Work is under way to set up a satellite-based mobile communications system that would allow a person to reach virtually anywhere in the world--even a remote tropical island--simply by using a cellular telephone. Several competing companies are currently testing plans and seeking necessary approval to begin launching satellites. Plans vary--and three are outlined at right--but the basic principle behind most of them is outlined below. Problems with such a global system include the cost and how to regulate it since calls will cross many international borders.

1. Call originates from within a cell. Entire globe is divided into a network of cells, or calling areas.

2. When a call is placed in the system, it seeks the most efficient available route. Sometimes calls may go to a switching station, or gateway, on the ground and then through a satellite dish to be uplinked into space.

3. Among a network of orbiting satellites, the one that is in position nearest to the caller (the one "responsible" for that cell area), picks up the transmission.

4. Since the call is overseas, it is passed on through the network of satellites until it reaches the one nearest to its destination and then is retransmitted earthward to the proper cell and to the receiver.

Satellite Systems

Each system would have a number of satellites employed in different orbits and at varying altitudes so that every point on Earth is in contact with the system.

Sun-synchronous, low-altitude orbits:

Projects: Iridium, Teledesic

Satellites: 66 to 840

Orbits: 6 to 21

Altitude: Approx. 400 miles

Cost: $3 billion to $9 billion


Medium-altitude orbits:

Projects: Odyssey

Satellites: 12

Orbits: 3

Altitude: Approx. 5,000 miles

Cost: $1.3 billion


Geo-synchronous (geo-static), high-altitude orbit:

Altitude: Approx, 22,000 miles

Projects: Current telephone and television transmission

Sources: AT&T; Teledesic; TRW; Iridium

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