Network Reference - Network Description
The Iridium system is one of a number of low-Earth orbit (LEO) satellite networks developed by Motorola® to provide personal mobile services. The original concept was visualized as far back as 1987 and was granted a full FCC license in January of 1995 for construction and operation in the United States. The Iridium satellite network is now owned and operated by Iridium Satellite LLC (ISLLC). There are four components to the network: (1) a constellation of 66 satellites, (2) three terrestrial gateways, (3) a satellite network operation center (SNOC) and (4) Iridium subscriber units (ISU).
The satellites are in 6 orbital planes separated by 31.6o at an altitude of 780km and 86.4o inclination MOVIE. The Iridium network is designed to operate in the L-band of 1616 to 1626.5 MHz for ground user links, in the Ka-band of 19.4 to 19.6 GHz and 29.1 to 29.3 GHz for gateway down- and up-links and in the Ka-band of 23.18 to 23.38 GHz for inter-satellite links (ISL). The exact L-band frequencies used depend on local regulating authorities and issued licenses in any particular region.
Each satellite projects 48 beams on the surface of the Earth, which may be viewed as providing coverage cells on the ground similar to terrestrial cellular systems MOVIE MOVIE. Each beam is approximately 600 km in diameter. The 66-satellite constellation has the potential to support a total of 3,168 spot beams; however, as the satellite orbits converge at the poles, overlapping beams are shut down to prevent interferences. The satellite footprint is ~4,700 km in diameter. Under each footprint, a satellite's power is limited to ~1,100 simultaneous circuits. A user is in view of a satellite for approximately 9 minutes, with about 1 minute under each beam, before being handed-off to the next satellite MOVIE.
Inter-satellite link or ISL is the network architecture employed by Iridium (versus bent-pipe1 employed by Globalstar® and Orbcomm®). A unique feature of the Iridium ISL capability is that the satellites not only can talk to ISU and gateways, but they can also talk to each other, forming a network aloft. When a signal is up-linked to a satellite by an ISU, it is down-linked immediately to a gateway located within the satellite's footprint and then gets distributed to the final destination. However, when a gateway is not visible to the satellite, information is passed through the network of satellites to the one that is immediately over a gateway. Iridium ISL also allows ISU to talk to each other without ever referencing to any ground stations at all thereby reducing signal latency that can adversely affect time-sensitive protocols such as TCP/IP.
ISL provides benefits such as enhanced system reliability and capacity and reduces the number of gateways required. The greatest advantage of using ISL is essentially the capability of truly worldwide coverage without signal latency in either voice or data mode. By eliminating the dependency on ground infrastructure for traffic links, an ISL-based system such as Iridium becomes more autonomy. Moreover, ISL can make communications virtually impervious to disasters on Earth, such as earthquakes, hurricanes, floods and other natural and man-made disasters.
1 The most commonly used satellite network architecture is the "bent-pipe" approach whereby signals from a hand-held communicator are transmitted up to an overhead satellite, relayed immediately back down toward a gateway located within the satellite's footprint, and then delivered to the final destination via landline networks. When a satellite does not have a gateway under its footprint after receiving an up-linked message, the message gets store on-board the satellite and will be delivered to the next gateway it passes over. This mode of operation is referred to as "store-and-forward" and can only be employed for transmitting short telegram messages. Latency depends highly on where the data originated from (geographical location) and the geographical locations of gateways around the globe.