Link strategy for the mobile satellite system Iridium

Abstract

This paper develops crosslink routing strategies for the mobile satellite system Iridium. Motorola's Iridium satellite constellation, based on the orbit design of Adams and Rider [1987] is to provide a 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 /spl theta/=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. Time-varying routing and antenna pointing angles are developed for both synchronized and counterrotating orbital places.