Geostationary Systems

Contributed by Miquele Dlodlo

INMARSAT’s 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.

Geostationary orbit

A geostationary satellite is in the circular geosynchronous orbit along the equatorial plane, a fact that makes it appear stationary in the sky from the ground. By geosynchronous orbit is meant a path whose period equals one sidereal day of 23h 56 min 3.31s on average (assuming a spherical earth with a radius of 6 378 km and a satellite height above the earth’s surface of 35 800 km) and is identical therefore to that of the earth’s rotation.

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.

Advantages and Limitations.

As just pointed out, GEOS systems for mobile communications are already in successful operation and are known to inter-work with the fixed satellite and terrestrial networks as well. This has given the operators the chance to capture a sizeable market share well ahead of the competition and build on the goodwill emanating from historical successes. In addition, operational experience should enable them to make realistic projections in their development efforts towards third generation systems. Unlike the fixed terrestrial network hardware, satellites are built to last no more than 10 to 15 years. That means plant replacement is an in-built cost factor which permits regular and timely updates. In fixed terrestrial networks, by contrast, swift evolution is impossible because it is more economical to have much longer plant replacement cycles. For instance, ISDN has been promised for decades, yet up to now only a handful of networks world-wide are fully digital. The height of the geostationary orbit itself is enough to permit the use of fewer, more powerful satellites while still promising global coverage.

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.

The Future of GEOS Systems.

In the short-to-medium term, the GEOS market appears established, secure and in a state of growth in the direction of FPLMTS (or UMTS). Its larger, powerful and more robust equipment has been time-tried and is still evolving with technological progress. While competition in the PCS marketplace is just round the corner, the GEOS system will beyond doubt continue to dominate the marine, fleet management and aeronautical markets for the foreseeable future.