Iridium is probably the most famous, or rather notorious, of the satellite systems on the market. They very publicly went bankrupt in 1999; one year after the service was launched. However, Motorola Inc. stepped in and settled their debts. Iridium was re-launched in 2001 and has been operating ever since.
Iridium’s mission was to create a satellite service that would provide truly global coverage, a service that Inmarsat and Globalstar have not yet realised. To achieve this the Iridium network uses 66 satellites, 450 miles high, orbiting the globe on 6 different planes. Travelling at 16,832 miles per hour, each satellite is able to circle the globe every 100 minutes.
This configuration ensures that an Iridium handset will be able to send and receive calls from any location on the planet (the only exceptions to this being countries who have declined to support the service for political reasons). As long as the handset has a clear line of site to the sky it will maintain communication with at least two satellites at any one time.
The main criticism that has been levelled at Iridium is the fact that they are limited in their data and fax capabilities. The service is available but not to the standards that Inmarsat and Globalstar can provide.
Also, as mentioned briefly in regards to Inmarsat’s service, Iridium run a higher risk of loosing a signal once it has been sent. This happens because they rely on the passing of signals via a number of satellites before routing them to their final destinations. The more links in the chain, the greater the chance of the signal being lost.