Network Concepts and Standards
INMARSATs geostationary satellite (GEOS) communication system was the only public global digital land-mobile satellite communication service available around 1995/1996. It provides global access to voice, data, facsimile (fax) and telex services not only for land-mobile users but also to marine and aeronautical terminals. It has a provision for interconnection to the public switched telecommunications network (PSTN) as well as the terrestrial public land mobile network (PLMN). The only other global communication satellite system at this high orbit is operated by INTELSAT to provide intercontinental links for PSTNs and broadcast television distribution services via large earth stations and gateways. INTELSAT services include voice, video, teleconferencing, television, fax, data and telex. Besides the global GEOS systems, there are some military, regional and national systems, such as Global Star, Astra, etc.
Existing as well as most planned future communication satellites carry a bank of transponders (repeaters) or frequency translating amplifiers. Functionally, then the receiving antenna collects the composite incident electromagnetic radiation, feeds it to a bank of frequency-selective bandpass filters which in turn feed an amplifier bank. A frequency translator for each selected band then transfers the signals to the corresponding power amplifier in the transmitting section. Output multiplex filters then recombine the signals that ultimately feed the transmitting antenna. Hence, the basic transponder concept is: pre-amplify, translate the carrier frequency and boost the power for onward transmission to earth.
The simplicity of a GEOS transponder translates into robust hardware components with a long life. In the INMARSAT system, for instance, the earth is divided into four zones along the equator, viz. the Indian, Atlantic East (AOE), Atlantic West (AOW) and Pacific Ocean Regions. Each zone is covered by one satellite with at least one spare close by. It may be of interest to figure out the range of distances between each satellite and the various mobile stations in its cell.
Most satellite-based communications research and development efforts prior to 1990 focused on the geostationary satellites. This has given GEOS systems a head start in the race for ubiquitous global communications, mainly in the business sphere.
A subtle advantage is the nature of INMARSAT itself as a public international co-operative in which most world governments already participate either directly at ministerial level or indirectly through state-authorised operators. Since these operators already own the fixed network, inter-operability and tariff structures are already part of the existing agreements.
A major technical limitation of the GEOS orbit is the remote location of the transponder. In view of the power loss law, high transmitter power is required. The smallest transceiver kit that can deliver the necessary quality of service in the field is the size of a briefcase, a far cry from the personal communication systems goal of a hand-held or wrist-worn unit.
An additional disadvantage from the distance is the perceptible 250 millisecond delay in a single hop transmission.