The Iridium project, implemented by Motorola Inc., was the first communications system for mobile satellitecommunications. It went into regular operation in September 1998 and was discontinued on March 18, 2000
Iridium was named after the chemical element with atomic number 77. Originally, the system was to include as many satellites. The number was later reduced to 66. The 66 fast-orbiting LEO satellites orbit the Earth at an altitude of 765 km on six circular orbits in about 1.7 hours, which corresponds to a speed of 27,000 kilometers per hour. All six orbits cross over the poles. Eleven satellites are equally spaced on each orbit. Each satellite uses a cellular concept to illuminate the Earth's surface with spot beams. Each satellite can emit up to 48 spot beams. This means that the entire surface of the earth can be broadcast with a maximum of 3,168 radio cells.
The communication links between the satellites
The satellites operate with various communication links among themselves, but in particular with mobile and stationary receiving facilities on Earth. In addition to links between mobile stations and satellites, there are links between satellites and stationary gateway stations, links between two satellites flying behind each other, links between two adjacent satellites, and links between satellites and control stations.
As satellites move across the user, radio cells also move across the user. If a radio cell can no longer serve the user, the cellular spot- beam concept passes the transmission to the next radio cell on the same satellite in the form of a handover. Only when all the radio cells of a satellite can no longer reach the user is there a handover via the Inter Satellite Link( ISL) to the subsequent satellite. The ISL link to the following satellite is also used for communication between two end users who are not in the same footprint of a satellite.
Transmission frequencies of Iridium
The Iridium system allows mobile communications to and from any point on Earth. The designated frequency band for the useful signals is between 1,610 MHz and 1,626.5 MHz. The modulation of the radio interface between mobile and satellite is quadrature phase shift ke ying( QPSK) with FDMA/ TDMA. Communication between the satellites and via the satellite gateways to the ground stations is in the Ka-band for the uplink at 29.2 GHz and the downlink at 19.5 GHz. Twelve ground stations provide connectivity to terrestrial, fixed and mobile networks. Thanks to the use of LEO satellites, the delay time, which is essentially characterized by the signal propagation times from the mobile data terminals to the satellites and back to earth, is only 180 ms.
In the Iridium network, a location-independent personal phone number for the user will be implemented. The network should be able to cover up to 12 million users, with one satellite being able to switch up to 1000 calls simultaneously. It thus forms the precursor to the global Personal Communications Network( PCN). The Iridium service was temporarily discontinued in February 2000.