Reliability considerations for multiple-spot-beam communication satellites

Reliability considerations associated with multiple-spot-beam satellite systems are explored. If each coverage area is serviced by a single transponder, then loss of transponders due to failure eliminates all service to the areas covered by those transponders. Thus, failures are quite costly compared to a system employing global coverage with multiple transponders, where a limited number of transponder failures results in a slight increase in the traffic demand upon the survivors. Since the total orbital weight of a satellite is fixed, any redundant hardware deployed to improve reliability reduces the number of active transponders that can be supported, and a highly efficient redundancy strategy must be employed. Cold standby redundancy with complete spare interconnectivity is studied and appropriate reliability formulas are derived. A specific satellite concept dominated by final power amplifier failures is studied in detail, and it is found, for typical failure rates, that a 27-percent reduction in capacity must be accepted to provide for a single satellite lifetime reliability of 99 percent. Various techniques for employing the in-orbit redundancy of a spare satellite are investigated to increase reliability while minimizing capacity reduction. Bounds are derived and a reliability of at least 99 percent is shown possible for a system containing three active satellites plus one spare, at a cost of 9 percent in potential capacity of the active satellites.