Satellite Mobile Systems

Satellite mobile systems are useful means of communication for long-distance travellers. Also, they are adopted in aircraft and ship navigational purposes. National Aeronautics and Space Administration (NASA) through the ATS-6 satellite undertook the premier efforts on mobile satellite communication.

In 1979, the INMARSAT (International Maritime Satellite) organisation was set up to establish world-wide aeronautical satellite communication standards governing the telephone and telex services (Standard A), ISDN (Standard B), and low data rate services (Standard C). Since 1983, the International Civil Aviation Organisation (ICAO) is also involved in the related activities.

The other satellite-based mobile systems include the following:

  • Radio Determination Satellite System (RDSS) of Geostar: This is a radio navigation/radio location system managed via a single satellite for US based domestic applications.
  • OmniTracs system of Qualcom Inc.: This is a two-way mobile satellite communication system intended for vehicle positioning and is adopted in the United States and Europe.
  • MobileSat: This is an Australian system which supports services such as circuit-switched voice, and data/packet-switched data transfers for land, aeronautical, and maritime users 􀂃 Telesat Mobile Inc. (TMI)/American Mobile Satellite Corporation (AMSC) systems: These systems provide mobile satellite services in the United States and in Canada.

Global communications are often based on satellite systems because of the relative ease with which they establish global coverage. Although fiber-optic networks yield world-wide coverage, it is only a parallel strategy to the satellitebased relaying. For the first time, the public satellite communication was deployed in 1962 with the efforts due to NASA through the Application Technology Satellite (ATS) program.

Satellite communication offers an extensive coverage or footprint on the earth and allows a seamless mobile service. The size of the footprint depends on the orbit of the geostationary (GEO) satellite deployed. For example, with an orbit at 36,000 km above the earth, the field-of-view (FoV) coverage is about 13,000 km diameter.

There are low orbit (LEO) satellites, typically at an altitude of about 800 km and an FoV of about 1,500 km diameter. The communications via LEO satellites face small path-losses, which allow for small-sized earth antennas.

Inmarsat was developed in 1973 to provide maritime communications worldwide through GEOs and it offers services to all modes of transportation by air, land, or sea. Iridium is a global satellite phone system, with voice and data traffic. It offers services like, digital voice, paging, faxing, global positioning service (GPS) etc. Iridium satellites are interconnected via microwave cross-links and interface gateways with the PSTN.