Finding a place in the geostationary orbit: aim is to ensure access for all.

The UNISPACE 82 Conference noted the explosive growth in recent years in the use of the geostationary orbit, especially for communication satellites. The Conference noted that the present system of registration and coordination might need to be improved to guarantee in practice, for all countries, equitable access to the geostationary orbit and frequency bands allocated to space services. Two sessions of a World Administrative Radio Conference (WARC) on the use of the geostationary orbit are scheduled to meet, in 1985 and 1988.

UNISPACE 82 also noted that most nations accept that the geostationary orbit is a part of outer space and, as such, is available for use by all States, in accordance with the 1967 Outer Space Treaty. However, equatorial countries consider that the geostationary orbit is a physical phenomenon related to the earth's gravity. For this reason, these countries maintain, it should not be included in the concept of outer space and its use should be regulated under a sui generis regime.

The study (A/AC.105/340)--"The Feasibility of Obtaining Closer Spacing of Satellites in the Geostationary Orbit"--was prepared with the assistance of a group of experts from Colombia, Czechoslovakia, Italy, Japan, Kenya, Pakistan, Sweden, USSR and United Kingdon.

Conclusions: Closer spacing of satellites in the geostationary orbit is feasible and certain technologies exist to allow greater overall efficiency in the orbit's use. Some technologies and techniques are already in the implementation phase; others could be implemented on a large scale in the next five to 10 years. The efficient use of the orbit is expected to increase noticeably. However, full advantage of the benefits can be achieved only when new techniques are widely used.

The most crowded parts of the orbit are the arcs from 49[deg.]E to 90[deg.]E (over the Indian Ocean), from 135[deg.]W to 87[deg.]W (serving North America) and from 1[deg.]W to 35[deg.]W (over the Atlantic Ocean). For some parts of the orbit, such as over the western Pacific, there would appear to be little prospect of congestion. Since each country or region can only use a portion of the orbit for its communication needs, any competition for positions will be between a certain number of countries and not global.

Under present procedures, although some countries have had difficulties adapting their proposed satellites to existing assignments, no country has been denied access to the geostationary orbit for any satellite. Technological advances, including those contributing to a reduction in spacing, could help to ensure continuing access.

Given that satellite and Earth station technology will continue to develop and that a growing number of systems using different technologies will be introduced, the potential minimum spacing between satellites will vry with time and with position in the geostationary orbit.

Though the possibility of collisions between satellites and other objects in the geostationary orbit is not yet serious, a systematic study of the problem of collisions may be needed to find ways to avert them.

Advantages offered by communication satellites for telecommunications and broadcasting have influenced planners in developing countries, who want access to space technology. A systematic effort must be made to assist developing countries achieve indigenous capability through transfer of know-how. White it is neither possible nor desirable for all countries to establish independent research and development programmes in the field of satellite communications, every country should be able to participate bilaterally, regionally or internationally in such programmes.

If a country or group of countries has decided to acquire a satellite system or ground system, a key question is the choice between designing and building satellites and Earth stations or buying systems from other countries. A decision to build domestically may have implications for satellite spacing in that a country new to the technology of satellite or Earth station design may have difficulty incorporating the most advanced technology that could maximize communication capacity or minimize spacing.

Nations and organizations with advanced technological capabilities should make a particular effort to provide technical assistance to developing countries in order to provide the greatest possible access to communications to all countries. Such assistance should also include education and training in the planning and design of communication satellite systems and operation and maintenance of ground systems.

Special efforts should be made by the International Telecommunication Union (ITU) and its members to assist developing countries assess future satellite communication requirements and identify optimum orbital positions and frequency bands for their satellite communication needs.

Communication uses

The geostationary orbit is a circular orbit around the Earth approximately 35,787 kilometres (22,300 miels) above the equator. At this altitude, a satellite orbits the Earth during a period of 23 hours 56 minutes, synchronously with the Earth's rotation. If the orbit is directly above the equator, with the satellite revolving in the same direction as the Earth rotates, from wet to east, the satellite appears from the ground to remain stationary. Such an orbit provides a constant orientation between the satellite and any fixed point on the ground within view of the satellite, so that there would be no need for ground antennas to move to track the satellite.

The orbit is used mostly for communication satellites for which continuity of service, large area of coverage and fixed pointing antennas are great advantages. The majority of communication satellites currently in use are in the geostationary orbit. Satellites in the fixed-satellite service relay telephone signals, telex messages, television programmes and data transmissions between fixed Earth stations.

Telephony has constituted the bulk of the traffic until recently, but television distribution form central distributing centres to large numbers of local receivers has been growing rapidly, and business services including data transmission, facsimile services and video conferencing are expected to grow rapidly in the future.

A large number of communication satellite systems are now in operation and additional systems are being planned and built. Two global systems, Intelsat and Intersputnik, provide world-wide service and are open to any country which wishes to join. Intelsat has 108 member States and owns and operates 18 satellites over the Atlantic, Pacific and Indian Oceans providing international voice, television and data communications. In addition to its international services, Intelsat leases spare capacity to 24 countries--most of them developing countries--for domestic communications. Intersputnik has 14 member States and leases capacity on USSR satellites over the Atlantic and Indian Oceans for telephone and television use.

On the regional level, the European Communications Satellite (ECS) system, developed by the ESA, is now being operated by Eutelsat for telephone and television communications to its 23 member States. Arabsat is planning to launch its first satellite in 1984 to provide services to States of the Arab League.

Domestic communication satellite systems are being operated by Canada (Anik), India (Insat), Indonesia (Palapa), Japan (CS), USSR (Ekran, Gorizont, Raduga, Molniya, Loutch) and the United States (Comstar, Galaxy, RCA Statcom, SBS, Westar). Brazil, France, the Federal Republic of Germany and Sweden have systems under construction; other countries are planning them. Indonesia leases use of its Palapa satellites to Malaysia, the Philippines and Thailand for domestic communications.

Such international and regional cooperation increases the efficient use of existing capacity, thereby reducing the need for additional satellites.

Other satellites: Geostationary satellites are also used operationally for maritime communications and broadcasting services. Satellite systems for aeronautical and land mobile communications are being studied. These differ from satellites in the fixed-satellite service primarily in that they have more powerful transmitters so that the signals can be received by smaller and less expensive Earth stations.

Inmarsat, currently with 40 member States, provides telephone services globally to ships and marine platforms and is studying the possibility of providing aeronautical and land mobile communication services. The USSR's Volna satellite system provides maritime and aeronautical mobile service.

Geostationary satellites can also be used to relay data from satellites in low Earth orbit to a central ground station thereby allowing real-time world-wide coverage by the orbiting satellites without a global network of receiving stations. The United States Tracking and Data Relay Satellite System (TDRSS), which has linked the Space Shuttle and Landsat 4 to a ground station, is an example of such a system. While these mobile satellite services will certainly grow over the next decade, the number of satellites involved will, for the foreseeable future, remain small compared to the number of satellites in the fixed-satellite service.

Direct broadcasting satellites for transmitting television programmes to small inexpensive home or community receivers have been used by some countries.

Meteorological satellites in geostationary orbit provide frequent observations of cloud cover, cloud and surface temperature and water vapour and allow determination of wind velocity at cloud level.

The orbit has also been used for space research satellites, including the International Ultraviolet Explorer (IUE) for astronomical research and the Geostationary Earth Observation Satellite GEOS-2, for studying the magnetosphere.

Although placing remote sensing satellites in the geostationary orbit has been proposed, there are no plans to do so currently. Such satellites could selectively observe areas of interest and provide virtually continuous coverage or rapidly changing phenomena such as floods. However, it is difficult to obtain high resolution images from the altitude of the geostationary orbit, and a number of satellites would be needed for global coverage.

Other technologies proposed for the geostationary orbit include: large multi-mission space platforms, solar power satellites, manned space stations and co-ordinated clusters of satellites. If and when implemented, these uses could either increase congestion in the orbit, as might be the case with large numbers of extensive solar power satellites, or reduce congestion, as might be the case with communication platforms using advanced technologies to maximize efficient use of available radio frequency bands.

Collisions: The number of objects in the geostationary orbit, including active satellites, dead satellites and associated fragments and debris, is increasing steadily and with it the probability of collisions. Any collision is likely to put an active satellite completely out of service.

The problem of collisions between active and dead satellites is somewhat greater than between active satellites. The orbits of inactive satellites are generally eccentric and inclined to the geostationary orbit so that they pass through the geostationary orbit periodically. When satellite tracking indicates that a dead object will pass dangerously near an active satellite, an evasive manoeuvre can be made by the active satellite to eliminate the risk. Large amounts of small untrackable debris in orbits intersecting the geostationary orbit--for example due to explosions of propulsion systems or through fragmenting collisions--will increase the risk of collision substantially.

The probability of collision can be reduced if inactive satellites are removed from the geostationary altitude, with the final thrust of station-keeping propulsion. An increase in altitude of about 200 kilometres is believed to be able to prevent a satellite from passing through the geostationary orbit. Some satellites have already been removed from the geostationary orbit at the end of their useful lives in this way.

Once satellites have lost their manoeuvring capability, they can be removed from the geostationary orbit only by expensive scavenging missions which are not possible with current technology. The danger of collisions, although very small at present, could in the future impose certain constraints on the number and sized of satellites in the geostationary orbit.

Radio interference: Spacing between satellites must take into account the possibility of radio interference between communication satellites.

In the foreseeable future, only communication satellites in the fixed-satellite service and broadcasting satellites are likely to be affected by congestion of the geostationary orbit and the available frequency bands.

In principle, minimum spacing between satellites will be the spacing at which the combined interference and noise into each communication system is just below the level that would seriously degrade the signal quality. If all satellites were of identical design and carried identical communications, the minimum spacing would be uniform and fairly easy to determine.

In practice, communication satellites and their associated Earth stations vary substantially in their characteristics, the technology is developing quite rapidly, and the number of systems and their geographical distribution is changing. Different systems will therefore require different spacing and the positions assigned must allow enough tolerance for changes in traffic patterns and the introduction of new systems.

Satellites can be assigned to the same nominal orbital position and the same frequency band in the signals are polarized differently or if the beams are limited to well separated areas, techniques referred to as "frequency re-use". Polarization of signals is used to approximately double the communication capacity of satellites in the fixed-satellite service within a given frequency band.

Satellites can also use directional antennas to focus signals on the desired service area while minimizing the power transmitted outside the service area. The communication capacity of a satellite can also be increased by techniques of signal processing.

The International Telecommunication Convention, which contains principles to govern international telecommunication, states that radio frequencies and the geostationary satellite orbit are limited natural resources and that they must be used efficiently and economically, so that countries or groups of countries may have equitable access to both, taking into account the special needs of the developing countries and the geographical situation of particular countries.

Recommendations made at ITU's World Administrative Radio Conference (WARC) in 1979 provide that existing frequency assignments should not provide any permanent priority to geostationary orbit positions nor be an obstacle to establishing satellite communication systems by other countries.

Currently, the position of each satellite and thereby the spacing between each pair of satellites is determined through bilateral co-ordination procedures as each new system is planned.