecraft has been launched. 
 
 The nominal orbital parameters of the TIROS-N satellite series 
 are summarized in Table 3-2. For comparison, the orbital parameters 
 of the previous ITOS series (NOAA-1 through NOAA-5) are also given. 
 The TIROS-N series has been designed to operate in a sun-synchronous 
 orbit. The two nominal altitudes shown in Table 3-2 have been chosen 
 to keep the orbital periods of two operational satellites in similar 
 orbits sufficiently different (1-minute) so that they do not both view 
 the same point on the Earth at the same time each day for prolonged 
 periods. 
 
 A satellite pass directly over an antenna site will be within 
 view of that antenna (horizon-to-horizon) for about 15.5 minutes when 
 the satellite is at 833 km and 16 minutes when it is at 870 km. The 
 user of APT or HRPT can therefore expect to receive data from a 
 
 3-4 
 
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 3-5 
 
circular area 6,200 km in diameter centered at the location of the 
 antenna. If one assumes that the spacecraft must be 5 degrees above 
 the horizon for useful data to be acquired, the contact times are 
 reduced to about 13.0 minutes (833 km satellite altitude) and 
 13.7 minutes (870 km); the area is reduced to 5,200 km in diameter. 
 
 Because the number of orbits per day is not an integer number, 
 the sub-orbital tracks do not repeat from day to day, although the 
 local solar time for passing any latitude is essentially unchanged. 
 For this reason, the orbital equator crossings will occur at varying 
 longitudes during the lifetime of the satellite. 
 
 NOAA/NESS currently operates two CDA stations, one in Virginia 
 and one in Alaska, to receive the environmental data from the 
 satellite. During three sequential orbits of the Earth (four, some 
 days), however, the satellite remains out of contact with one of these 
 sites. To eliminate delay in receipt of high priority vertical 
 temperature profile data during this period, a data receipt-only 
 station has been established to receive data in Lannion, France, by 
 CNES. This station will acquire stored TIP data and transmit it to 
 the central processing facility in the United States. When this 
 station is in use, the satellite will be out of contact with the 
 ground for no more than one orbital period per day. 
 
 3.3 Equator Crossing Time 
 
 The TIROS-N satellite series has been designed to operate with a 
 southbound equator crossing between 0600 and 1000 Local Solar Time 
 (LST), or a northbound equator crossing between 1400 and 1800 LST 
 (Figure 3-2). Power and thermal constraints preclude normal operation 
 within two hours of noon or (midnight) LST. The time of day actually 
 chosen for the initial injection condition for a particular satellite 
 depends on several factors: 
 
 • time of day that data are needed for input to synoptic map 
 analyses, 
 
 • subpoint solar angles for visible channel instruments, 
 
 • orbital plane separation from second satellite in orbit, 
 
 3-6 
 
6 AM 
 
 DESCENOI 
 NODE 
 
 6 PM 
 
 ORBIT 
 NORMAL 
 
 AM DESCENDING NODE ORBIT 
 
 6AM 
 
 ORBIT 
 NORMAL 
 
 6 PM 
 
 NDNG 
 NODE 
 
 PM ASCENDING NODE ORBIT 
 
 Figure 3-2 TIROS-N/NOAA Equator Crossing Times 
 
 3-7 
 
• expected drift from sun-synchronous conditions and 
 spacecraft time of day (solar angle) constraints, and 
 
 • time of year launch will occur. 
 
 3.4 Effects of Precession of Orbital Plane 
 
 3.4.1 Sun-synchronous Orbits with Earth-Oriented Sensors 
 
 A satellite traveling in a sun-synchronous orbit can acquire 
 images of any given area at the same local sun time each day. To 
 maintain this sun-synchronous operation from week to week, it is 
 necessary that precession 0, of the orbital plane be 360° in 365 days 
 to compensate for the earth's seasonal travel around the sun. 
 Precession is a function of satellite altitude and orbital inclination 
 and is given by the expression 
 
 Q - 10.0 a * cos i (degrees/24 hours) (5) 
 
 where i is the orbital inclination to the equator, a = (1 + h/R) , 
 R = 6,364 km, and h is average satellite height. The desired 
 precession is achieved when the sun-synchronous inclination i is 
 
 S S 
 
 such that: 
 
 cos i = 0.0986 fl + £) 3 * 5 (6) 
 
 ss \ Ry 
 
 Note that if the standard sign convention is followed, sun-synchronous 
 
 precession is achieved when Q has the negative value -0.986 and i 
 
 is greater than 90° (a retrograde orbit). Hov ver, sun-synchronous 
 
 may be taken to be positive, and the symbols i and i for 
 
 s s 
 
 inclination are then defined to be the acute angle between the 
 equatorial and orbital planes for a retrograde orbit. 
 
 The orbital inclination also defines the maximum geocentric 
 latitude (north and south) reached by the satellite subpoint on each 
 orbital pass. The maximum poleward excursions for sun-synchronous 
 inclination for various satellite altitudes are: 
 
 3-1 
 
• at h = 833 km (450 nm) , i = 81.1°, 
 
 • at h = 1,110 km (600 nm), i , = 79.9°, 
 
 s s 
 
 • at h = 1,388 km (750 nm), i =78.6°, and 
 
 ss ' 
 
 • at h = 1,850 km (1,000 nm), i = 75.9°. 
 
 s s 
 
 3.4.2 Drift from Sun-synchronous Conditions 
 
 Orbit parameter errors result from several causes associated with 
 the booster operation. These errors may be characterized as either 
 orbital plane (inclination) or altitude related. Orbital parameters 
 for the TIROS-N series are expected to be within the following three 
 sigma limits: 
 
 • altitude (average): +18.5 km, 
 
 • inclination: 0.15°, and 
 
 • apogee/perigee difference: less than 56 km. 
 
 After injection, solar forces (gravity and event effects) will 
 cause the inclination to change from that originally achieved. The 
 effect of this change, combined with the initial injection errors, 
 will probably cause the nodal precession rate to deviate from that 
 required for true sun synchronism. Whether the local solar time 
 becomes earlier or later depends on the sign of the error. It is 
 apparent that initial drift will be a factor of the accuracy of 
 injection into orbit; changes after this drift is defined will be 
 caused by solar effects. 
 
 3.5 Spacecraft Track over the Earth 
 
 In the early 1600s, Johann Kepler discovered the physical laws 
 that control the paths and motions of satellites in orbit. Those laws 
 forming the basis of computations at the local site are as follows. 
 
 • The orbit of each planet (satellite) is an ellipse with the 
 sun (earth) always located at one focus. 
 
 3-9 
 
• Each planet (satellite) moves in an orbit so that an 
 
 imaginary line drawn between the planet (satellite) and the 
 sun (earth) sweeps out equal areas in equal times (see 
 Figure 3-3). 
 
 .ORBITAL PATH 
 
 APOGEE 
 
 BOOfl TRIANGLE A 
 
 Figure 3-3 Diagram of Elliptical Orbit 
 
 The area of A will equal the area of B when the time required for 
 moving from 1 to 2 is equal to the time required for moving from 3 
 to 4. 
 
 In a perfectly circular orbit at any given altitude, a spacecraft 
 moves above the surface of the earth at a constant velocity. In 
 actual practice, no earth orbit is perfectly circular, although some 
 may approach this condition. In an earth satellite's orbital ellipse, 
 the point of closest approach to the earth is called perigee, whereas 
 the point farthest from the earth is the apogee. The initial 
 positions of apogee and perigee are determined by the launch 
 characteristics; subsequently, the positions of apogee and perigee 
 advance gradually around the orbit as a result of the gravity effect 
 of the equatorial bulge of the earth. 
 
 The change in the positions of perigee and apogee cause the 
 height at which the spacecraft passes over a given region of the earth 
 to vary with time (see Figure 3-4). Daily messages will reflect 
 changes in the position of apogee and perigee. 
 
 3-10 
 
For convenience and ease of computations, the time the spacecraft 
 passes over a given geographical location is frequently expressed as 
 the elapsed time between the northbound equator crossing (ascending 
 node) and the time it reaches the particular location. In this way, 
 time is considered zero when the spacecraft passes the equator 
 northbound and increases through a complete orbital period. 
 
 The time required for a spacecraft to move through a given 
 geocentric angle of its ellipse is considered to be constant for any 
 particular day so that the time required for the spacecraft to reach a 
 particular latitude is the same for every circuit of the earth on that 
 day. For example, if it reaches latitude 30 degrees North .ten minutes 
 after the northbound equator crossing (ascending node) on one circuit 
 of the earth, it will reach the same latitude (30 degrees N) at 
 ten minutes after the ascending node on each succeeding circuit of the 
 earth during that day. The movement of apogee and perigee causes a 
 very gradual change in the time necessary to reach this latitude on 
 each circuit. These changes are included in the Daily Messages. 
 
 10 MINUTES AFTER ASCENDING 
 NODE. LATITUDE 30* N. 
 
 ASCENDING NODE 
 ( TIME REFERENCE - ) 
 
 DAY I CONDITION < PERIGEE IS LOCATED AT THE EQUATOR. 
 
 APOGEE 
 
 10 MINUTES AFTER ASCENDING 
 NODE. LATITUDE 15* 
 
 EQUATOR 
 ( TIME REFERENCE - ) 
 
 PERIGEE 
 DAY N CONDITION : PERIGEE IS NEAR THE SOUTH POLE . 
 
 Figure 3-4 Diagram of Change in Perigee and Apogee 
 
 3-11 
 
4. REAL-TIME DATA SYSTEMS FOR LOCAL USERS 
 
 Local users in line of sight of the TIROS-N satellite can receive 
 real-time data directly from the spacecraft. TIROS-N broadcasts 
 visible (VIS) and IR data from the AVHRR as well as other data from 
 the TIROS Operational Vertical Sounder (TOVS), the DCS, the SEM, and 
 the spacecraft housekeeping telemetry. This section will focus on the 
 direct readout imaging systems of TIROS-N. 
 
 4.1 APT System 
 
 The AVHRR provides the data for transmission to both the APT and 
 HRPT users. The HRPT data are transmitted at full resolution 
 (1.1 km), whereas the APT output is transmitted at reduced resolution 
 (4 km) to maintain bandwidth constraints. The APT system transmits 
 any two of the AVHRR channels (see Table 2-1); the visible channel is 
 used to provide visible APT imagery during daylight, and one IR 
 channel is used constantly (both day and night). A second IR channel 
 can be scheduled to replace the visible channel observations during 
 the nighttime portions of the orbit. 
 
 4.2 HRPT System 
 
 The HRPT system transmits all of the channels. To avoid future 
 changes on the spacecraft and on-ground receiving equipment, the 
 TIROS-N HRPT data format has been designed as though the AVHRR were 
 already a five-channel instrument. When operating with the initial 
 four-channel instrument, the data from Channel 4 (11 um) are inserted 
 in the data stream twice so that the basic data format will be the 
 same for both the four- and five-channel versions. Output from the 
 low data-rate system onboard the spacecraft is multiplexed with the 
 AVHRR data and becomes a part of the HRPT output available to direct 
 readout users. The low data-rate system includes the three 
 instruments of the TOVS, the SEM, the DCS, and the spacecraft 
 housekeeping telemetry. 
 
 4-1 
 
4.3 Direct Sounder Broadcasts (DSB) 
 
 Data from the TOVS instruments are available from two separate 
 real-time data transmission links. Onboard the satellite, the TOVS 
 data are collected and formatted by the TIP. Parallel outputs are 
 provided for: 
 
 • the real-time VHF beacon transmission link, and 
 
 • MIRP, which combines the TIP data with the output from the 
 AVHRR. This combined data stream is: 
 
 broadcast in real-time as the HRPT service and 
 
 stored onboard the satellite for later transmission to 
 
 the ground. 
 
 Note that all data from the TOVS are digital and that the data 
 included within the HRPT format are identical to that broadcast on the 
 beacon. Data from the TOVS are available on both very high frequency 
 (VHF) and S-band data links. Those users receiving the high 
 resolution transmission from the AVHRR (HRPT system) will probably 
 find it most desirable to extract the TOVS data from this data 
 stream. Using two frequencies for these data eliminates interference 
 between transmissions from two satellites within view of a station. 
 This problem is particularly severe in high latitudes where 
 overlapping coverage is routine. 
 
 The VHF beacon data, also with two possible frequencies, are 
 available for users who do not intend to install the more complex 
 equipment necessary to receive data on the S-band frequencies. The 
 lower data rates (8 kilobits/second versus 665.4 kilobits/second for 
 HRPT) permit the user to install less complex and less costly 
 equipment to receive the data without degrading its quality. 
 
 4-2 
 
4.4 Data Collection System (DCS) 
 
 The Data Collection System (DCS) onboard TIROS-N is provided by 
 CNES of France. This system, also referred to as the ARGOS Data 
 Collection and Platform Location System (ARGOS DCLS), provides a means 
 to locate and/or collect data from fixed and free floating buoy and 
 balloon platforms. It provides two new services not currently present 
 in the geostationary satellite (GOES) data collection systems. First, 
 it is able to determine platform location using an inverse doppler 
 technique; second, it is able to acquire data from any place in the 
 world, but particularly in the polar regions beyond transmission range 
 of the geostationary satellites. 
 
 The ARGOS System will receive data from the platforms at UHF 
 401.65 megahertz (MHz) . The platforms will transmit independently 
 of any interrogation from the spacecraft that uses a random access 
 receiving system. Each platform will make continual transmissions, 
 varying in length from 360 to 920 milliseconds (ms) and a repetition 
 interval from 40 to 200 seconds. As the spacecraft passes within 
 range of a platform during its orbit, it will receive and record the 
 successive transmissions. Once the spacecraft comes within range of a 
 CDA, it will play back on command the recorded data to the ground 
 facility. On the ground, the data will be forwarded to Suitland, 
 where the ARGOS data will be separated from the other spacecraft 
 telemetry data and relayed to the CNES processing center in Toulouse, 
 France. There, the platform locations will be computed and the data 
 processed and prepared for relay to the user. The location 
 information is determined by measuring the platform carrier frequency 
 received by the DCS instrument in the spacecraft at each 
 transmission. When several transmissions are obtained from a 
 particular platform, differential doppler techniques are used to 
 determine the platform location within an accuracy of 3 to 5 km. 
 
 With two spacecraft operating in the TIROS-N operational system, 
 a platform will be in contact with the spacecraft during at least four 
 intervals each day, about six hours apart. During any interval, the 
 platform will be "seen" by the spacecraft on at least two successive 
 
 4-3 
 
orbits, which would provide a minimum of eight reports a day. In the 
 polar areas, as the orbital paths converge, many more contacts will be 
 made. Up to twenty or more reports may be acquired, approximating the 
 frequency of coverage available through the geostationary satellites. 
 
 The ARGOS system will also provide for the immediate rebroadcast 
 of data received from platforms. These data reports will be included 
 in the same data stream that contains the direct broadcast of sounder 
 data. The spacecraft VHF beacon transmits the DCS data at the low 
 rate and the S-band beacon (HRPT) transmits the DCS data at the higher 
 speed. Only data from platforms so located that both the platforms 
 and the receive site are simultaneously in view of the satellite will 
 be available from this direct broadcast. The DCS data in the direct 
 broadcast will only permit platform location computations when a 
 computer with the proper software is available for processing the data. 
 
 For more details on the ARGOS system contact Service ARGOS at the 
 following address: 
 
 Service ARGOS 
 
 Centre Spatial de Toulouse 
 
 18, Avenue Edouard Belin 
 
 31055 Toulouse CEDEX 
 
 France 
 
 4-4 
 
5. TRACKING PROCEDURES FOR DIRECTIONAL ANTENNAS USED TO ACQUIRE 
 DATA FROM REAL-TIME TRANSMISSIONS SYSTEM SENSORS 
 
 5.1 Introduction 
 
 All operators of satellite stations designed to receive any type 
 of direct readout transmissions from polar orbiting satellites need to 
 know the satellite's position in time and space in order to know when 
 to operate their equipment and to permit them to locate geographically 
 the features seen in the images. Operators of stations using 
 directional antennas also need this information to determine antenna 
 pointing angles. Satellite tracking procedures are not required for 
 receiving stations equipped with omnidirectional antennas. 
 
 The primary source of information concerning a satellite's 
 position in time and space is the APT Predict bulletin. The 
 information in the bulletin can be used with an APT plotting board and 
 a tracking diagram to determine the antenna azimuth and elevation 
 angles necessary to follow a polar orbiting satellite passing within 
 receiving range of a given station. The content and primary sources 
 of the APT Predict bulletin and the use of the plotting board and 
 tracking diagram are described in Sections 5.2 through 5.6. Alternate 
 sources and forms of satellite prediction information are identified 
 in Section 5.7. 
 
 The code form of the APT Predict (TBUS) bulletin, an example of 
 an actual TIROS-N TBUS bulletin, and a decoding exercise are given in 
 Appendix A. Exercises in the application of the TBUS bulletin 
 (Plotting Board preparation, tracking, and gridding) are given in 
 Appendices B through D. 
 
 5.2 APT Predict (TBUS) Bulletin 
 
 The APT Predict (TBUS) bulletin contains information on satellite 
 equator crossing times and longitudes, orbit numbers, orbital period, 
 longitudinal time, and longitude increments between successive orbits; 
 also, satellite positions at two-minute intervals (for a reference 
 
 5-1 
 
orbit), transmission frequencies, and other information related to 
 satellite tracking and performance. The information identifies those 
 orbits that will occur three days after the date of the bulletin (for 
 example, information disseminated on the tenth of the month pertains 
 to orbits that will occur on the thirteenth of the month). 
 
 The bulletins are prepared by NESS and transmitted through the 
 National Weather Service Communications Center (KWBC) to major 
 communications centers and relay points around the world. These 
 transmissions occur daily, at about 1908 GMT, by landline and 
 radio-teletype links to the World Weather Watch's Global 
 Telecommunications Service (GTS). The GTS primarily serves the 
 international meteorological community; therefore, most 
 nonmeteorological services, academic and commercial institutions, and 
 other organizations and individuals cannot receive the complete TBUS 
 bulletins unless they have the proper receiving and printing devices 
 and are able to intercept radio teletype broadcasts. Still, it is 
 important for those who are not connected with this service to 
 understand the derivation and content of the TBUS bulletin, because 
 nearly all other orbital predict data are extracts from this message 
 form. 
 
 There are two forms of the TBUS bulletin. One form, identified 
 as TBUS-1, is used to convey information about satellites that are 
 descending in daylight (traveling north-to-south on the sunlit portion 
 of the orbit). The second form, TBUS-2, provides data for satellites 
 ascending in daylight (northbound on the sunlit portion of the 
 orbit). A schematic representation of the TBUS-1 and TBUS-2 bulletins 
 is given in Figure 5-1. 
 
 Both bulletins consist of four parts. Part 1 is quite short. It 
 identifies a reference orbit on a given day (three days after the date 
 of the bulletin) and gives the equator crossing time and longitude for 
 this reference orbit. This is followed by an orbital nodal period and 
 a longitudinal increment — the separation between successive equator 
 crossings, measured in degrees. The orbit numbers of the fourth and 
 eighth orbits following the referenced orbit are then listed along 
 with the equator crossing times and longitude of these orbits. 
 
 5-2 
 
EARTH 
 
 DAY 
 PART II 
 
 DAY 
 PART2T 
 
 NIGHT 
 PARTH 
 
 TBUS-I 
 
 NORTH TO SOUTH DAYLIGHT ORBIT 
 
 ASCENDING 
 NODE 
 
 NIGHT 
 PART it 
 
 EARTH 
 NIGHT \ DAY 
 TERM\INATOR 
 
 NIGHT 
 F&RTU 
 
 TBUS-2 
 
 SOUTH TO NORTH DAYLIGHT ORBIT 
 
 DAY 
 PART n 
 
 ASCENDING 
 NODE 
 
 NIGHT 
 
 PARTTLT 
 
 DAY 
 PART HI. 
 
 Figure 5-1 Schematic representation of information conveyed in TBUS-1 and TBUS-2 
 bulletins 
 
 5-3 
 
By itself, Part I contains sufficient information to permit the 
 user to calculate future equator crossing times and longitudes several 
 days in advance with considerable accuracy. During periods of maximum 
 solar activity, however, the accuracy of crossing times and longitudes 
 extrapolated from Part I information diminishes if the extrapolations 
 are carried much more than a week ahead. 
 
 Parts II and III of the bulletins are quite lengthy. Part II 
 (Day) contains predicted subpoint and height data at two-minute 
 intervals for the portion of the orbit that is sunlit north of the 
 equator. Part II (Night) contains predicted subpoint and height data 
 at two-minute intervals for the portion of the orbit in darkness north 
 of the equator. Part III (Day) contains predicted subpoint and height 
 data at two-minute intervals for the portion of the orbit that is 
 sunlit south of the equator. Part III (Night) contains predicted 
 subpoint and height data at two-minute intervals for the portion of 
 the orbit in darkness south of the equator. All times are referenced 
 to the ascending node (northbound equator crossing) and are given as 
 minutes after or before this time as shown in Figure 5-1. 
 
 Part IV is relatively short, and usually consists of three 
 items: a code group, transmission frequencies of each operating 
 direct readout sensor system, and remarks. The code group contains 
 the orbital parameters used to generate Parts I, II, and III. It is 
 intended for use by those station operators needing more precision in 
 satellite tracking and having appropriate computer programs to ingest 
 such data to produce both equator crossings and antenna pointing 
 angles. 
 
 Remarks in Part IV are in plain language and advise the ground 
 station operator of problems or changes in the mode of operating the 
 satellite. In particular, erratic behavior or failure of a sensor, 
 transmitter, or spacecraft will be indicated under "Remarks." An 
 example of an actual TIROS-N TBUS bulletin, the code form, and an 
 exercise in decoding are given in Appendix A. 
 
 For NOAA-1 through NOAA-5, NASA provided NESS with the orbital 
 parameters on a weekly basis. For TIROS-N series spacecraft (such as 
 TIROS-N and NOAA-6), the information is obtained on a daily basis from 
 
 5-4 
 
United States Air Force NORAD stations; the NORAD stations determine 
 the orbits and provide NESS with high-precision orbital elements on a 
 daily basis, and low-precision orbital elements as needed 
 (high-precision orbital elements are expected to provide an in-orbit 
 positioning accuracy of +3 km). The elements are used to provide a 
 satellite ephemeris file, which is then accessed to produce the TBUS 
 bulletin. 
 
 Transmission frequencies for each operating direct readout 
 service (APT, HRPT, and DSB) also are included in Part IV. The APT 
 will always operate on a frequency of either 137.50 or 137.62 MHz; the 
 HRPT will transmit on 1698, 1702.5, or 1707 MHz (1702.5 MHz would be 
 used for HRPT in the event of failure of primary transmitters); the 
 DSB will operate on the (beacon) frequencies, either 137.77 MHz or 
 136.77 MHz. The selection of a particular frequency for any of these 
 transmissions depends on whether other satellites in this same series 
 are operating and the frequencies they are using. Another factor, of 
 course, will be transmitter performance. 
 
 Channel selection for APT transmissions is shown in Part IV of 
 the TBUS bulletin. The two-channel output of the APT is derived from 
 the five-channel AVHRR on TIROS-N series spacecraft. Two of the five 
 channels have responses in the visible region of the electromagnetic 
 spectrum; the other three channels are in the IR region. Data from 
 one visible and one IR channel will be transmitted continuously, both 
 day and night. A second IR channel can be programmed to replace the 
 visible channel data at night, however. 
 
 5.3 Plotting Board 
 
 The APT Plotting Board is a polar projection diagram of the earth 
 centered at either pole and extended 30 degrees of latitude past the 
 equator into the other hemisphere. (Examples of plotting boards for 
 the Northern and Southern Hemispheres are shown in Figures 5-2 and 
 5-3, respectively.) The Board has radials from the pole representing 
 one-degree intervals of longitude; each fifth radial is accentuated. 
 Concentric circles on the projection represent latitudes. The 
 equatorial latitude (zero degrees) is represented by a 
 
 5-5 
 
APT SYSTEM 
 
 METC080LOOICAL SATCLLITC 
 
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 TRACKING DIAGRAM 
 
 
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 Figure 5-2 Plotting Board—Northern Hemisphere 
 
 5-6 
 
APT SYSTEM 
 
 METCOftOUMKM. SftTELLITC 
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 TRACKINS DlMBAK 
 
 
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 Figure 5-3 Plotting Board-Southern Hemisphere 
 
 5-7 
 
heavier circle for clarity. A transparent overlay is centered at the 
 pole on the Plotting Board. The subpoint track for the spacecraft 
 from which data are to be acquired is plotted on this overlay. (An 
 exercise in the preparation of the Plotting Board is given in 
 Appendix B. ) 
 
 5.4 Tracking Diagram 
 
 5.4.1 Theory of Determining Azimuth and Elevation 
 
 Azimuth is a function of the direction of the spacecraft from the 
 antenna site and can be measured on any convenient map on which the 
 station and spacecraft location can be plotted. 
 
 Spacecraft elevation angle is a function of the spatial location 
 (position and height) of the spacecraft and the ground station antenna 
 (see Figure 5-4). If the spacecraft altitude (h) and the distance 
 from the ground station antenna to the spacecraft subpoint are known, 
 the elevation angle (6) of the spacecraft can be computed. 
 
 SPACECRAFT 
 
 SSP- SUBSATELUTE POINT 
 9- ELEVATION ANSLE 
 h - ALTITUOE 
 
 Figure 5-4 Diagram for Computing Spacecraft Elevation Angle 
 
 5-8 
 
5.4.2 Application of Tracking Diagram 
 
 The tracking diagram (Figure 5-5) was designed for use with the 
 Plotting Board and is constructed to show azimuth and distance of the 
 spacecraft from the antenna site for a given subpoint position. The 
 concentric curves (near ellipses) are isopleths of great circle arc 
 distance drawn at two-degree 222 km (120 nmi) intervals. 
 
 Azimuth is used directly in tracking; arc distance (geocentric 
 degrees) must be converted to elevation angle. Tables 5-1 and 5-2 
 provide a convenient conversion from arc length to elevation angle. 
 Note here that the arc distance on the diagram can be converted to and 
 labeled directly as elevation angles. This conversion is particularly 
 desirable if a circular orbit is achieved because, when labeled, the 
 diagram need not be changed during the lifetime of the spacecraft. An 
 elliptical orbit requires frequent changes of labels so use of the 
 tables is recommended in place of labeling. 
 
 A tracking diagram is available for each five-degree latitude 
 belt. The diagram drawn for the latitude closest to the antenna 
 location should be used because the scale changes significantly with 
 latitude. 
 
 5.5 Clock 
 
 A station clock must be readily available when tracking (if a 
 directional antenna is used) and when acquiring data. This clock 
 should be accurate to one second and easily read if accurate data 
 location is to result. Because the spacecraft moves over the earth at 
 approximately 380 km (210 nmi) per minute, a one-second time error 
 results in a location error of 6.5 km (3.5 nmi). 
 
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 5-9 
 
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 5-10 
 
TABLE 5-1 
 GREAT CIRCLE ARC LENGTH AT ZERO-DEGREE ELEVATION 
 
 Orbit Height (nm) 
 
 100 
 125 
 150 
 175 
 200 
 225 
 250 
 275 
 300 
 325 
 350 
 375 
 400 
 425 
 450 
 475 
 500 
 525 
 550 
 575 
 600 
 625 
 650 
 675 
 700 
 725 
 750 
 775 
 800 
 825 
 850 
 875 
 900 
 
 Arc Length 
 
 Height (km) 
 
 Arc Length 
 
 13.6 
 
 200 
 
 14.2 
 
 15.2 
 
 250 
 
 15.8 
 
 16.6 
 
 300 
 
 17.3 
 
 17.9 
 
 350 
 
 18.6 
 
 19.1 
 
 400 
 
 19.8 
 
 20.2 
 
 450 
 
 20.9 
 
 21.2 
 
 500 
 
 22.0 
 
 22.2 
 
 550 
 
 23.0 
 
 23.1 
 
 600 
 
 23.9 
 
 24.0 
 
 650 
 
 24.9 
 
 24.8 
 
 700 
 
 25.7 
 
 25.6 
 
 750 
 
 26.5 
 
 26.4 
 
 800 
 
 27.3 
 
 27.1 
 
 850 
 
 28.1 
 
 27.8 
 
 900 
 
 28.8 
 
 28.5 
 
 950 
 
 29.5 
 
 29.2 
 
 1000 
 
 30.2 
 
 29.8 
 
 1050 
 
 30.9 
 
 30.4 
 
 1100 
 
 31.5 
 
 31.0 
 
 1150 
 
 32.1 
 
 31.6 
 
 1200 
 
 32.7 
 
 32.2 
 
 1250 
 
 33.3 
 
 32.7 
 
 1300 
 
 33.9 
 
 33.3 
 
 1350 
 
 34.4 
 
 33.8 
 
 1400 
 
 34.9 
 
 34.3 
 
 1450 
 
 35.5 
 
 34.8 
 
 1500 
 
 36.0 
 
 35.3 
 
 1550 
 
 36.5 
 
 35.8 
 
 1600 
 
 36.9 
 
 36.2 
 
 1650 
 
 37.4 
 
 36.7 
 
 1700 
 
 37.9 
 
 37.1 
 
 1750 
 
 38.3 
 
 37.6 
 
 1800 
 
 38.8 
 
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 5-12 
 
5.6 Tracking Procedures 
 
 The procedures for satellite tracking are described in the 
 following sections. A tracking exercise, with all procedures 
 explained in detail, is given in Appendix C. 
 
 5.6.1 Initial Preparation 
 
 Center the appropriate tracking diagram on the Plotting Board at 
 the location (latitude and longitude) of the antenna site. The zero- 
 degree azimuth line must always point toward the pole and the 0-180 
 radial must be parallel to a longitude line. 
 
 NOTE: Southern Hemisphere stations must relabel azimuth lines 
 after the diagram is in place so that the zero-degree azimuth refers 
 to North . 
 
 5.6.2 Post Launch Preliminary Preparation 
 Transparent Orbital Overlay 
 
 • Plot the orbital path on the Plotting Board overlay using 
 the data in the first valid TBUS bulletin. Subdivide the 
 track into one-minute intervals starting with minute zero at 
 the equator. All points, therefore, will be plotted at a 
 time relative to the ascending node (northbound equator 
 crossing). Stations with the capability of receiving and 
 displaying both day and night transmissions should plot the 
 orbital subpoint track for both times of day as indicated in 
 the Daily Message. 
 
 • Subdivide the equatorial line (on the orbital overlay) into 
 divisions equal to the nodal increment. Begin at the 
 reference orbit and progress westward through 360 degrees 
 numbering sequentially from the reference orbit. Because 
 the equator crossing for every fourth orbit is given in 
 
 5-1: 
 
Part I of the bulletin, with the reference orbit set 
 properly, these points may be plotted directly to eliminate 
 possible error. 
 
 Equator Line on Plotting Board 
 
 Determine and indicate on the equator line of the Plotting Board 
 the acquisition zone of equator crossings (ascending nodes) that 
 willbring the spacecraft within range of the antenna site. There will 
 be two zones — one for the sunlit portion of the orbit and one for the 
 night portion. These zones should be labeled appropriately. 
 
 • From the data in Table 5-1, determine the arc distance for a 
 zero-degree antenna elevation when the spacecraft is at 
 apogee (this distance provides an estimate of the maximum 
 range from which data acquisition is possible). 
 
 • Indicate the zero degree line directly on the tracking 
 diagram. 
 
 • Rotate the previously plotted orbital overlay diagram until 
 the subpoint track (for the portion of the orbit where the 
 spacecraft is heading northward) until it is tangent to the 
 zero-degree elevation circle east of the antenna site. 
 Indicate the longitude of the equator crossing (ascending 
 node) of the plotted subpoint track. Rotate the orbital 
 overlay until the subpoint track is tangent to the 
 zero-degree elevation circle west of the antenna site. 
 Again, determine longitude of the equator crossing 
 (ascending node). Orbits whose ascending nodes fall between 
 the two longitudes determined are (assuming no local 
 obstruction) generally within line of sight of the 
 spacecraft during the northward portion of these orbits. 
 
 • Repeat the process (b) for the portion of the orbit where 
 the spacecraft is headed southward to determine the 
 corresponding longitudinal range for southbound acquisition. 
 
 5-14 
 
For practical application, the elevation of the spacecraft will 
 have to exceed zero to five degrees above the horizon for at least 
 four to five minutes during a pass before useful data can be 
 expected. Exact limits can be determined by experience, and the 
 longitudinal belts can be adjusted accordingly. 
 
 5.6.3 Daily Preparation — Derivation of Tracking Data 
 
 • Determine from the TBUS bulletin the exact equator crossing 
 (longitude and time) of the orbits from which data are to be 
 acquired. 
 
 • Place the ascending node of the plotted subpoint track 
 overlay for the Plotting Board at the longitude of the 
 ascending node of the reference orbit. 
 
 e Read directly the longitude of the ascending nodes and the 
 
 number of orbital increments from the reference orbit to the 
 equator crossings within the acquisition zone determined in 
 the previous section. 
 
 Mathematically determine the equator crossing time of those orbits 
 from which data are to be acquired by adding the proper number (as 
 determined above) of nodal periods (TBUS bulletin) to the equator 
 crossing time of the reference orbit. The exact longitude of the 
 equator crossings may be determined similarly if desired. For most 
 purposes, however, the graphical solution will prove to be of 
 sufficient accuracy. 
 
 • Rotate the transparent orbital overlay until the ascending 
 node of the plotted subpoint track is at the exact longitude 
 of the orbit from which data are to be acquired. 
 
 • Read directly the azimuth and arc distance of the satellite 
 (from the station) at the plotted points (one-minute 
 intervals) along the track when the track is within the 
 zero-degree elevation circle. The point of maximum 
 elevation angle (minimum arc distance) may be included as a 
 supplementary datum point. 
 
 5-15 
 
• Convert arc distance to elevation angle (Table 5-2). 
 
 • Convert the time for each minute (which is relative to the 
 equator crossing time) to Greenwich Mean Time (GMT). 
 
 • Check the accuracy of the clock. Set it to be accurate to 
 within one second. 
 
 Note that when a relatively circular orbit is achieved and absolute 
 pointing accuracy is not required, antenna predictions (with time 
 relative to the equator crossing) may be reused for future orbits with 
 similar equator crossing longitudes. It has been found for instance 
 that it is entirely feasible to use a single antenna prediction for 
 satellite equator crossings within 1 degree of the nominal predict. 
 GMT (clock) time must be updated for the actual equator crossing time 
 as determined from the daily message. 
 
 5.6.4 Tracking — Step Tracking 
 
 Because most receiving antennas will have beam widths of 20 to 
 30 degrees, the antenna must only move at one-minute intervals rather 
 than track continuously. It is necessary, therefore, to keep as close 
 to the predetermined values as possible; if, for instance, the 
 computed values were: 
 
 Time Azimuth Elevation 
 
 15:50:25Z 150° 10° 
 
 15:51:25Z 140° 18° 
 
 The antenna should be pointed at 18 degrees elevation, 140 degrees 
 azimuth at 15:50:55Z so that the antenna would be "ahead of" the 
 satellite for 30 seconds and "behind" for a similar period. The next 
 adjustment would then be at 15:51:55Z. When the elevation angle 
 exceeds 50 degrees, the user may find that the antenna position must 
 be changed at 30-second intervals, because azimuth will be changing 
 rapidly during "this period. 
 
 5-16 
 
5.7 Alternate Sources and Forms of Satellite Prediction Position 
 Information 
 
 The primary source of orbital prediction information for TIROS-N 
 series satellites is the Global Telecommunications System (GTS), and 
 the main form of this information is the TBUS bulletin. Many APT 
 station operators, and some government agencies, do not have access to 
 the GTS and must be able to obtain orbital prediction information from 
 other sources. These alternate sources include, but are not limited 
 to, the Aeronautical Fixed Telecommunications Network (AFTN), WEFAX 
 broadcasts from U.S. geostationary satellites, radio/CW broadcasts, 
 and mail. 
 
 5.7.1 Aeronautical Fixed Telecommunications Network 
 
 The AFTN primarily serves world-wide aviation interests. 
 Satellite orbital prediction information is transmitted on the AFTN as 
 an addressed message to those overseas agencies that cannot receive 
 TBUS bulletins via the GTS. 
 
 Message traffic on the AFTN is limited to 200 five-word groups, 
 and for this reason, only Part I and Part IV of the Predict bulletin 
 are transmitted via this network. 
 
 5.7.2 WEFAX 
 
 The complete TBUS bulletin, except the Remarks portion of 
 Part IV, is sent via GOES WEFAX once each day. Obviously, only those 
 stations having both an APT and WEFAX receiving capability would need 
 to, or be able to, copy these WEFAX broadcasts. 
 
 The format of this bulletin is described in Section 5.2 and in 
 Appendix A. It is the same format used to transmit APT Predict (TBUS) 
 bulletins via the GTS network. 
 
 5-17 
 
5.7.3 Radio/CW/Teletype 
 
 The American Radio Relay League (ARRL), a nonprofit organization 
 serving the amateur radio community, broadcasts satellite equator 
 crossing times and longitudes daily via radio (voice), CW, and 
 radio-teletype. These broadcasts contain crossing information only 
 for satellite passes over the North American continent and can only be 
 received in northern and eastern areas. 
 
 For additional information on broadcast times and frequencies, 
 write to: 
 
 American Radio Relay League 
 225 Main Street 
 Newington, CT 06111 
 
 5.7.4 Mail 
 
 Many nongovernment satellite readout stations are unable to 
 receive satellite orbital prediction information via government 
 telecommunications networks. Others do not have the equipment to 
 receive, or are not within range of U.S. geostationary satellites that 
 broadcast the TBUS messages. In these instances, the only source of 
 prediction data is a monthly letter prepared and distributed by NESS. 
 
 This information appears in the following form: 
 
 June 1, 1979 
 PREDICT DATA 
 for calculating satellite equator crossing times and longitudes: 
 
 Reference Orbit Information Increment 
 
 Satellite Orbit No. Date/ Time Longitude Nodal Period Between Orbit 
 
 (Ascending 
 Node) 
 
 TIROS-N 3252 01 June 150.28 102.0949 25.52 
 
 0109.24 
 
 5-lj 
 
Other Methods 
 
 In Africa, a number of countries receive orbital prediction 
 information through Agency Pour la Seceirite de la Navigation Ericanne 
 (ASECNA). 
 
 With the proper receiving equipment and teleprinter, some station 
 operators are able to intercept the GTS radio-teletype transmissions 
 of TBUS messages. Most major communications centers on the GTS (such 
 as Offenbach and New Delhi) relay this information via landline and 
 radio-teletype. Frequencies and schedules would have to be obtained 
 from the center nearest to the station. In the area of the United 
 Kingdom, it is known that this information is transmitted by station 
 GFL, which operates on frequencies of 4489, 9886.5, and 14356 KHz 
 24 hours per day, and on 6835 KHz between 1800 and 0600Z. 
 
 As a general rule, satellite readout station operators who do not 
 have access to the GTS, AFTN, WEFAX, or other such sources of predict 
 data are urged to contact an office of their national meteorological 
 agency or service to see if arrangements can be made to obtain copies 
 of these messages. (This is not possible in the United States, 
 because very few weather stations are linked to the GTS.) 
 
 Note that APT ground station operators who do not have a 
 requirement to grid images can use any of the above sources of orbital 
 information to predict the passage of satellites over their region. 
 Operators of APT, HRPT, or other types of direct readout ground 
 stations that require precise orbital information for tracking and/or 
 gridding would need to receive orbital information via the GTS, AFTN, 
 or geostationary satellites. 
 
 5-19 
 
6. GEOGRAPHIC REFERENCING TECHNIQUES 
 
 This section provides a detailed discussion of techniques for the 
 geographic referencing of the TIROS-N AVHRR continuous line scan data 
 system. In this discussion, it is assumed that spacecraft roll, 
 pitch, and yaw are all zero. Under these conditions, the scan is 
 perpendicular to the heading line and contains the instantaneous 
 subpoint at the bisector of the data line between earth horizons. 
 Thus, the subpoint track can be represented by a straight line 
 connecting the midpoint of each line. 
 
 6.1 Grid Format 
 
 A geographic referencing grid for the APT data transmitted by the 
 TIROS-N series satellites has been prepared and is available to APT 
 users on film. To fit the line-scan display on the ground, the grid 
 must be enlarged to the size of the image. The grid can be obtained 
 by contacting the Coordinator, Direct Readout Services. 
 
 A reproduction of the TIROS-N grid for an 840-km orbital height 
 (sun-synchronous orbit, inclination 98.8 degrees) is shown in 
 Figure 6-1. The solid line through the center, parallel to the edges, 
 represents the subpoint track. Image edges are indicated by solid 
 lines parallel to and at equal distances from the subpoint track. 
 With correct orientation, a single grid drawn for one-quarter of the 
 orbit may be used throughout the orbit. Orientation is proper for the 
 orbital segment for which the legend may be read directly. Two copies 
 of the grid may be combined (if desired by the user) to provide a 
 continuous grid from pole to pole. 
 
 6.2 Grid Labels 
 
 Latitude lines are labeled directly on the grid, whereas the 
 longitude lines are universal. The user assigns the proper longitude 
 labels to conform with orbital data computed from the Daily Predict 
 Bulletin. Latitude lines are drawn and labeled for five-degree 
 
 6-1 
 
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 6-2 
 
intervals. Longitude interval is a function of latitude, with a 
 spacing of five degrees from the equator to 50 degrees north or south, 
 10 degrees between 50 and 70 degrees, and 20 degrees from 70 to 
 85 degrees. Longitudes are not indicated above 85 degrees. 
 
 When assigning correct labels to the longitudes, the grid must be 
 positioned so that the grid and image edges match and the individual 
 lines of data are at their proper latitudinal position. It is 
 incorrect to move the grid perpendicular to the track so that 
 longitude values coincide with the grid lines. 
 
 Two techniques are available for labeling longitude lines. 
 
 • Determine the longitude of the ascending (descending) node 
 mathematically from information in the daily message and 
 label this intersection directly on the grid. Other 
 longitude lines should be labeled relative to this point. 
 
 • Determine the actual latitude and longitude of the 
 subsatellite point for several lines of data during 
 acquisition. Label the grid to conform to these points 
 during the gridding procedure. 
 
 If an overlay type grid is used for geographical orientation of 
 the APT data, the first technique will probably be the one most 
 readily adaptable to image gridding. The grid may be prepared before 
 data acquisition. If a projection method is used for gridding, the 
 second technique will be most readily adaptable. 
 
 6.3 Fitting Grids to APT Images (Overlay Technique) 
 
 The following procedure is suggested for fitting grids to APT 
 images using the overlay technique. A practical exercise in grid 
 fitting is given in Appendix D. The exercise procedures, based on an 
 earlier satellite grid, will be the same for the TIROS-N grid. 
 
 1) Preparation: 
 
 a) determine subpoint data (latitude and height) directly 
 from the daily message and extrapolate the ascending 
 (descending) node longitude for the orbits from which 
 data are to be acquired, 
 
 6-3 
 
b) choose the 840-km height grid (already magnified to the 
 correct size), 
 
 c) label the longitudes to conform to the ascending 
 (descending) node determined in Step la, and 
 
 d) sketch the even five-degree intervals for convenience 
 in using the data. 
 
 2) Determine time of receipt of several lines of data separated 
 by 4 to 6 minutes. For convenience, use a time after 
 (before) ascending node found in the daily message 
 (example: 23 minutes after ascending node). 
 
 3) Determine directly latitude of subsatellite point at each of 
 the times in Step 2. 
 
 4) Orient the grid properly (based on direction of spacecraft 
 movement) and place it on the image so that the edges (grid 
 and image) match and the latitudes of the subsatellite 
 points, determined in Step 3, are correctly located. 
 
 6.4 Fitting Grids to Images (Projection Technique) 
 
 1) Determine subpoint data (latitude, longitude, and height) 
 for the orbit on which data are to be acquired. 
 
 2) Determine time of receipt of several lines of data separated 
 in time by 4 to 6 minutes. For convenience, those lines of 
 data should be found which are acquired at an even minute 
 interval after (before) the ascending node (use a minute 
 value found in the daily message). 
 
 3) Determine the subsatellite point position of each line of 
 data for which time was determined. 
 
 4) Enlarge the TIROS-N grid to the size of the image display. 
 
 5) Orient the grid properly to agree with the direction of 
 motion of the spacecraft. 
 
 6) Project the grid onto the image so that the edges (grid and 
 image) match and the latitude of the subsatellite points 
 (Step 3) are correctly displayed. 
 
 6-4 
 
7) Assign longitude values to agree with the data determined in 
 Step 3. When using either of these techniques, landmarks 
 should be checked to ensure a proper fit. Near the center 
 of the grid, landmarks should appear in the correct 
 location; if they do not, the grid should be adjusted to 
 correct the errors. The most common causes of erroneous 
 grid-fit are miscomputations, time, or attitude errors. 
 
 )-5 
 
7. FORMAT OF DIRECT READOUT DATA 
 
 Examples of TIROS-N APT and HRPT direct readout images are shown 
 in the following sections. These images are included in the User's 
 Guide to demonstrate the format of the direct readout data transmitted 
 by the TIROS-N series satellites and to demonstrate the differences 
 between the APT and the HRPT images. 
 
 In some of the examples shown, the data have been processed to 
 enhance certain features in the images. Although sophisticated types 
 of data enhancement require expensive processing equipment, some 
 enhancement of APT data can be performed using relatively inexpensive 
 equipment available to APT users. For further information on 
 techniques and equipment for image enhancement, it is suggested that 
 the user contact the Coordinator, Direct Readout Services or contact 
 directly the APT equipment manufacturers. 
 
 In the example images shown in the following sections, certain 
 features of interest are pointed out. However, it is not the purpose 
 of this User's Guide to present a complete discussion of the 
 meteorological interpretation of direct readout data. References on 
 data interpretation and analysis are given in the bibliography. 
 
 7.1 APT Data 
 
 An example of unenhanced APT data is shown in Figure 7-1. These 
 data were received and recorded from the TIROS-N satellite on an 
 afternoon ascending pass using an Alden APTS-3B Satellite Ground 
 Receiving Station equipped with VHF omni-directional antenna 
 (Model 9330-9-2A). The image on the left is Channel 4 (thermal 
 infrared, 10.5-11.5 pm) and on the right Channel 1 (visible, 
 0.55-0.90 urn) . It should be noted that these images have been 
 inverted to facilitate interpretation of the data (north is at the top 
 of the page). These images cover the eastern part of North America; 
 Hudson Bay is near the top of the image, and Hispaniola can be 
 identified near the bottom of the image. 
 
 7-1 
 
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The channel identification on APT images may be accomplished by 
 referring to the gray-scale wedge included along the edge of each 
 image. The identification technique requires a visual comparison of 
 the gray-level density of the "channel ID" step with the remaining 
 eight steps. The step that matches the density of the channel ID step 
 is the channel number. 
 
 Enhanced APT images are shown in Figure 7-2a (Channel 1) and 
 Figure 7-2b (Channel 4). These enhanced images were, processed from a 
 tape recording of the same TIROS-N pass using a Leonessa Engineering 
 K 300-140B signal processor. In the enhanced images, considerably 
 greater detail can be seen than in the images processed with no 
 enhancement. In the Channel 1 data (Figure 7-2a), for example, a 
 distinct snowline can be seen extending southwestward from the 
 southern end of Chesapeake Bay on the United States east coast. In 
 the enhanced Channel 4 data (Figure 7-2b), clouds over the 
 snow-covered ground can be more easily seen because of their lower 
 temperatures; also, variations in sea surface temperature associated 
 with the Gulf Stream can be detected off the east coast. 
 
 The thermal infrared image (Channel 4) provides a good 
 illustration of the enhancement technique. In the data as received 
 directly from the satellite, a full range of temperature is displayed 
 in a limited number of gray-scale steps. It is difficult, therefore, 
 to detect the subtle temperature differences in the display. In the 
 enhanced data, the relationship between the temperature and the 
 gray-scale levels can be varied. For example, the higher temperatures 
 can be displayed in a greater number of gray-scale steps than in the 
 original data (note the difference in the corresponding gray-scale 
 wedges in Figures 7-1 and 7-2b). In this way, the subtle temperature 
 differences can be displayed. 
 
 Examples of the other TIROS-N channels are shown in Figures 7-3 
 and 7-4. In Figure 7-3, Channel 2 (0.725-1.1 pm) is on the left and 
 Channel 1 on the right. In this view of the central United States, 
 greater contrast between water (lakes, rivers) and land can be seen in 
 the Channel 2 image. In Figure 7-4, Channel 3 (3.55-3.93 um) is on 
 
 7-3 
 
Figure 7-2a 
 
 TIROS-N enhanced Channel 1 (visible) APT image of the east- 
 ern United States and the Atlantic Ocean. The snowline is 
 much easier to discern as compared to the unenhanced Channel 
 1- image shown in Figure 7-1 (b) . Refer to Figure 7-1 for 
 identification of features. 
 
 7-4 
 
 $ 
 
Figure 7 -2b 
 
 TIROS-N enhanced Channel 4 (thermal infrared) APT image show- 
 ing the same area as Figure 7-2a. This enhanced APT image 
 shows greater detail in both cloud structure and variations 
 in sea surface temperature (A) than does the unenhanced image 
 in Figure 7-1 (a) . The Great Lakes (B) and the edge of snow- 
 line (C) can be seen in this image. 
 
 7-5 
 

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the left and Channel 4 on the right. Some cloud and land features are 
 difficult to identify in the Channel 3 image because in the daytime 
 the energy detected at that wavelength is a combination of reflected 
 solar energy and emitted energy (temperature). The data shown in 
 these two figures have also been enhanced, similar to the data shown 
 in Figures 7-2(a and b). 
 
 With regard to the enhancement of APT data, it is important to 
 remember that only the raw data received directly from the satellite 
 can be enhanced at the APT receiving station using a device such as 
 the Leonessa Engineering K300-140B signal processor. Images 
 transmitted by WEFAX have already been processed at a central 
 processing facility and cannot be enhanced further at the APT station. 
 
 7.2 HRPT Data 
 
 Images received in the HRPT (or LAC) operational mode are the 
 full-resolution (1.1 km) AVHRR data, whereas images received in the 
 APT (or GAC) mode are the degraded resolution (4 km) data. These 
 operational modes are described in detail in earlier sections. 
 
 Examples of daytime HRPT images are shown in Figures 7-5 
 (a and b). The Channel 1 (visible) image is in Figure 7-5a, and the 
 Channel 4 (IR) image in Figure 7-5b. The area viewed is the eastern 
 United States and part of Canada. On this date (17 February 1979), 
 snow covered most of the eastern part of North America, and an 
 unusually extensive ice cover existed on the Great Lakes; some ice can 
 also be detected in bays along the east coast of the United States. 
 The improved detail of the HRPT as compared to the APT can be seen by 
 comparing Figure 7-5 with Figures 7-1 and 7-2, which view 
 approximately the same area. 
 
 Examples of all four AVHRR channels received in the LAC mode are 
 shown in Figures 7-6 (a through d). The LAC images are the same 
 resolution as HRPT, except are recorded on-board the satellite for 
 later transmission on commanded readout to the Command and Data 
 
 7-8 
 
6Z33JZT Nl SI A IddHN T 96ZT T0:Tfr:6T:8tt) Z "Wl 
 
 Figure 7-5 (a) TIROS-N HRPT Channel 1 (visible--0 . 55-0 .90 ym) daytime image 
 viewing the eastern United States and Canada. Snow covers 
 most of the land areas viewed, and considerable ice cover 
 exists on the Great Lakes. 
 
 7-9 
 
5232321 Nl SI IdHH N — i 952T TO:Tfr:ST:SWJ 2 U 
 
 Figure 7-5 (b) TIROS-N HRPT Channel 4 (infrared--10. 5-11.5 urn) daytime HRPT image 
 
 taken at the same time and over the same area shown in Figure 7-5 (a) 
 
 7-10 
 
Figure 7-6(a) TIROS-N HRPT Channel 1 (0.55-0.90^m) daytime image viewing 
 India and Sri Lanka. 
 
 7-11 
 
Figure 7-6(b) TIROS-N HRPT Channel 2 (0. 725-1. l>«m) daytime image, same time 
 and area as shown in Figure 7-6(a). 
 
 7-12 
 
Figure 7-6(c) TIROS-N HRPT Channel 3 (3.55-3.93ywm) daytime image, same time 
 and area as shown in Figure 7-6(a). 
 
 7-13 
 
Figure 7-6(d) TIROS-N HRPT Channel 4 (10.5-ll.5yum) daytime image, same time 
 and area as shown in Figure 7-6(a). 
 
 7-14 
 
Acquisition Station for relay to the NOAA central processing 
 facility. These images show India and Sri Lanka in daylight; 
 Channels 1 through 4 are shown in Figures 7-6a through 7-6d, 
 respectively. 
 
 7.3 Enhancement of Digitized AVHRR Data 
 
 At the NOAA/NESS central receiving station at Wallops Station, 
 Virginia; Fairbanks, Alaska; and San Francisco, California; the AVHRR 
 analog signal can be converted to digital form and stored on computer 
 compatible tapes. Using the digital data the sensitivity of the 
 satellite sensor can be fully exploited such that the more subtle 
 temperature differences of the input signal can be made readily 
 apparent to the analyst. This type of sophisticated data processing 
 cannot be accomplished at APT stations, but the user may see examples 
 of the enhanced digitized data in various publications. A reference 
 on digital processing is given in the bibliography. 
 
 7-15 
 
REFERENCES FOR ORBIT PREDICTION, SATELLITE TRACKING, 
 AND IMAGE GRIDDING 
 
 Anon., Lehigh University. "Aptrack Users' Guide," (Aptrack is a 
 Fortran IV Program which computes Azimuth and Elevation look 
 angles for selected APT ground stations). S.S.D.O. Student 
 Report, 1971, 28 pp. 
 
 Brush, R. J. H. "Real Time Linearization and Gridding of NOAA Scanning 
 Radiometer Pictures," University of Dundee, Department of 
 Electrical Engineering and Electronics (Dundee, Scotland), 1976, 
 16 pp. 
 
 Davidoff, M. R. "Using Satellites in the Classroom: A Guide for 
 
 Science Educators," Cantonsville Community College, Cantonsville, 
 MD 21228. 
 
 Eber, L. E. "Subpoint Prediction for Direct Readout Meteorological 
 Satellites," NOAA Technical Report NMFS SSRF-669 , August 1973, 
 7 pp. 
 
 Forber, A. E. "Geographical Annotation of Reduced Resolution Satellite 
 Imagery," Internal Report No. MSRB-77-5 (Satellite Data 
 Laboratory, AES, 4905 Dufferin Street, Downsview, Ontario), 
 October 31, 1977. 
 
 Hall, J. "Bearing and Distance Calculations by Slight of Hand," QST , 
 August 1973, p. 24. 
 
 Keegan, T. J. and R. S. Hawkins. "A Simplified Method of Programming 
 APT Tracking Angles," Reprinted from Journal of Applied 
 Meteorology , Vol. 5, No. 6, December 1966, pp. 902-903. 
 
 Larson, L. R. "Bearing and Elevation Calculations for Satellites," 
 Technical Correspondence, QST , May 1974, p. 57. 
 
 Taggart, R. E. Appendix I, "Satellite Orbital Data," Weather Satellite 
 Handbook (A 73 publication), p 61. 
 
 Taggart, R. E. Appendix II, "A Cookbook Procedure for Generating 
 Orbital Data," Weather Satellite Handbook (A 73 publication), 
 p. 62. 
 
 Tetrick, N. J. and W. W. Knapp. "Computer Program for the Calculation 
 of Tracking Data for Automatic Picture Transmission Satellites," 
 New York State College of Agriculture and Life Sciences at Cornell 
 University (Ithaca, New York), 1976, 27 pp. 
 
 Thompson, P. D. "A General Technique for Satellite Tracking," QST , 
 November 1975, pp. 29-34. 
 
 R-l 
 
GENERAL REFERENCES 
 
 Anderson, R. K. 1974. Application of Meteorological Satellite Data 
 in Analysis and Forecasting . ESSA Technical Report NESC 51. 
 Washington, DC. 
 
 Argos 1978. User's Guide Satellite-Based Data Collection and Location 
 System . Service Argos, France. 
 
 Argos NASA/NOAA/CNES. Location and Data Collection Satellite System . 
 Service Argos, France. 
 
 Hussey, W. J. 1978. The Tiros-N Twins Are Coming . NOAA Reprint 
 Volume 8(2). U.S. Department of Commerce, Washington, DC. 
 
 Hussey, W. J. 1977. The Tiros-N Polar Orbiting Environmental Satellite 
 System . NESS, Washington, DC. 
 
 Johnson, J. D., F. C. Parmenter, and R. Anderson 1976. Environmental 
 Satellites: Systems Data Interpretation and Applications . 
 U.S. Department of Commerce, Washington, DC. 
 
 Koffler, R. Digital Processing of NOAA's Very High Resolution 
 Radiometer (VHRR) Data . NESS/NOAA, Washington, DC. 
 
 National Environmental Satellite Center 1969. ESSA Direct Transmission 
 System User's Guide . U.S. Department of Commerce, Washington, DC. 
 
 Nelson, G. J. 1978. APT IR Channel Calibration . U.S. Department of 
 Commerce, Washington, DC. 
 
 NOAA 1978. Tiros-N APT/HRPT IR Calibration Information . APT Informa- 
 tion Note 78-5, Washington, DC. 
 
 NOAA 1978. Tiros-N Launch and Operating Plans . APT Information 
 Note 78-4, U.S. Department of Commerce, Washington, DC. 
 
 NOAA 1977. They've Got You Covered . Volume 7(4). U.S. Department 
 of Commerce, Washington, DC. 
 
 NOAA/NESS 1977. Present and Future U.S. Meteorological Satellite 
 Programs . U.S. Department of Commerce, Washington, DC. 
 
 NOAA/NESS. Direct Reception of Tiros-N Digital Dat£ .- U.S. Department 
 of Commerce, Washington, DC. 
 
 Schwalb, A. 1978. The Tiros-N/NOAA A-G Satellite Series . NOAA TM 
 NESS 95. U.S. Department of Commerce, Washington, DC. 
 
 R-2 
 
Shelton, E. F. "Individual AZ/EL Coordinates in Minutes," AMSAT 
 Newsletter, March 1976, p. 18. 
 
 Shuch, H. P. "Calculating Antenna Bearing for Geostationary 
 Satellites," to be published in Ham Magazine . 
 
 Stevenson, M. R., F. R. Miller, and R. W. Wagner. "Design of an 
 
 Inexpensive Programmable Antenna Tracking System" (Diseno de un 
 Sistema de Programacion, Poco Costosa, de una Antena Detectora), 
 Technical Report, May 1974, Inter-American Tropical Tuna 
 Commission, La Jolla, CA 92037. 
 
 R-3 
 
APPENDIX A 
 APT PREDICT (TBUS) BULLETIN 
 
 APT Predict (TBUS) Bulletin Code 
 
 The TBUS is a national practice code form used by the United States to 
 transmit information for predicting the path or locating the position of polar 
 orbiting environmental satellites. It is transmitted daily, at about 1908Z, by 
 KWBC Washington, DC, on the Global Telecommunications Service network. 
 
 The TBUS-1 code form is used to convey information about satellites that 
 are descending in daylight (i.e., north to south direction of travel in 
 daytime), while the TBUS-2 code form relates to satellites that are ascending in 
 daylight (south to north). 
 
 This Appendix contains: < 
 
 Figure A-l -- Schematic representation of the difference 
 
 between TBUS-1 and TBUS-2 A-2 
 
 Section A.l - The TBUS-1 code form A-3 
 
 Section A. 2 - The TBUS-2 code form A-5 
 
 Section A. 3 - Explanation of code symbols A-7 
 
 Figure A-2 -- Global octant map relating to the code 
 
 symbol Q in the code A-13 
 
 Section A. 4 - Sample APT Predict (TBUS) bulletin A-14 
 
 Section A. 5 - Decoding exercise applicable to the APT Predict 
 (TBUS) bulletin and to the exercises contained 
 in the following Appendices A-15 
 
 A-l 
 
EARTH 
 
 DAY 
 PARTIT 
 
 DAY \ NIGHT 
 
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 NIGHT 
 PART It 
 
 + 52 J 
 
 56 X 
 
 58 X 
 
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 / EQUATOR \ 
 
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 | 2 ASCENDING 
 
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 + 64 -V 
 
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 PARTUT 
 
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 NIGHT 
 PART It 
 
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 NIGHT V DAY 
 
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 NIGHT 
 FftRT U 
 
 TBUS-2 
 
 SOUTH TO NORTH DAYLIGHT ORBIT 
 
 DAY 
 PART n 
 
 ASCENDING 
 NODE 
 
 NIGHT 
 
 PARTTH 
 
 DAY 
 
 PART nrx: 
 
 Figure A-l. Schematic representation of difference between 
 
 TBUS-1 and TBUS-2 
 
 A-2 
 
Section A.l THE TBUS-1 CODE FORM 
 
 U. S. NATIONAL PRACTICE CODE TBUS-1 FOR SATELLITE EPHEMERIS PREDICT MESSAGE 
 
 (DAYLIGHT DESCENDING SATELLITES) 
 
 TBUS 1 KWBC 
 APT PREDICT 
 MMYYSS 
 
 PART I 
 
 ON r N r N r N r OY r Y r G r G r Og r g r s r s r Q r L L Q l l Tggss LL^l^ 
 
 N 4 N 4 N 4 N 4 G 4 G 4 g 4 g 4 s 4 s 4 Q 4 L L Q 1 Q l Q 
 
 N 8 N 8 N 8 N 8 G 8 G 8 g 8 g 8 s 8 s 8 Q 8 L L 1 Q l 
 
 N 12 N 12 N 12 N 12 G 12 G 12 g 12 g 12 s 12 s 12 Q^LoVo 
 
 NIGHT PART II 
 
 02h 02 h 02 Q 02 L a L a l a L o L Q l Q 04h 04 h 04 Q 04 L a L a l ^1 Q 
 
 06h 06 h 06%6 L a L a ] a^o 1 o <>gh gg h gg Q gg ^a 1 aW o 
 
 ...and continuing north, at two-minute intervals, to day/night terminator in 
 N. Hemisphere. 
 
 NIGHT PART III 
 
 02h 02 h 02%2 L a L a ] a L o L o ] o 04h 04 h 04%4 W & l oW o 
 
 06h n .h n< -Q n . L L 1 L 11 ggh n „h Q nn L L 1 L L 1 
 
 06 06 M 06 a a a o o o gg gg gg a a a o o o 
 
 ...and continuing south, at two-minute intervals, to day/night terminator in 
 S. Hemisphere. 
 
 DAY PART II 
 
 ...begins near day/night terminator in N. Hemisphere, two minutes after last 
 position given in NIGHT PART II, continuing south at two-minute intervals and 
 ending close to and north of the equator, repeating the code form: 
 
 ,ggh h Q L L 1 L L 1 
 
 gg gg gg a a a o o o 
 
 DAY PART III 
 
 ...begins two minutes after last position given in DAY PART II. First two code 
 groups give satellite time, height, octant, and latitude/longitude of the first 
 position south of the equator; following groups give the same information at 
 two-minute intervals until spacecraft reaches day/night terminator in 
 S. Hemisphere; repeating code form: 
 
 .... g g gg gg^gg a a a o o o.... 
 
 A-3 
 
PART IV 
 
 AAAAAAAAA BBBBB CCCCCCCCCCCC DDEEFFGGHHI 1 1 1 1 JJJJJJJ 
 
 KKKKKKKK LLLLLLLL MMMMMMMM NNNNNNNN 00000000 PPPPPPPP 
 
 QQQQQQQQ RRRRRRRR SSSSSSSSSS TTTTTTTTTT UUUUUUUUUU VVVVVVVVV 
 
 WWWWWWWWW XXXXXXXXX YYYYYYYYY ZZZaaabbb cccc dddddddddd 
 
 eeeeeeeee fffffffff ggggggggg SPARESPARE 
 
 APT TRANSMISSION FREQUENCY XXX. XX MHz 
 
 HRPT TRANSMISSION FREQUENCY XXXX.XX MHz 
 
 BEACON (DSB) TRANSMISSION FREQUENCY XXX. XX MHz 
 
 APT DAY X/X APT NIGHT X/X 
 
 DCS CLOCK TIME DAY XXX XXXXX.X 
 
 (ADDITIONAL PLAIN LANGUAGE REMARKS WHEN NEEDED) 
 
 CODE SYMBOLS FOR HEADING AND PARTS I -1 1 1 
 
 MM - month 
 
 YY - day 
 
 SS - satellite (see Table A-l , below) 
 
 NNNN - orbit number 
 
 GG - hour 
 
 gg - minutes 
 
 ss - seconds 
 
 Q - Octant of Globe (see Figure A-2, page A-13) 
 
 L L - Longitude (tens and units) 
 
 1 - Longitude (tenths) 
 
 T - Group indicator 
 
 L - Group indicator 
 
 hh - Height in hundreds and tens of kilometers 
 
 L - Latitude (tens) 
 1 - Latitude (tenths) 
 
 a 
 
 TABLE A-l 
 
 Numbers used in TBUS, SATOB, SATEM, SAREP bulletins to identify satellites: 
 
 10 - 19 ITOS series satellites 
 20 - 29 SMS/GOES series satellites 
 30 - 39 TIROS-N series satellites 
 
 A-4 
 
Section A. 2 THE TBUS-2 CODE FORM 
 
 U. S. NATIONAL CODE TBUS-2 FOR SATELLITE EPHEMERIS PREDICT MESSAGE 
 
 (DAYLIGHT ASCENDING SATELLITES) 
 
 TBUS 2 KWBC 
 APT PREDICT 
 MMYYSS 
 
 PART I 
 
 ON r N r N r N r OY r Y r G r G r Og r g r s r s r Q^l^ Tggss LL L 1 1 
 N 4 N 4 N 4 N 4 G 4 G 4 g 4 g 4 s 4 s 4 Q^l^ 
 N 8 N 8 N 8 N 8 G 8 G 3 g 8 g B s 8 s 8 0^1 1 Q 
 
 N 12 N 12 N 12 N 12 G 12 G 12 g 12 g 12 s 12 s 12 Q 12 L o L o 1 o 1 o 
 
 DAY PART II 
 
 02h 02 h 02 Q 02 L a L a l a L L Q l Q 04h 04 h 04 Q 04 L^l ^1 Q 
 
 06h 06 h 06%6 ^a 1 a l oW o ggh gg h gg Q gg ^a 1 a l oW o 
 
 ...and continues north, at two-minute intervals, to day/night terminator in 
 N. Hemisphere. 
 
 DAY PART III 
 
 02h 02 h 02 Q 02 L a L a l a L L Q l Q 04h 04 h 04 Q 04 L a L a l aL() L l Q 
 
 06h 06 h 06%6 L a L a 1 a L o L o 1 o ggh gg h gg Q gg L a L a 1 a L o L o 1 o 
 
 ...and continuing south, at two-minute intervals, to day/night terminator in 
 S. Hemisphere. 
 
 NIGHT PART II 
 
 ...beginning near day/night terminator in N. Hemisphere, two minutes after 
 last position given in DAY PART II, continuing at two-minute intervals and 
 ending close to and north of the equator; repeating code form: 
 ...ggh h Q L L 1 L L 1 
 
 gg gg gg a a a o o o 
 
 NIGHT PART III 
 
 ...beginning two minutes after last position given in NIGHT PART II. First 
 two code groups give satellite time, height, octant, and latitude/longitude of 
 the first position south of the equator; following groups give the same 
 information at two-minute intervals until spacecraft reaches day/night 
 terminator in S. Hemisphere; repeating code form: 
 ...ggh h Q I L 1 L L 1 
 
 y gg gg w gg a a a o o'o 
 
 A-5 
 
PART IV 
 
 AAAAAAAAA BBBBB CCCCCCCCCCCC ODEEFFGGHHI I I I I JJJJJJJ 
 
 KKKKKKKK LLLLLLLL MMMMMMMM NNNNNNNN OOOOOOOO PPPPPPPP 
 
 QQQQQQQQ RRRRRRRR SSSSSSSSSS TTTTTTTTTT UUUUUUUUUU VVVVVVVVV 
 
 WWWWWWWWW XXXXXXXXX YYYYYYYYY ZZZaaabbb cccc dddddddddd 
 
 eeeeeeeee fffffffff ggggggggg SPARESPARE 
 
 APT TRANSMISSION FREQUENCY XXX. XX MHz 
 
 HRPT TRANSMISSION FREQUENCY XXXX.XX MHz 
 
 BEACON (DSB) TRANSMISSION FREQUENCY XXX. XX MHz 
 
 APT DAY X/X APT NIGHT X/X 
 
 DCS CLOCK TIME DAY XXX XXXXX.X 
 
 (ADDITIONAL PLAIN LANGUAGE REMARKS WHEN NEEDED) 
 
 CODE SYMBOLS FOR HEADING AND PARTS I -I I I 
 
 MM - month 
 
 YY - day 
 
 SS - satellite (see Table A-l, below) 
 
 NNNN - orbit number 
 
 GG - hour 
 
 gg - minutes 
 
 ss - seconds 
 
 Q - Octant of Globe (see Figure A-2, page A-13) 
 
 L L - Longitude (tens and units) 
 
 1° ° - Longitude (tenths) 
 
 T - Group indicator 
 
 L - Group indicator 
 
 hh - Height in hundreds and tens of kilometers 
 
 L - Latitude (tens) 
 
 l a - Latitude (tenths) 
 
 a 
 
 TABLE A-l 
 
 Numbers used in TBUS, SATOB, SATEM, SAREP bulletins to identify 
 satellites: 
 
 10 - 19 ITOS series satellites 
 20 - 29 SMS/GOES series satellites 
 30 - 39 TIROS-N series satellites 
 
 A-6 
 
Section A. 3 EXPLANATION OF CODE SYMBOLS 
 
 TBUS-1 (or TBUS-2) - APT Bulletin originating in the United States: 
 
 TBUS-1 is North to South (descending) daylight 
 orbit. TBUS 2 is South to North (ascending) 
 daylight orbit. 
 
 KWBC - Traffic entered at Washington, D. C. 
 
 APT PREDICT - Identifies message content. 
 
 MMYYSS - Message serial number 
 
 MM - Month 
 YY - Day of Month 
 
 SS - Number of spacecraft to which predict 
 applies (See Table 1, page A-4) 
 
 PART I - Equator crossing reference information follows 
 
 - Code group indicator for first three groups 
 
 N r N- N N^ - Number of reference orbit. (Note: Information 
 
 in Parts II and III also are related to this 
 ■i reference orbit.) 
 
 Y„Y G G g g s s - Reference orbit equator crossing time (GMT), 
 
 rrrrrrrr satellite northbound: 
 
 YY - Day of Month 
 GJ.GJ. - Hour 
 g g - Minute 
 
 s,s - Second 
 
 r r 
 
 NOTE: In TBUS-1, northbound equator crossing takes place on NIGHT side of 
 orbit. In TBUS-2, northbound equator crossing takes place on DAY side 
 of orbit. 
 
 Q - Octant satellite is entering after crossing 
 
 equator on reference orbit (See Appendix B) 
 
 ^o^o^o^o " ^ e ^ erence or^it equator crossing longitude in 
 
 degrees and hundreths. 
 
 T - Indicator: nodal period follows (always be 
 
 shown as "T"). 
 
 gg - Nodal period, minutes 
 
 ss - Nodal period, seconds. [NOTE: Hundreds group 
 
 will not be included. Example: 100 minutes 
 .13 seconds will be coded as 0013. 
 
 - Indicator, nodal longitude increment follows 
 (always shown as "L"). 
 
 A-7 
 
L rt L-l A l ft " Degrees and hundreths of degrees longitude 
 
 '0000 
 
 between successive equator crossings. 
 
 N4N4N4N4 - Orbit number of fourth orbit following 
 
 reference orbit. 
 
 G4G4 - Hour of northbound satellite equator crossing 
 
 four orbits after reference orbit. 
 
 9 4 9 4 
 
 - Minute 
 
 s^s* - Second 
 
 0. - Octant satellite is entering after crossing 
 
 equator on fourth orbit after reference orbit. 
 
 L L 1 1 - Equator crossing longitude of fourth orbit 
 
 0000 after reference orbit. 
 
 Above information is repeated for eighth (NoNgNgNg) and twelfth ( N i? N i2 N i2 N 12^ 
 orbits following reference orbit. 
 
 NIGHT PART II (TBUS-1) or - Contains satellite altitude and subpoint 
 DAY PART II (TBUS-2) coordinates at two-minute intervals after time 
 
 of equator crossing; satellite northbound. 
 
 02 - Indicator; satellite altitude and subpoint 
 
 coordinates at two minutes after time of 
 equator crossing. 
 
 h 2 n Qo - Altitude, in hundreds and tens of kilometers, 
 
 at two minutes after equator crossing. 
 (Thousands figure understood; hence 1440 km 
 is encoded as 44.) 
 
 '02 
 
 - Octant of globe at two minutes after equator 
 crossing. 
 
 L L 1 - Latitude of satellite subpoint in degrees and 
 
 tenths of degrees at two minutes after equator 
 crossing. 
 
 L L 1 - Longitude of satellite subpoint in degrees and 
 
 tenths of degrees at two minutes after equator 
 crossing. 
 
 (Above information is repeated at two-minute intervals over the NIGHT portion of 
 the orbit north of the equator for TBUS-1, and DAY portion of the orbit north of 
 the equator for TBUS-2.) 
 
 NOTE: Should the time after ascending node become greater than 99, the 
 hundreds will be assumed (example, minute 102 will be encoded as 02). 
 
 A-8 
 
JJjffll-^yT,! 1 !.!™?: 1 ^ or " Sate l lite altitude and subpoint coordinates 
 DAY PART III (TBUS-2) at two-minute intervals south of equator on 
 
 the descending side of the orbit. 
 
 02 - Indicator; satellite altitude and subpoint 
 
 coordinates at two minutes after time of 
 equator crossing follows. 
 
 0? 0? 
 h h - Satellite altitude in hundreds and tens of 
 
 kilometers at gg minutes after equator 
 
 crossing. 
 
 '02 
 
 - Octant of globe at two minutes after equator 
 crossing. 
 
 L L la - Latitude of satellite subpoint in degrees and 
 
 a tenths of degrees at two minutes after equator 
 
 crossing. 
 
 L L 1 - - Longitude of satellite subpoint in degrees 
 
 and tenths of degrees at two minutes after 
 equator crossing. 
 
 (Above information is repeated at two-minute intervals over the night portion of 
 the orbit south of the equator for TBUS-1, and sunlit portion of the orbit north 
 of the equator for TBUS-2.) 
 
 DAY PART II (TBUS-1) - Satellite altitude and subpoint coordinates 
 
 NIGHT PART II (TBUS-2) at two-minute intervals after time of equator 
 
 crossing follows. 
 
 gg - Information pertinent to gg minutes after 
 
 equator crossing follows. 
 
 h h - Satellite altitude in hundreds and tens of 
 
 99 99 kilometers at gg minutes after equator 
 
 crossing. 
 
 Q - Octant of globe at gg minutes after equator 
 
 99 crossing. 
 
 L a L a la - Latitude of satellite subpoint in degrees and 
 
 tenths of degrees at gg minutes after equator 
 crossing. 
 
 a a 
 
 L L 1 - Longitude of satellite subpoint in degrees 
 
 and tenths of degrees at gg minutes after 
 equator crossing. 
 
 (Above information is repeated at two-minute intervals over the sunlit portion 
 of the orbit north of the equator for TBUS-1, and night portion of the orbit 
 north of the equator for TBUS-2.) 
 
 A-9 
 
DAY PART III (TBUS-1) or 
 NIGHT PART III (TBUS-2) 
 
 02 
 
 h 
 ^02 
 
 Wa 
 
 ooo 
 
 Satellite altitude and subpoint coordinates at 
 two-minute intervals south of the equator on 
 the descending side of the orbit. 
 
 Indicator: satellite altitude and subpoint 
 coordinates at two minutes after time of 
 equator crossing. 
 
 Satellite altitude in tens of kilometers, 
 two minutes after equator crossings. 
 
 at 
 
 - Octant of globe at two minutes after equator 
 crossing. 
 
 - Latitude of satellite subpoint in degrees and 
 tenths of degrees at two minutes after equator 
 crossing. 
 
 - Longitude of satellite subpoint in degrees and 
 tenths of degrees at two minutes after equator 
 crossing. . 
 
 (Above information is repeated at two-minute intervals over the sunlit portion 
 of the orbit south of the equator for TBUS-1 and night portion of the orbit 
 south of the equator for TBUS-2.) 
 
 NOTE: Should the time after ascending node become greater than 99, the hundreds 
 will be assumed (example, minute 102 will be encoded as 02). 
 
 A-10 
 
PART IV 
 
 (Contains high precision orbital elements, 
 transmission frequencies, and remarks). 
 
 Symbol 
 AAAAAAAAA 
 
 
 BBBBB 
 CCCCCCCCCCCC 
 
 DD 
 
 EE 
 
 FF 
 
 GG 
 
 HH 
 
 1 1 1 1 1 
 
 JJJJJJJ 
 
 KKKKKKKK 
 
 LLLLLLLL 
 
 MMMMMMNIM 
 NNNNNNNN 
 
 00000000 
 
 PPPPPPPP 
 
 QQQQQQQQ 
 RRRRRRRR 
 
 SSSSSSSSSS 
 TTTTTTTTTT 
 
 Explanation 
 
 Spacecraft identification (International 
 designator- see "COSPAR Guide to .Rocket and 
 Satellite Information and Data Exchange 
 Information Bulletin #9, July 1962). 
 
 Orbit number at epoch. 
 
 Time of ascending node (days from January 1 at 
 00Z, to nine decimal places. 
 
 Epoch year 
 
 Epoch month 
 
 Epoch day 
 
 Epoch hour 
 
 Epoch minute 
 
 Epoch second, to three decimal places 
 
 Greenwich Hour Angle at Aries at epoch, to four 
 decimal places. 
 
 Anomalistic period (minutes), to four decimal 
 places. 
 
 Nodal period (minutes), to four decimal places. 
 
 Eccentricity, to eight decimal places. 
 
 Argument of perigee (degrees), to five decimal 
 places. 
 
 Right Ascension of the ascending node (degrees), 
 to five decimal places. 
 
 Inclination (degrees), to five decimal places. 
 
 Mean anomaly (degrees), to five decimal places. 
 
 Semi -major axis (km), to three decimal places. 
 
 Sign and epoch X position component (km), to four 
 decimal places. 
 
 Sign and epoch Y position component (km), to four 
 decimal places. 
 
 A-ll 
 
Symbol (Cont.) 
 UUUUUUUUUU 
 
 VVVVVVVVV 
 
 WWWWUWWWW 
 
 xxxxxxxxx 
 
 YYYYYYYYY 
 
 111 
 aaa 
 bbb 
 cccc 
 
 dddddddddd 
 
 eeeeeeeee 
 
 fffffffff 
 
 ggggggggg 
 
 SPARESPARE 
 
 Explanation (Cont) 
 
 *Sign and epoch Z position component (km), to 
 four decimal places. 
 
 *Sign and epoch X velocity (Xdot) component 
 (km/sec), to six decimal places. 
 
 *Sign and epoch Y velocity (Ydot) component 
 (km/sec), to six decimal places. 
 
 *Sign and epoch Z velocity (Zdot) component 
 (km/sec), to six decimal places. 
 
 Ballistics coefficient CD-A/M (m /kg), to eight 
 decimal places. 
 
 7 2 
 Daily solar flux value (10.7 cm) (10 watt/nr). 
 
 -7 2 
 90-day running mean of solar flux (10 watts/m ) 
 
 Planetary magnetic index (2x10" gauss). 
 
 Drag modulation coefficient, to four decimal 
 places. 
 
 Radiation pressure coefficient (m /kg), to ten 
 decimal places. 
 
 Sign and perigee motion (deg/day), to five 
 decimal places. 
 
 Sign and motion of Right Ascension of the 
 ascending node (deg/day), to five decimal places, 
 
 Sign and rate of change of mean anomaly at epoch 
 (deg/day), to two decimal places. 
 
 spares 
 
 *-- All signed values in PART IV are preceeded by a "P" or "N" to denote a plus 
 (+) or minus (-)value. 
 
 APT TRANSMISSION FREQUENCY XXX. XX MHz 
 
 HRPT TRANSMISSION FREQUENCY XXXX.XX MHz 
 
 BEACON (DSB) TRANSMISSION FREQUENCY XXX. XX MHz 
 
 APT DAY X/X APT NIGHT X/X where X will identify channels being used. 
 
 DCS CLOCK TIME DAY XXX XXXXX.X 
 
 Followed by PLAIN LANGUAGE messages when necessary. 
 
 A-12 
 
»-'■ 
 
 
 +J 
 
 C 
 
 to 
 
 4-> 
 
 o 
 o 
 
 03 
 
 o 
 
 5 
 
 CM 
 I 
 
 S-. 
 C7> 
 
 A-13 
 
Section A.4 SAMPLE APT PREDICT (TBUS) BULLETIN 
 
 The following encoded APT Predict (TBUS) Bulletin example is referred to 
 throughout the remaining appendices. The major features of the message are 
 decoded on the following pages. 
 
 TBUS2 KWBC 211900 
 APT PREDICT 
 062430 TIROS N 
 
 PART I 
 
 08749 02416 01653 01146 T0202 L2550 
 
 37532 30503 11349 
 
 87570 55311 24446 
 
 87611 24121 34243 
 
 DAY PART II 
 
 02840 070132 04840 140149 06840 21066 
 
 08840 28084 10840 350204 12840 419227 
 
 14840 488255 16840 55689 18840 623336 
 
 20840 688407 22840 749529 24840 796776 
 
 26841 808200 28841 774538 30841 717705 
 
 32851 654794 34842 588749 36842 520710 
 
 38842 452679 
 
 DAY PART III 
 
 02845 070100 04855 140083 06855 210066 
 
 08855 280048 10855 350028 12845 419005 
 
 14868 488022 
 
 NIGHT PART II 
 
 40852 383655 42852 313633 44842 244614 
 
 46842 174596 48842 104580 50852 034563 
 
 NIGHT PART III 
 
 52857 035547 54857 105531 56857 175514 
 
 58857 244497 60857 314478 62857 383456 
 
 64857 452431 66857 520401 68857 588362 
 
 70857 654306 72857 717217 74857 773050 
 
 76858 808714 78858 797290 80858 749043 
 
 82855 689079 84845 624150 86845 557197 
 
 88845 489232 90845 420259 
 
 PART IV 
 
 1979 057A 09345 105066560150 810414203210007 1509616 
 
 01011681 01012254 00124732 17142454 13773458 09867899 
 
 18869440 07189253 M053313427 P048448725 M000019396 
 
 P00759127 P00825300 P07350534 003263350 245206018 9449 
 
 0000499998 M00290091 P00098722 P00512415 SPARESPARE 
 
 APT TRANSMISSION FREQUENCY 137.62 MHZ 
 
 HRPT TRANSMISSION FREQUENCY 1707 MHZ 
 
 BEACON (DSB) FREQUENCY 137.77 MHZ 
 
 APT DAY/NIGHT 1/4 APT IR CHANNEL 4 (10.5 to 11.5 MICROMETERS) 
 
 AND VIS CHANNEL 1 (0.55 to 0.90 MICROMETERS) 
 
 WILL BE TRANSMITTED CONTINUOUSLY. 
 
 DCS CLOCK TIME DAY 088 41126.4 
 
 A -14 
 
Section A. 5 Decoding Excercise 
 
 TBUS2 KWBC 211900 
 
 TBUS2 
 
 KWBC 
 211900 
 
 APT PREDICT 
 062430 
 
 TIROS N 
 
 PART I 
 
 - Bulletin heading—identifies bulletin for 
 satellite southbound in daylight 
 
 - Bulletin source—Washington, D. C. 
 Communications Center 
 
 - 21 -Day of the month— 21st 
 
 - 1900 -Bulletin broadcast time— 1900z 
 
 - Bulletin identifier 
 
 - Message serial number 
 06 — month -- June 
 
 24 --Day for which bulletin applies -- 24th 
 30 --Satellite identifier (see code table 
 A-l, page A-4.) 
 
 - Plain language satellite identifier 
 
 - Identifies reference orbit information and 
 the orbital height, equator crossing, 
 longitude and equator crossing time for the 
 fourth, eighth, and twelfth orbits after 
 the reference orbit. 
 
 08749 02416 01653 01146 T0202 L2550 
 
 08749 
 02416 
 
 01653 
 
 01146 
 
 T0202 
 L2550 
 
 
 8749 
 
 
 
 24 
 16 
 
 
 
 16 
 53 
 
 
 1146 
 
 T 
 0202 
 
 L 
 2550 
 
 Group indicator 
 Reference orbit number 
 
 Group indicator 
 
 Day of month of equator crossing 
 
 Hour 
 
 Group indicator 
 
 Minutes 
 
 Seconds 
 
 (time of equator crossing 16:16:53z) 
 
 Octant (0° to 90°), N. Hemisphere 
 011.46°W (equator crossing longitude 
 for orbit 8749 in octant 0) 
 
 Group indicator 
 
 Orbital period 102 minutes 02 seconds 
 
 Group indicator 
 
 Nodal longitudinal increment 25.50° 
 
 A-15 
 
 
87532 30503 11349 
 
 87532 30503 
 
 11349 
 
 87570 55311 24445 
 87611 24121 WHS 
 
 DAY PART II 
 
 02840 070132 04840 140149 
 
 02840 
 
 070132 
 
 04840 
 
 140149 
 
 8753 
 2 30503 
 
 1 
 1349 
 
 02 - 
 
 84 - 
 - 
 
 070 - 
 132 - 
 
 04 - 
 
 84 - 
 - 
 
 Orbit number 8753 (4th orbit after 
 reference orbit) 
 
 Time (23:05:03z) of ascending node 
 for orbit 8753 
 
 Octant 1 (90°W to 
 113.49°W (equator 
 8753 in octant 1) 
 
 180°) 
 crossing 
 
 for orbit 
 
 Decoded in same manner as previous 
 underlined line of data 
 
 Contains satellite altitude and 
 subpoint coordinates at two-minute 
 intervals after time of Northbound 
 (ascending) equator crossing. 
 
 Minute 02 after Northbound equator 
 crossing 
 
 Spacecraft height 840km 
 
 Octant (0° to 90°W), N. Hemisphere 
 
 Latitude 07.0°N 
 Longitude 013. 2°W 
 
 Minute 04 after Northbound equator 
 
 crossing 
 
 Spacecraft height 840 km 
 
 Octant (0° to 90°W), N. Hemisphere 
 
 140 - Latitude 014. 0°N 
 149 - Longitude 14.9°W 
 
 Remainder of DAY PART II decoded in same manner. Data for DAY PART II are 
 continuous at 2-minute intervals from equator North to Northern terminator. 
 
 DAY PART III 
 
 -. Contains satellite altitude and 
 subpoint coordinates at two-minute 
 intervals South of the equator, 
 satellite Northbound in the Southern 
 Hemisphere (Points are plotted 
 Southward from the equator) . 
 
 A-16 
 
02845 070100 04855 140083 
 
 02845 
 
 070100 
 
 04855 
 
 02 - Minute 02 before Northbound equator 
 crossing 
 
 84 - Spacecraft height 840 km 
 
 5 - Octant 5 (0 to 90°W, S. Hemisphere) 
 
 070 - Latitude 07.0°S 
 
 100 - Longitude 010. 0°W 
 
 04 - Minute 04 after equator crossing 
 
 85 - Spacecraft height 850 km 
 
 5 - Octant 5 (0 to 90°W, S. Hemisphere) 
 
 140083 
 
 Remainder of DAY PART II decoded in same manner 
 
 140 - Latitude 14.0°S 
 083 - Longitude 08.3°W 
 
 NIGHT PART II 
 
 40862 383655 42862 313633 
 
 40852 
 
 383655 
 42852 
 
 - 40 
 
 - 85 
 
 - 2 
 383 
 
 ■ 655 
 
 - 42 
 
 - 85 
 
 - 2 
 
 313 
 633 
 
 Satellite altitude and subpoint 
 coordinates at two-minute intervals 
 beginning at the day/night terminator 
 in the N. Hemisphere and continuing 
 southward toward the equator. 
 
 Minute 40 after Northbound equator 
 
 crossing 
 
 Spacecraft height 860 km 
 
 Octant 2 (90°E to 180°, N. Hemisphere) 
 
 Latitude 38.3°N 
 
 Longitude 165. 5°E 
 
 Minute 42 after equator crossing 
 Spacecraft height 860 km 
 Octant 2 (90°E e to 180°, N. 
 Hemisphere) 
 Latitude 31.3°N 
 Longitude 163. 3°E 
 
 Remainder of NIGHT PART II decoded in same manner, 
 intervals. 
 
 Data continuous at 2-minute 
 
 NIGHT PART III 
 
 - Satellite altitude and subpoint co- 
 ordinates at two-minute intervals 
 south of the equator on the descending 
 side of the orbit. 
 
 A-17 
 
52867 035547 54867 105531 
 52857 
 
 035557 
 
 54857 
 
 105531 
 
 Remainder decoded in 
 first point South of 
 
 PART IV 
 
 AAAAAAAAA 
 BBBBB 
 
 CCCCCCCCCCCC 
 DDEEFFGGHHIIIII 
 
 JJJJJJJ 
 
 KKKKKKKK 
 
 LLLLLLLL 
 
 MMMMMMMM 
 
 NNNNNNNN • 
 
 00000000 
 
 PPPPPPPP 
 
 QQQQQQQQ 
 
 RRRRRRRR 
 
 - 52 - Minute 52 after Northbound equator 
 
 crossing 
 
 - 85 - Spacecraft height 860 km 
 
 - 7 - Octant 7 (90°E to 180°, S. Hemisphere) 
 
 - 035 - Latitude 03.5°S 
 
 - 547 - Longitude 154. 7°E 
 
 - 54 - Minute 54 before equator crossing 
 
 - 85 - Spacecraft height 860 km 
 
 - 7 - Octant 7 (90°E to 180°, S. Hemisphere) 
 
 - 105 - Latitude 10.5°S 
 
 - 531 - Longitude 153. 1°E 
 
 same manner. Data continuous at 2-minute intervals from 
 equator to Southern terminator. 
 
 - Indicator -- orbital elements, 
 transmission frequencies, and remarks 
 follow. 
 
 1979-057A 
 09345 
 
 105066560150 
 810414203210007 
 
 1509616 
 
 01011681 
 
 01012254 
 
 00124732 
 
 17142454 
 
 13773458 
 
 09867899 
 
 18869440 
 
 07189253 
 
 1979-057A International designator 
 
 revolution 9345 
 
 105.066560150 days 
 
 81 — 1981 year 
 04—04 months 
 14—14 days 
 20—20 hours 
 32—32 minutes 
 10007—10.007 seconds 
 
 150.9616 degrees 
 
 101.1681 minutes 
 
 101.2254 minutes 
 
 0.00124732 no units 
 
 171.42454 degrees 
 
 137.73458 degrees 
 
 98.67899 degrees 
 
 188.69440 degrees 
 
 7189.253 km 
 
 A-18 
 
ssssssssss 
 
 TTTTTTTTTT 
 UUUUUUUUUU 
 VVVVVVVVV 
 
 wwwwwwwww 
 xxxxxxxxx 
 
 YYYYYYYYY 
 ZZZaaabbb 
 
 cccc 
 
 dddddddddd 
 eeeeeeeee 
 fffffffff 
 
 ggggggggg 
 
 SPARESPARE 
 
 M053313427 
 
 P048448725 
 
 M000019396 
 
 P00759127 
 
 P00825300 
 
 P07350534 
 
 003263350 
 
 245206018 
 
 9449 
 
 0000499998 
 
 M00290091 
 
 P00098722 
 
 P00512415 
 
 SPARESPARE 
 
 -5331.3427 km 
 
 +4844.8725 km 
 
 -1.9396 km 
 
 +0.759127 km/sec 
 
 +0.825300 km/sec 
 
 +7.350534 km/sec 
 
 0.03263350 m2/kg 
 
 245-245 x I0~l watt/m* 
 206—206 x 10"' watt/nT 
 018--36 x 10 gauss 
 
 0.9449 no units 
 
 0.000499998 m 2 /kg 
 
 -2.90091 degrees/day 
 
 +0.98722 degrees/day 
 
 +5124.15 degrees/day 
 
 No information at this time 
 
 APT TRANSMISSION FREQUENCY 137.62 MHZ 
 HRPT TRANSMISSION FREQUENCY 1707 MHZ 
 BEACON (DSB) FREQUENCY 137.77 MHZ 
 
 APT DAY/NIGHT 1/4 APT IR CHANNEL 4 (10.5 TO 11.5 MICROMETERS) 
 AND VIS CHANNEL 1 (0.55 TO 0.90 MICROMETERS) 
 WILL BE TRANSMITTED CONTINUOUSLY. 
 
 DCS CLOCK TIME DAY 088 41126.4 - DATA COLLECTION SYSTEM CLOCK TIME 
 
 RESET FOR JULIAN DAY 088, 41126.4 
 SECONDS 
 
 NOTE: The classical elements (Keplerian) from MMMMMMMM to RRRRRRRR are (Brower) 
 mean elements; the position and velocity components SSSSSSSSSS to XXXXXXXXXX are 
 actual. The Greenwich Hour Angle initially will be put out as mean siderial 
 time; it is expected to eventually become apparent siderial time. 
 
 The paramenter "time of the ascending node" (i.e., CCCCCCCCCCCC) is presently 
 being reevaluated. It may be to changed to "epoch time minus time of 
 the ascending node crossing prior to epoch (in minutes)". Thus, the user could 
 compute the time of the northbound equator crossing prior to epoch instead of 
 the time for an arbitrary northbound equator crossing. Naturally any changes 
 would be announced. • 
 
 A-19 
 

 
APPENDIX B 
 Plotting Board Preparation 
 
 B.l General Procedures 
 
 a. Obtain an APT plotting board and appropriate tracking diagram (see 
 details in B.3, below). 
 
 b. Attach a clear plastic overlay covering the area encompassed by the 
 equatorial circle. 
 
 c. Obtain a TBUS bulletin from the Global Telecommunications Service 
 teletype network or request a copy of satellite subpoints from the 
 Coordinator, Direct Readout Services. 
 
 d. Plot satellite subpoint positions at two-minute intervals on the 
 plastic overlay. 
 
 e. Determine the equator crossing zones for satellites passing within 
 receiving range of the station. 
 
 B.2 Orbital Track 
 
 The first APT "plotting boards" were Northern and Southern Hemisphere polar 
 stereographic projections reproduced on one-eighth inch hard plastic. They were 
 designed and produced in the early 1960 's by a commercial company under contract 
 to the Government, for use by U. S. Weather Bureau and other professional 
 meteorological stations. Copies of these original boards are available today 
 from a commercial company for approximately $150 (US). Plotting boards provided 
 by the National Earth Satellite Service to APT station operators are reproduc- 
 tions of the original "board" printed on high quality paper. 
 
 The first step in preparing the plotting board for satellite tracking is to 
 mount the board on a firm surface such as a stiff piece of cardboard or 
 heavy-weight paper. Next, overlay the plotting board with a clear piece of 
 plastic or film. The plastic should be at least 18 x 18 inches (46 x 46 cm) 
 square -- large enough to cover the entire area within the equatoral circle. 
 Attach the overlay to the plotting board with a pin or screw through the center, 
 or Pole, so that it can be easily rotated. 
 
 Next, plot orbital subpoints at two-minute increments. An information 
 sheet containing the orbital subpoint positions for a single orbit of any one of 
 the TIROS-N series satellites can be obtained upon request from the Coordinator, 
 Direct Readout Services. Additionally, the same information can be obtained by 
 decoding a TBUS bulletin. Two-minute subpoints for a satellite travelling south 
 in daylight are contained in TBUS-1 NIGHT PART II and DAY PART II. For a satel- 
 lite northbound in daylight, TBUS-2 DAY PART II and NIGHT PART II can be used. 
 
 B-l 
 
Plot these points starting at the equator; continue toward the Pole. The 
 track of the orbit will pass near, but not over, the Polar region. Continue 
 plotting the track until the equatorial line on the other side of the map 
 projection is reached. Draw a smooth curve through these points. It would also 
 help to put one or two arrowheads on this curve to indicate direction of travel. 
 
 Number each point to indicate two-minute intervals. On a Northern 
 Hemisphere map projection, the equator crossing should be marked as minute "0", 
 the first point north of the equator would be minute 02, followed by 04, 
 
 06 etc. On a Southhern Hemisphere map projection, the equatorial crossing 
 
 should be marked minute 51, followed by 53 (first point below equator), 55, 57, 
 etc. 
 
 The orbital plot which has just been completed is representative of a 
 typical orbit for a given satellite with an orbital period of approximately 102 
 minutes. The time markers are only indicators showing the position of the 
 satellite "X" minutes after an equator crossing. In practice, these numbers 
 would have specific time values for a given orbit. For example, for an equator 
 crossing at 16:10:55Z, minute 02 would have the value 16:12:55Z. This numbering 
 will become more obvious later. 
 
 Before moving the plotted track, mark off increments 25.50°, 51.0°, and 
 76.5° east and west of the equator crossing position. Place small tic-marks on 
 the line of the equator at these points. These represent equator crossing 
 positions -1, -2, and -3 (east) and +1, +2, and +3 (west) before and after the 
 reference orbit crossing. These are nodal longitude increments (each 25.50° 
 apart) determined from PART 1, code group L, of the TBUS bulletin. 
 
 B.2 Tracking Diagram 
 
 A tracking diagram is used in conjunction with the plotting board to permit 
 the operator to graphically compute azimuth and distance of a spacecraft from 
 the antenna site to any point along the satellite track. The concentric curves 
 near-ellipses) are isopleths of great circle arc distance drawn at two-degree 
 222 km or 120 nm) intervals. Section 5.4.1 of the text discusses the theory of 
 determining azimuth and elevation using this diagram. 
 
 ♦Tracking diagrams have been computed for each five degrees of latitude. 
 In the exercise in this report, the antenna site is at 38.0°N, 75.2°W (Wallops 
 Island, Virginia readout site). Therefore, the tracking diagram for 40° 
 latitude is used. The diagram is permanently attached to the plotting board, 
 under the plastic overlay, with the center of the diagram on the latitude and 
 longitude of the receiving antenna and the 0°-180° line parallel to the 
 longitude line of the station. 
 
 *These diagrams can be ordered, Coordinator, Direct Readout Services, S/DS21 
 without charge, from: National Earth Satellite Service 
 
 National Oceanic and Atmospheric 
 Administration 
 
 Washington, D. C. 20233 
 
 B-2 
 
Important Notice : These tracking diagrams originally were prepared for use 
 with the early TIROS, ESSA, and ITOS series of satellites having orbital alti- 
 tudes between 1400-1600 km. The outer circle represents an arc length of 36°, 
 the zero-elevation angle for a satellite at a height of 1500 km (see Table 5-1). 
 The average height, or altitude, of TIR0S-N series satellites is 850 km (425nm). 
 Therefore, a circle (ellipse) representing an arc length of 28.1° (from Table 
 5.1) indicates the zero-elevation angle for TIR0S-N series satellites. This is 
 the "acquisition circle"; it has been drawn heavier than other circles on the 
 diagram. 
 
 B.3 Determining orbits within reception zones. 
 
 Figure B-l shows the APT Plotting Board with the reference orbit plotted 
 from the APT Predict (TBUS) Bulletin in Appendix A. The equator crossing 
 longitude of the Reference Orbit (#8749) is 11.46°, according to the TBUS 
 bulletin. The tracking diagram has been properly positioned for the station 
 location as described above. 
 
 Using a Northern Hemisphere Plotting Board, turn the overlay so that the 
 orbit track is east of the station and tangent to the zero-elevation circle 
 (28.1° arc length) , as shown in Figure B-2a. The direction of travel should be 
 northbound, with the minute markers increasing in value. Note the equator 
 crossing longitude. For the station position used in this exercise, the equator 
 crossing longitude is 143°E. Now turn the track clockwise so it is tangent to 
 the zero-elevation circle west of the station, as shown in Figure B-2b, and note 
 the longitude. For the sample exercise station, this would be 67°E. 
 
 For this station, any TIR0S-N series satellite southbound in daylight will 
 pass within receiving range of the station if the equator crossing occurs 
 between 67°E and 143°E. This range, or zone, is therefore marked by a heavy 
 line on the equator. (It is also true that a satellite crossing the equator in 
 this zone in daylight will pass with range of the station at night). 
 
 To determine the equatorial crossing zone for nighttime passes, the same 
 technique is used. The track is positioned so it is tangent to the 
 zero-elevation circle west of the station. The equator crossing longitude is 
 24°W. The track is then rotated so it is tangent east of the station; the 
 longitude noted is 102°W (figures B-2c and 2d, respectively). The zone is 
 marked as a heavy line on the equator. Any satellite crossing the equator 
 within this zone northbound, day or night, will pass in range of the station. 
 
 The same technique is used in the Southern Hemisphere, using a Southern 
 Hemisphere Plotting Board. 
 
 B-3 
 
B.4 Maximum spacecraft acquisition period during one full orbit 
 
 The maximum acquisition period occurs when the spacecraft passes directly 
 over the station. To determine when this would occur, position the track so 
 that it passes directly through the center of the tracking diagram (and 
 therefore over the station). For a satellite passing over the station 
 southbound, the equator crossing will occur at about 108°E and the satellite 
 will be within range from minute 32 to minute 48 after the equator crossing 
 time. If the satellite is passing over the station northbound the equator 
 crossing longitude for a full pass over the station would be 65°W and the 
 spacecraft would be within range from 03 minutes to 19 minutes from the time of 
 equator crossing. 
 
 B-4 
 
APT SYSTEM 
 
 APT STATION. 
 
 LOCATION: L.AT. LONG 
 
 vaTEOROLOGICAL SATELLITE 
 PLOTTING BOARD 
 
 AND 
 TRACKING DIAGRAM 
 
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 fLEVftTiON SN&lES 
 
 4 
 
 _:.J 
 
 Figure B-l. Plotting board with the plotted subpoint track set at 11.46°W, 
 the equator crossi-ng longitude of the Reference Orbit (#8749). Satellite 
 northbound. 
 
 B-5 
 
APT SYSTEM 
 
 METEOROLOGICAL SATELLITE 
 
 PlOTTINO BOARD 
 
 MO 
 TRACKING DIAGRAM 
 
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 apt station: 
 
 LOCATION: L.AT. LONG. 
 
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 Figure B-2a.. Plotting board with the plotted subpoint track set tangent to 
 the zero-degree elevation circle, east of the station. Satellite northbound 
 The equator crossing longitude is observed to be at 143°E. 
 
 B-6 
 
APT SYSTEM 
 
 UfTCOROlOfclCSL SATELLITE 
 Pi OTTING BOARD 
 
 ANO 
 TRACKING DIAGRAM 
 
 \oi 
 
 "'V 
 
 APT STATION 
 
 LOCATION L.AT. LONG. 
 
 * 
 
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 i i: I 
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 180 
 
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 FLEV*fiO« ANGLES 
 
 
 Figure B-2b. Plotting board with the plotted subpoint track set tangent to 
 the zero-degree elevation circle, west of the station. Satellite northbound 
 The equator crossing longitude is observed to be at 67°E. 
 
 B-7 
 
APT SYSTEM 
 
 MfTCOROlOGICAL SATELLITE 
 PLOTTING HUO 
 
 AND 
 TRACKING DIAGRAM 
 
 apt station: 
 
 location! lat., long. 
 
 & 
 
 •> 
 
 «' 
 
 V 
 
 V 
 
 ft 
 
 jft !a-, 
 
 Fl6v*TiON ANCLES 
 
 ■ *i*i ik.low!uii 
 
 i .; 
 
 
 Figure B-2c. Plotting board with the plotted subpoint track set tangent to 
 the zero-degree elevation circle, east of the station. Satellite southbound 
 The equator crossing longitude is observed to be at 24°W. 
 
 B-8 
 
APT SYSTEM 
 
 <l-TEOROLOGICAL SATELLITE 
 
 plotting board 
 
 ANO 
 
 TRACKING DIAf-RAM 
 
 V 1 
 
 * Og 
 
 apt station 
 location: 
 
 .l.AT. LONG. 
 
 
 E.EVATiQN ANCLES 
 
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 Figure B-2d. Plotting board with the plotted subpoint track set tangent to the 
 zero-degree elevation circle, east of the station. Satellite southbound. 
 The equator. crossing longitude is observed to be at 102°W. 
 
 B-9 
 
APPENDIX C 
 
 TRACKING EXERCISE 
 
 Given an orbit number, equator crossing time and longitude, the period of 
 revolution, and the longitudinal increment between successive orbits, several 
 methods can be used to determine future equator crossing times and longitudes. 
 Once these have been determined, the plotting board and tracking diagram can be 
 used to graphically compute the antenna azimuth and elevation angles from any 
 station to a polar orbiting satellite passing within reception range. 
 
 This tracking excercise illustrates the use of the APT Predict (TBUS) 
 Bulletin contained in Appendix A (page A-14), and the plotting board and 
 tracking diagram arrangement described in Appendix B, to track a satellite 
 passing within receiving range of a station located at Wallops station, 
 Virginia . In this excercise, azimuth and elevation angles will be determined at 
 one-minute increments. 
 
 First, examine PART I, line 1, of the APT Predict (TBUS) Bulletin. This 
 line identifies the Reference Orbit (#8749), the equator crossing time 
 (16:16:53Z), longitude (11.46°W), period (102 minutes, 02 seconds), and 
 longitudinal increment between successive orbits (25.50°). 
 
 Next, position the track at the equator crossing longitude of the Reference 
 Orbit, as shown in Figure C-l. Remember that the equator crossing longitude of 
 the Reference Orbit is always the northbound , or Ascending Node, position. Note 
 that the satellite's path for this Reference Orbit does not fall within the 
 "above zero degree elevation area", or acquisition circle . £The "above zero 
 degree elevation area" is a circle (or ellipse) 28.1° from the center of the 
 diagram. This area should already be outlined, in accordance with instructions 
 in Appendix B.] 
 
 Observe the positions of the tic-marks at +1, +2 and +3. These tic marks 
 correspond to the equator crossing positions of the next three orbits west of 
 the Reference Orbit, and are 25.50° apart -- the longitudinal increment between 
 
 orbits indicated in PART I. The position at +1 is 11.46°W + 25.50°= 36.96°W 
 (37.0°is close enough, since fractions of degrees are difficult to read on this 
 scale map) and corresponds to the ascending node of orbit #8750; +2 is at 11.46 
 + 2x25.50° or 62.46°W, corresponding to orbit #8751; +3 is at 11.46°W + 3x25.50° 
 or 87.96°W (88.0° is close enough) and corresponds to orbit #8752. 
 
 When the overlay on which the track is plotted is rotated to the equatorial 
 positions corresponding to +1 and +2, the satellites' path passes within the 
 acquisition circle. At +3, the path is west of the circle. Return the track to 
 the position corresponding to +1. This will show the satellite crossing into the 
 acquisition circle 09 minutes after the time of the ascending node, and 
 remaining in range until minute 19. 
 
 The shape of the tracking diagram will be different for different latitudes, 
 and the position of the tracking diagram on the plotting board will depend upon 
 the operator's station location. The method used to determine az/el angles, 
 however, will be the same for any location. 
 
 C-l 
 
Since orbital period equals 102 minutes 02 seconds, and since the Reference 
 Orbit equator crossing time was 16: 16:53Z, then the Reference Orbit +1 equator 
 crossing time will be 17: 58:55Z. The satellite signal will be acquired 9 
 minutes later -- at 18:07:55Z. It will remain within acquisition range for ten 
 more minutes (until minute 19 AAN), or until 18: 17:55Z. 
 
 Times for Orbit +2 after the Reference Orbit are computed the same way. Set 
 the track to the equator crossing position of this orbit (62.46°W), as shown in 
 Figure C-2. Observe that the satellite will be within the acquisition circle 
 from minute 04 to minute 19. The equator crossing time for this orbit is 
 19:40:57Z (two times the orbital period added to the equator crossing time of 
 the Reference Orbit). The satellite will come within acquisition range 4 
 minutes later, at 19:44:57Z, and pass out of range 19 minutes after the time of 
 the Ascending Node, or at 19:59:57. 
 
 Look at the APT TRACKING WORKSHEET on the following page. The information 
 at the top identifies the receiving station, spacecraft, and orbit number, date, 
 equator crossing longitude and time for the orbit. The first column marked TIME 
 (AAN) shows the minutes After Ascending Node (AAN) during which the satellite is 
 within receiving range. The second TIME column is the Z-time, or the Greenwich 
 Mean Time corresponding to the number of minutes after the ascending node. 
 
 Elevation angles are obtained first by listing Great Circle distance values 
 at each minute marker along the satellite's path, then converting these 
 distances to elevation angles by referring to Table 5.1 (page 5-12). 
 
 Each of the concentric circles radiating from the center of the tracking 
 diagram represent two degrees of Great Circle distance. A satellite passing 
 directly over the station would have a Great Circle distance of 0° and a 
 corresponding elevation angle of 90° (from Table 5.1) at the instant when the 
 satellite is directly overhead. When the Great Circle distance equals 28.1°, 
 the elevation angle from the station to the satellite would be zero (for a 
 satellite orbiting the earth at an altitude of 850 km). To compute the 
 elevation angle record the Great Circle distance for each minute the satellite 
 is within acquisition range, then convert these values to elevation angles by 
 extrapolating between the values listed in Table 5.1. 
 
 Azimuth angle is determined directly from the plotting board. Each line 
 radiating from the center of the tracking diagram represents 10° of azimuth. 
 Observe and record the azimuth angle at each one-minute marker along the path of 
 the satellite. 
 
 The APT Worksheet on page C-3 shows how az/el has been computed for orbits 
 #8750 and #8751. 
 
 C-2 
 
APT TRACKING WORKSHEET 
 
 STATION: Wallops Station, Virginia 
 
 SPACECRAFT: TIROS-N 
 
 DATE: 
 
 6/24 ORBIT: #1850 EQUATOR CROSSING 
 
 LONG: 36. 
 
 96°W TIME 
 
 : 17:58:55Z 
 
 TIME 
 
 TIME 
 
 
 AZIMUTH 
 
 ELEV. 
 
 GREAT 
 
 HEIGHT 
 
 
 *AAN 
 
 (Z) 
 
 
 (Deg.) 
 
 (Deg.) 
 
 CIRCLE 
 DISTANCE 
 
 (km) 
 
 
 09 
 
 18 h 07 m 55 S 
 
 97.0 
 
 12.5 
 
 26.0 
 
 
 
 10 
 
 18 
 
 08 
 
 55 
 
 90.0 
 
 15.4 
 
 24.1 
 
 840 
 
 
 11 
 
 18 
 
 09 
 
 55 
 
 82.0 
 
 17.8 
 
 22.4 
 
 
 
 12 
 
 18 
 
 10 
 
 55 
 
 75.0 
 
 18.5 
 
 22.0 
 
 840 
 
 
 13 
 
 18 
 
 11 
 
 55 
 
 66.0 
 
 19.4 
 
 21.6 
 
 
 
 14 
 
 18 
 
 12 
 
 55 
 
 53.0 
 
 20.2 
 
 21.0 
 
 840 
 
 
 15 
 
 18 
 
 13 
 
 55 
 
 44.0 
 
 18.7 
 
 21.8 
 
 
 
 16 
 
 18 
 
 14 
 
 55 
 
 36.0 
 
 18.1 
 
 22.4 
 
 840 
 
 
 17 
 
 18 
 
 15 
 
 55 
 
 27.0 
 
 15.8 
 
 24.2 
 
 
 
 18 
 
 18 
 
 16 
 
 55 
 
 20.0 
 
 12.5 
 
 26.0 
 
 840 
 
 
 19 
 
 18: 
 
 17 
 
 55 
 
 15.0 
 
 9.8 
 
 28.0 
 
 
 
 APT TRACKING WORKSHEET 
 
 
 ' 
 
 
 
 
 STATION: Wallops 
 
 Station, Virginia 
 
 SPACECRAFT: TIROS-N 
 
 
 DATE: 
 
 6/24 ORBIT: # 
 
 8751 EQUATOR CROSSING 
 
 LONG: 62. 
 
 46°W TIME: 
 
 19:40:57Z 
 
 TIME 
 
 TIME 
 
 
 AZIMUTH 
 
 ELEV. 
 
 GREAT 
 
 HEIGHT 
 
 
 *AAN 
 
 (Z) 
 
 
 (Deg.) 
 
 (Deg.) 
 
 CIRCLE 
 DISTANCE 
 
 (km) 
 
 
 04 
 
 19 h 44 m 57 s 
 
 
 158.0 
 
 1.0 
 
 26.8 
 
 840 
 
 
 05 
 
 19:45:57 
 
 
 158.0 
 
 4.6 
 
 23.6 
 
 
 
 06 
 
 19:46:57 
 
 
 157.5 
 
 10.1 
 
 19.4 
 
 840 
 
 
 07 
 
 19:47:57 
 
 
 155.0 
 
 16.0 
 
 15.8 
 
 
 
 08 
 
 19:48:57 
 
 
 151.5 
 
 23.8 
 
 12.2 
 
 840 
 
 
 09 
 
 19:49:57 
 
 
 148.5 
 
 34.4 
 
 8.0 
 
 
 
 10 
 
 19:50:57 
 
 
 135.0 
 
 51.2 
 
 5.2 
 
 840 
 
 
 11 
 
 19:51:57 
 
 
 100.0 
 
 60.9 
 
 2.6 
 
 
 
 12 
 
 19:52:57 
 
 
 35.0 
 
 61.5 
 
 3.6 
 
 840 
 
 
 13 
 
 19:53:57 
 
 
 10.0 
 
 46.7 
 
 6.0 
 
 
 
 14 
 
 19:54:57 
 
 
 358.0 
 
 30.8 
 
 9.8 
 
 840 
 
 
 15 
 
 19:55:57 
 
 
 355.0 
 
 21.4 
 
 13.2 
 
 
 
 16 
 
 19:56:57 
 
 
 352.0 
 
 14.9 
 
 16.4 
 
 840 
 
 
 17 
 
 19:57:57 
 
 
 351.5 
 
 9.2 
 
 20.0 
 
 
 
 18 
 
 19:58:57 
 
 
 351.0 
 
 3.9 
 
 23.8 
 
 840 
 
 
 19 
 
 19:59:5 
 
 >7 
 
 
 350.0 
 
 1.7 
 
 26.2 
 
 
 
 * AAN -- After Ascending Node 
 
 C-3 
 
APT SYSTEM 
 
 MtTEOROLOGICAL SATELLITE 
 
 apt station: 
 
 LOCATION! LAT., LONG. 
 
 PLOTTING BOARD 
 
 
 
 
 AND 
 
 
 
 * 
 
 TRACKING DIAGRAM 
 
 
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 Figure C-l. Track set for Reference Orbit (#8749) 
 
 C-4 
 
 
APT SYSTEM 
 
 METEOROLOGICAL SATELLITE 
 
 APT STATION 
 
 LOCATION: LAT., LONG 
 
 PLOTTING BOARD 
 
 AND 
 TRACKING DIAGRAM 
 
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 Figure C-2. Track set for Reference Orbit + 2 (#8751) 
 
 C-5 
 
APPENDIX D 
 GRIDDING EXCERCISE 
 
 The gridding technique described in this section is based on the assumption 
 that the station operator has a grid. Figure 0-1 is a paper print of a grid 
 produced by the National Earth Satellite Service (NESS) for use with TIROS-N 
 series satellites. The original grid was produced on a 35mm film strip in both 
 positive and negative form. (A copy of this film grid can be obtained from the 
 Coordinator, Direct Readout Services). The user must first photographically 
 enlarge the grid to the scale of his image, then produce a transparent film copy 
 to use as an overlay on the image*. 
 
 This transparent grid overlay has a center line marking the subpoint track 
 of the satellite, and one line along each edge parallel to the center line. 
 These outer lines mark the picture edge, or horizon. Near-vertical dashed lines 
 are longitude lines five degrees apart; latitude lines, also five degrees apart, 
 are marked along the subpoint track. Numbers along the edge denote one-minute 
 intervals beginning at time=0 at latitude 0°. Broad arrows indicate the 
 direction of travel of the spacecraft. 
 
 On TIROS-N orbit #8751, an APT image was acquired at a satellite readout 
 station close to Wallops Station, Virginia. The following exercise shows the 
 steps used to fit a grid to this image. 
 
 The image acquisition time of orbit #8751 has already been determined in 
 Appendix C. In brief, the time of the Reference Orbit (#8749) is given in PART 
 I of the APT Predict (TBUS) Bulletin as 16:16:53Z, the period of a single orbit 
 is 102 minutes 02 seconds, the equator crossing longitude is 11.46°W, and the 
 longitudinal increment between orbits is 25.50°. Orbit #8751 is +2 orbits after 
 the Reference orbit. Therefore, the equator crossing time and longitude of this 
 orbit is easily calculated to occur at 19:40: 57Z and 0° latitude, 62.46°W 
 longitude. 
 
 Mark this point on the grid. The near-vertical dotted line intersecting 
 the 0° latitude and 0-minute marker in Figure D-2 is the 62.45°W longitude line. 
 Adjacent dotted longitude lines are five degrees apart and are appropriately 
 marked 72.46° ,67.46° , 57.46°, and 52.46°. 
 
 Now look at Figure D-3. Most users prefer to work with a conventional 
 five-degree longitudinal grid system where 0° is the Greenwich Meridian, and 
 successive five-degree longitudinal increments are marked off at 5°, 10°, 15°, 
 etc. For this reason, longitude lines have been re-drawn on this "work 1 grid at 
 55°W, 60°W, 65°W, etc., by interpolating between the longitude lines indicated 
 in Figure D-2. 
 
 *This is not a universal APT grid, and cannot be used with all APT display 
 devices due to differences in aspect ratios. With care and patience, APT station 
 operators can develop their own grid overlays from a series of pictures. 
 
 D-l 
 
Figure D-4 is the APT image acquired on orbit #8751. Figure D-5 shows this 
 image with the "work" grid overlayed. Figure 0-6 shows a retrace of the "work" 
 grid overlayed on the image, smoothed to show only the conventional grid lines. 
 
 Some professional APT station operators prefer to have the grid electroni- 
 cally superimposed on the image during acquisition. This is an obvious 
 advantage in offices where products used to prepare weather forecasts must be 
 examined quickly. The only disadvanage is that the grid points can obscure 
 significant featuuress on the image. Using a grid overlay has the advantage 
 that the grid can be raised to examine image features. 
 
 D-2 
 
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PENN STATE UNIVERSITY LIBRARIES 
 
 ADDDD70 1 
 
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