Design of the control systems operations for the Iridium satellite system

The Iridium satellite system as proposed by Motorola, Inc. provides continuous global communications coverage by supplementing prevailing telephone services. The satellite system is comprised of 66 Low Earth Orbiting (LEO) satellites which are able to accommodate Iridium handsets or existing telephone links. These satellites are monitored and controlled by ground control facilities for approximately 6 years ( the estimated life of the vehicles) and are then replaced.

The ground control facilities are required to provide software upgrades, monitor the health of the vehicles, remove a satellite from the constellation when necessary, and coordinate launch and deorbit of the satellites. The approach used to code and modulate the telemetry processed by the ground stations is analyzed in order to develop support software for a recommended data network configuration used at the ground stations. A Reed-Solomon coding scheme is prepared for the telemetry channel since this coding scheme corrects bit errors and protects against burst errors. The Groupe Speciale Mobile (GSM), a combination of time division mUltiple access, TDMA, and frequency division multiple access, FDMA, is recommended for use by Iridium. A bus topology with a fiber backbone is suggested to support the operations at each facility and a separate landline is considered to support operations between the facilities when possible. The support software is available in the appendix and is written in Fortran 77.

A data security coding algorithm is introduced; however, this is not a requirement or a concern for Motorola and is an option that may be considered by the ground facilities. The data security for a commercial satellite system is considered insignificant at this time.