Iridium is probably the most well-known, or rather notorious, of the satellite systems available. They were bankrupt in 1990 and known to public; 1 year after the service was announced. However, Motorola Inc. walked in and resolved their loans. Iridium was re-launched in 2001 and has been operating since.
Iridium's aim was to generate a satellite system that would offer truly global reception, a service that Inmarsat and Globalstar have not yet realized. To achieve this the Iridium coverage makes use of 66 satellites, 450 miles high, and orbiting the globe on six various planes. Traveling at 16,832 miles per hour, for each satellite is capable to orbit the planet every 100 minutes.
This settings assures that an Iridium handset will be capable to send and receive calls from any area on the planet (the only exclusions to this regions who have diminished to help support the service for political reasons). So long as the handset has a clear of site to the sky it will surely maintain communication with at least two satellites at any one time.
The main grievance that has been leveled at Iridium is the truth that they are restricted in their data and fax capabilities. The service is available but not to the requirements that Inmarsat and Globalstar can offer.
Furthermore, as pointed out briefly in regards to Inmarsat's service, Iridium function a greater risk of losing a coverage once it had been sent. This occurs since they rely on the passing of receptions via a number of satellites before heading to their final locations.