Interference problems of co-channel communication satellite systems

The anticipated growth of satellite communication may lead to numbers of independent systems, analogous to the many overseas airlines. Some such systems may need to share frequency channels. This paper examines orbital constraints on channel sharing, toward controlling or avoiding intrusions of “foreign” satellites into the interference beamwidth of earth station antennas which are tracking satellites of their own system. Such an intrusion is termed an “eclipse.” In general, two systems can avoid eclipse interference by interchange of their “up” and “down” frequency channels. This, however, is not true sharing. Additionally, such interchange complicates interference coordination between earth stations of the two systems. Orbit crossings create eclipse conditions. Hence, random orbit systems and systems using orbits of different inclinations can avoid eclipse interference by eclipse-prediction and premature hand-over to an uneclipsed and available satellite. If co-channel systems use circular equatorial orbits of sufficiently different heights, eclipses would be intolerable near the equator, but would not occur at higher latitudes. Eclipses always can be avoided if systems share the same orbit(s) with phased satellites, maintaining adequate separation between satellites of the several systems. In particular, any stationary satellite is “phased” to the accuracy with which it is kept on station. Thus, many co-channel satellite systems can share the stationary orbit.