I. Introduction
The Iridium satellite system consists of an interconnected network of 66 satellites (plus 6 spares) in low Earth orbit (LEO) plus the ground-based infrastructure required to command and control the satellites and connect them to the public switched telephone network (PSTN). Figure1 shows the system architecture with the rf connections between satellites and the Earth. The satellites orbit the earth at an altitude of 780m. Their orbits are inclined at an angle of 86 deg with respect to the equator, providing global Earth coverage. The constellations configured as six planes with eleven operational satellites and one spare satellite in each plane. The satellites are equally spaced and arranged so that their coverage footprints overlap to provide continuous coverage by handing communications traffic from satellite to satellite as they pass over the Earth. The spare satellites are flown at a slightly lower altitude until they are needed to replace worn-out operational satellites, at which point the depleted satellite is deorbited and the spare maneuvered into the appropriate slot in the constellation [1][2].