Subpoint Prediction for Direct Readout 

 Meteorological Satellites 



L. E. EBER' 



ABSTRACT 



The National Environmental Satellite Service (NESS) provides orbital information on me- 

 teoroloKical satellites with direct transmission systems, through APT (Automatic Picture Trans- 

 mission) Predict messages sent over standard weather communications networks. With peri- 

 odic access to this information, operators of independent APT ground receiving stations can 

 extrapolate, by means of nodal period and nodal increment, to determine future orbits within 

 receiving range of their station. A technique for the prediction of subpoint location along an 

 orbit as a function of time after ascending node was developed from consideration of Kepler's 

 laws and derived expressions for the force due to the earth's gravitational potential. Subpoint 

 latitudes and longitudes computed by this technique are within 0.1 degree of those given in NESS 

 predictions. 



INTRODUCTION 



Users of APT (Automatic Picture Transmis- 

 sion) and Direct Readout Scanning Radiometer 

 data from meteoroio,<?ical satellites, who operate 

 APT ground receiving stations, need orbital in- 

 formation for scheduling transmission pickup and 

 for tracking and locating subpoints for data iden- 

 tification. Such information is provided by the 

 National Environmental Satellite Service (NESS) 

 through APT Predict messages transmitted over 

 meteorological teletype circuits.' In cases where 

 these messages are not regularly available, or 

 where advance scheduling is desired, the dates, 

 times, and longitudes of the ascending nodes for 

 selected orbits can be extrapolated by means of 

 nodal period and nodal increment as much as a 

 month ahead with acceptable accuracy. Subpoint 

 locations at selected time invervals along the 

 track also can be predicted for future orbits if 

 certain other orbital characteristics are known. 



' Southwest Fislieries Center, National- Marine Fislieries 

 Service. NOAA, La Jolla, CA 92037. 



-ESSA (Environmental Science Services Administration) 

 Direct Transmission System Users Guide, prepared by the 

 National Environmental Satellite Center, U.S. Department 

 of ('(immerce, 19()9. 



These include the semimajor axis and eccentric- 

 ity of the orbital ellipse, the orbital inclination, 

 and the geocentric angle of the perigee measured, 

 at a known reference time, from the ascending 

 node in the orbital plane. 



UNDISTURBED ORBITAL MOTION 



The orbit of a meteorological satellite is an el- 

 lipse with the earth's center at one focus. Accord- 

 ing to Kepler's laws, the motion of a satellite is 

 such that a ray from the focus to the satellite 

 sweeps out equal areas in equal times. Conse- 

 quently the angular velocity of the satellite is 

 greatest at perigee when the satellite is at its 

 nearest approach to the earth and least at apogee 

 when it is farthest away. The area swept out per 

 unit time by the satellite can be expressed by: 



i?2rf 



TiAB 



2dt 



where d a/dt is the angular velocity, R is the dis- 

 tance from the center of the earth to the satel- 

 lite, A and B are, respectively, the semimajor 

 and semiminor axes of the orbital ellipse, and P 

 is the orbital period. Rearrangement of terms 



