Link Strategy for the Mobile Satellite System Iridium

This paper developes crosslink routing strategies for the Mobile Satellite System Iridium [1,3,6,8]. Motorola’s Iridium satellite constellation, based on the orbit design of Adams and Rider [2] is to provide mobile satellite service for hand-held personal telephones. The Iridium system will have 66 satellites in 6 orbital planes at an altitude of h = 780 km and a minimum elevation angle of θ = 8.2 degrees for an optimal satellite distribution around the globe. It will be a system with direct connections between satellites (Intersatellite Links ISLs). These connections can be established with Intra-Orbit ISLs and with Inter-Orbit ISLs. The problem of routing messages through the satellite network is complicated by the time-varying satellite system geometry. The relative motion between the synchronized orbital planes is relatively small. However, orbital planes 1 and 6 are counterrotating and have large relative motion. Different Inter-Orbit ISL routing strategies are needed for these two cases. The time-varying behaviour of relative azimuth and elevation angles between satellites is analysed using the software systems SatLab [10] and BONeS DESIGNER [11]. Time-varying routing and antenna pointing angles are developed for both synchronized and counterrotating orbital planes.