Voice and Data
The Iridium system is a satellite-based, wireless personal communications network providing a robust suite of voice and data features all over the globe. It is comprised of three principal components -- the satellite network, the ground network and the Iridium subscriber products, including phones and data modems.
The design of the Iridium network allows voice and data messages to be routed anywhere in the world. Voice and data calls are relayed from one satellite to another until they reach the satellite above the Iridium handset or terminal and the signal is relayed back to Earth. When an Iridium customer places a call from a handset or terminal, it connects to whatever satellite happens to be overhead, and is relayed among satellites around the globe to whatever satellite is above the appropriate Earth gateway, which downlinks the call and transfers it to the global public voice network or Internet so that it reaches the recipient.
The satellites are in a near-polar orbit at an altitude of 485 miles (780 km). The 66 active satellites fly in formation in six orbital planes, evenly spaced around the planet, each with 11 satellites equally spaced apart from each other in that orbital plane.
Iridium Satellite Network
A single satellite completely circles the Earth once every 100 minutes, traveling at a rate of 16,832 miles per hour, and traveling from horizon to horizon across the sky in about ten minutes. As a satellite moves out of reach, the call is seamlessly handed over to the next satellite coming into view.
Since Iridium is a LEO satellite system, voice delays are typically unnoticeable. Other mobile satellite systems use Geostationary Earth Orbits (GEOs), which, by comparison, are about 22,300 miles above the equator. As a result, latency can be quite high, causing speakers to have to wait for each other to finish. GEOs are also largely ineffective in more northern or southern latitudes. The curvature of the Earth disrupts message transmission when attempted at the edge of a GEO satellite's footprint.
Each Iridium satellite is cross-linked to four other satellites - two satellites in the same orbital plane and two in an adjacent plane. These links create a dynamic network in space - calls are routed among Iridium satellites without touching the ground, creating a highly secure and reliable connection. Cross-links make Iridium particularly impervious to natural disasters - such as hurricanes, tsunamis and earthquakes - that can damage ground-based wireless towers since cross-links are space-based.
The Iridium ground network is comprised of the system control segment and telephony gateways used to connect into the terrestrial telephone system. With centralized management of the Iridium network, the system control segment supplies global operational support and control services for the satellite constellation, delivers satellite tracking data to the gateways, and controls the termination of Iridium messaging services.
The system control segment consists of three primary components -- four telemetry tracking and command/control (TTAC) stations, the operational support network, and the satellite network operation center (SNOC). Ku-Band feeder links and cross-links throughout the satellite constellation supply the connections among the system control segment, the satellites and the gateways.
Gateways are the ground-based antennas and electronics that provide voice and data services, messaging, prepaid and postpaid billing services, as well as other customer services. The gateways are responsible for the support and management of mobile subscribers and the interconnection of the Iridium network to the terrestrial phone system. Gateways also provide management functions for their own network elements and links.