Satgen 457 Formation Flying (1) by GM4IHJ (BID SGEN457) 27Dec97 Over the next two years 460 or more mobile communications satellites are planned to be in orbit. 4 will be geostationary(Inmarsat). 22 will be in high circular orbits around 10350kms (10 I-CO; 12 ? Odyssey), and the rest will be in low altitude orbits around 750 or 1400kms (66 Iridium; 288 Teledesic; 48 Globalstar and 36 OrbComm). All except OrbComm will handle wide band data and voice. OrbComm with be low data rate messages only. Among the more interesting features of these various commercial systems are the orbits employed, and the restrictions placed upon them by their operating frequencies and the more difficult characteristics of mobile operation as opposed to fixed ground station working. Considering the orbits first. There are 2 principal features which determine the choice of, orbit type, inclination and separation :- 1. Orbital height. Which affects firstly the time delay in the round trip user to satellite to user. With the delay experienced with a geostationary satellite amounting to a rather awkward 1/4 of a second which can upset some communications protocols. Where as the delay to and back from a high circular orbiter is less than 1/10 of a second and that from a low earth orbiter a mere 1/50 of a second approx . More importantly height affects the size of the satellite footprint, ie that part of the world the satellite can see at any one time. A geosat can cover 40% of the earth , but gives no cover at the poles of the earth. 4 geosats can link permanently around the earth thereby providing seamless coverage , unlike satellites in lower orbits where the user may have to shift from a satellite going out of range to one coming in range during a call. A high 10350km orbiter can cover nearly as much of the earth in its footprint as a geosat but it moves continously ( and so cannot use a directional ground station antenna), and will eventually drift out of range ( albeit slowly ), sometimes requiring a shift to another satellite during a call. If placed in say 45 degree inclination orbits 10 high orbiter could give useful coverage of the whole earth. But many more than 10 satellites will be needed if as will probably be necessary multi satellite space diversity is used to avoid fading and screening effects at the ground mobile equipment. Low altitude orbiters (LEOs) have much smaller footprint coverage . So in order to see other satellites and thereby offer worldwide links , they must be much less than twice their footprint radius apart to give adequate connection at the Equator ( at higher latitudes the natural bunching effect as the longitude lines close up will in theory call for less satellites , but if complete coverage is necessary this wastage of satellites at higher latitudes is a price which must be paid). Though in most cases the price can be reduced by covering low latitudes with satellites in planes close together in low inclination 45 or 50 degree inc orbits and supplementing these with a much smaller number of satellites in polar orbits. 2. Orbit plane separation. As shown above, High orbiters with big footprints can be widely separated both in the same orbit plane and between orbit planes ,Eg Odyssey plan to use 3 orbit planes , inc 50 degrees , footprint radius at 0deg = 67.5 degrees allowing a practical separation inplane and between planes of perhaps 110 degrees. But LEO satellites with footprints of less than 30 degrees radius must use orbit planes and, separations in the orbit plane, of much less than 60 degrees .