The Iridium Communications Network’s system is based on a network of 66 satellites. The satellite constellation is distributed on 6 near polar planes and each plane carries 11 operational satellites, as well as one spare satellite.
Each satellite weighs approximately 689 kilograms (1,500 pounds) and orbits at an altitude of 780 kilometres (420 nautical miles) above the surface of the Earth travelling around the planet once every 100 minutes. Unlike geostationary communications satellites which are located 36,000 kilometres above the Earth, the Iridium satellites’ low Earth orbit makes it possible to communicate directly with a hand-held telephone, in the meantime avoiding the typical delay of geostationary satellite transmissions.
The satellite is linked to callers by a main mission antenna that offers 16 decibels of link margin, a signal that is strong enough to deliver high quality communications to a hand-held telephone. In contrast with traditional wireless systems, coverage is achieved by satellites that project a grid of cells over the surface of the Earth. Each satellite covers an area 4,000 kilometres wide, providing links to all Iridium subscribers roaming in such an area.
Developed by Motorola’s Satellite Communications Division (SATCOM) in Chandler, Arizona, U.S.A., Iridium satellites required impressive standards of manufacturing. The satellite constellation is controlled by a sophisticated ground control network, located in the U.S.A. and in Italy.
True global connectivity is provided because each satellite is connected by radio transmission to four others through the use of crosslink antennas. By routing a call through the satellite network, the Iridium system is not dependent on availability of extensive terrestrial telecommunications facilities to deliver global telecommunications. These intersatellite links are essential for providing truly global communication coverage.
As the only proposed mobile satellite service using intersatellite links, the IRIDIUM system allows users to make and receive calls from anywhere on Earth, including polar and ocean regions.
Intersatellite crosslinks reduce the number of Earth stations required and allow for lower operating costs compared to other systems. The transmissions take place in the Ka frequency band between 23.18 and 23.38 GHz.
The IRIDIUM system utilizes a combination of Frequency Division Multiple Access and Time Division Multiple Access (FDMA/TDMA) signal multiplexing to make the most efficient use of limited spectrum.
The L-Band serves as the link between the satellite and IRIDIUM subscriber equipment. The Iridium subscriber equipments have been authorized by the European Administrations to operate in the band 1621.35 – 1626.5 MHz for both uplink and downlink.
The Ka-Band (19.4-19.6 GHz for downlinks; 29.1-29.3 GHz for uplinks) serves as the link between the satellite and the gateways and Earth terminals.
Gateways make communications possible between Iridium telephones and any other telephone in the world. Interconnection to the existing wired and wireless networks, domestic and international, public and private, is performed through ground-based Iridium gateways which are located in key regions of the world.
These stations consist of equipment to communicate with the satellite constellation and with the international switching centers of the domestic fixed network. Their functions include registering of Iridium subscribers and providing access to existing ground-based telephone networks, so achieving the connectivity of the telecommunications networks worldwide. Gateways are owned, operated and financed by one or more Iridium investors or their affiliates. Eleven gateways are scheduled to be in operation at the commencement of commercial service.
Iridium Italia and Iridium Communication Germany manage the European gateway which is installed at Telespazio’s Space Centre in the province of L’Aquila, approximately 130 km from Rome. The Fucino station – the largest civilian telecommunications centre in the world – was chosen for its geographic position, optimum for serving the European continent and outlying regions. The Fucino station is also to be home to a GBS (Gateway Business System) centre, an information centre for the processing and administration of the service’s data (contacts, consumption, bills).
At the heart of the Iridium gateway is a Siemens EWSD-based D900 switch. Based on the GSM (Global System for Mobile Communications) cellular standard, the D900 switch ensures a high quality integration of Iridium services with land-based telecommunication systems.
The System Control serves as the central management component for the Iridium satellite constellation. Two System Control Facilities are planned to be installed, one in the United States and one in Italy, in Rome. Located just outside Washington, D.C., the primary function of the Master Control Facility is to control and monitor the Iridium satellite network. Day-to-day management of a satellite constellation involves a variety of tasks, including analyzing telemetry data, to ensure that every subsystem aboard the spacecraft is working properly.
In addition, three Telemetry, Tracking, and Control Centres (TTACs) located in Hawaii and Canada, are linked directly with the Master Control Facility. The TTAC facilities communicate with the satellite network to, among many other functions, regulate the positioning of satellites during launch placement and subsequent orbit, monitor and control the position of the satellites during the launch phase and orbit phase.
Motorola’s Engineering Control Center in Chandler, Arizona, serves as launch control center, responsible for monitoring and controlling the initial ascent and early orbit phase activities of the Iridium constellation of 66 satellites plus spares.
The Engineering Control Center will hand over responsibility for control and monitoring of the satellite network to the Master Control Facility after the satellites are placed in orbit.
IRIDIUM – How it works. (n.d.). Retrieved June 04, 2016, from https://geoborders.com/iridium/en/iridium.htm