Iridium Satellite Constellation - Satellites

Satellites

The satellites each contain seven Motorola/Freescale PowerPC 603E processors running at roughly 200 MHz, connected by a custom backplane network. One processor is dedicated to each cross-link antenna ("HVARC"), and two processors ("SVARC"s) are dedicated to satellite control, one being a spare. Late in the project an extra processor ("SAC") was added to perform resource management and phone call processing.

The cellular look down antenna has 48 spot beams arranged as 16 beams in three sectors. The four inter-satellite cross links on each satellite operate at 10 Mbit/s. The inventors of the system had previously worked on a government study in the late 1980s that showed that microwave cross links were simpler and had fewer risks than optical cross links. Although optical links could have supported a much greater bandwidth and a more aggressive growth path, microwave cross links were favored because the bandwidth was more than sufficient for the desired system. Nevertheless, a parallel optical cross link option was carried through a critical design review, and ended when the microwave cross links were shown to support the size, weight and power requirements allocated within the individual satellite's budget. In recent press releases, Iridium Satellite LLC has stated that their second generation satellites would also use microwave, not optical, inter-satellite communications links. Such cross-links are unique in the satellite telephone industry, as other providers do not relay data between satellites.

The original design envisioned a completely static 1960s "dumb satellite" with a set of control messages and time-triggers for an entire orbit that would be uploaded as the satellite passed over the poles. It was found that this design did not have enough bandwidth in the space-based backhaul to upload each satellite quickly and reliably over the poles. Therefore, the design was scrapped in favor of a design that performed dynamic control of routing and channel selection late in the project, resulting in a one-year delay in system delivery.

Each satellite can support up to 1100 concurrent phone calls and weighs about 1,500 lb (680 kg). The Iridium System presently operates within a 1618.85 to 1626.5 MHz band adjacent to the 1610.6-1613.8 MHz Radio Astronomy Service (RAS) band.

Owing to the highly reflective antennas, Iridium satellites cause a phenomenon known as Iridium flares, watched by enthusiasts and sometimes visible in daylight.