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How the Iridium Network Works

The Iridium satellite network includes three principal components which include the satellite network, the ground stations and the satellite phones and data units.

Voice and data messages can be routed anywhere in the world by the Iridium network. Calls are relayed from the satellite phone or data unit on the ground to one of the Iridium satellites. It is then relayed from one satellite to another then down to an appropriate ground station. The call is then transferred to the public voice network or Internet when it reaches the recipient.

The Iridium constellation includes 66 satellites flying in six orbital planes with 11 satellites equally spaced apart from each other in that orbital plane. The satellites have polar orbits at an altitude of 485 miles.

The satellites communicate with each other using Ka-band intersatellite links. Each satellite has four intersatellite links, two to the fore and aft satellites in the same orbital plane and two to the satellites in orbital plane to either side. These intersatellite links allow calls to be routed among the Iridium satellites before being transferred to a ground station. This intersatellite links makes Iridium impervious to natural disasters like hurricanes or earthquakes which can damage a ground station. If a ground station is damaged the Iridium satellites can rout the call to another ground station.

Each satellite completely orbits the earth in 100 minutes traveling at 16,832 miles per hour. From horizon to horizon it takes 10 minutes. As satellites move out of view from the satellite phone user the call is handed over to the next satellite coming into view.

Each Iridium satellite contains seven Motorola Freescale PowerPC 603E processors running at 200MHz. One processor is dedicated to each cross link antenna and two processors are dedicated to satellite control with one as a spare.

Each Iridium satellite can support up to 1100 phone calls.

Communication with the Iridium network is done using a TDMA and FDMA based system using L band spectrum between 1616 and 1626.5MHz. Iridium controls 7.775MHz of this.

The Iridium network uses three different types of handoff. When a satellite travels over the horizon the call is handed to adjacent spot beams. At the equator where the satellites are spaced the furthest apart, a satellite stays in view for seven minutes. This handoff may be noticeable by the satellite phone user by a quarter second gap in the call. The constellation is able to transfer the call to different channels and timeslots within the same spot beam.

TheIridium gatewaysinclude the system control segment and telephony gateways used to connect into the public telephone system. As a satellite leaves the area of a ground station and looses it line-of-site, the routing tables change and frames are forwarded to the next satellite coming into view of the gateway. Gateways are located in Tempe, Arizona and Wahiawa, Hawaii.

Like other satellite networks, Iridium satellite phones must have line-of-site to the sky in order to place a call. They will not work consistently indoors or under trees. There are fixed site units that include an external antenna giving you line-of-site for use on a building. There are also car kits giving you an external antenna.

Network Performance

Iridium operates the most reliable satellite constellation currently available. One of the reasons Iridium works so well is their system has more satellites than any other satellite provider, giving coverage to every part of the planet. With no service gaps, Iridium users can place and receive calls from virtually every part of the planet as long as they have line-of-site to one of the satellites. With cellular systems only covering 15 percent of the planet Iridium is the only wireless service available to many parts of the world.

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