Coverage and overlap of satellites in circular equatorial orbits. With applications to multiple-access communication-satellite systems

This paper considers the earth areas within which many stations, all using the same satellite, can have multiple-access intercommunication without interruptions. These areas are the effective earth coverage in the case of a stationary satellite, or the coverage overlap at handover in the case of single-earth-track medium-altitude circular-equatorial-orbit systems. An equation is derived for the overlap of the coverage areas of two satellites at equal heights, for given angular separation and given minimum angle above the Earth´s horizon. Families of coverage and overlap curves are given. The relations between coverage and the earlier `mutual-visibility area¿ are explained. Diagrams of the boundaries of one-hop multiple-access systems, as viewed from above the pole, are shown and discussed, both for a six-satellite stationary-orbit system and for a 12-satellite 14000km phased circular-equatorial-orbit system; these are used and com pared in discussing routing principles. All such equatorial-orbit systems provide a flexibility of routing, via a choice of satellites, which decreases and disappears at higher latitudes. Specific illustrative long circuits using surface-communication `tails¿ are discussed using Mercator maps. It is shown that these longest circuits will probably be multihop systems, when using medium-altitude satellites, while single-hop use of stationary satellites (with surface tails when necessary) will be preferable and possible. The stationary-orbit system has the economic advantages that its earth stations do not require complex tracking facilities, fully steerable antennas or duplication of the installation for performing periodic handovers. Additionally, the use of relay stations and of more than one satellite hop does not appear to be necessary or probable, as with lower-orbit systems.