Part 4 of Section 1 (SATELLITE COMMUNICATIONS - A SHORT COURSE) of SATELLITE COMMUNICATIONS, prepared by Dr. Regis Leonard for NASA Lewis Research Center
The nature of future satellite communications
systems will depend on the demands of the marketplace (direct home distribution of
entertainment, data transfers between businesses, telephone traffic, cellular telephone
traffic, etc.); the costs of manufacturing, launching, and operating various satellite
configurations; and the costs and capabilities of competing systems - especially fiber
optic cables, which can carry a huge number of telephone conversations or television
channels. In any case, however, several approaches are now being tested or discussed by
satellite system designers.
One approach, which is being tested experimentally, is the "switchboard in the
sky" concept. NASA's Advanced Communications Technology Satellite (ACTS) consists of
a relatively large geosynchronous satellite with many uplink beams and many downlink
beams, each of which covers a rather small spot (several hundred miles across) on the
earth. However, many of the beams are "steerable". That is to say, the beams can
be moved to a different spot on the earth in a matter of milliseconds, so that one beam
provides uplink or downlink service to a number of locations. Moving the beams in a
regular scheduled manner allows the satellite to gather uplink traffic from a number of
locations, store it on board, and then transmit it back to earth when a downlink beam
comes to rest on the intended destination. The speed at which the traffic is routed and
the agility with which the beams move make the momentary storage and routing virtually
invisible to the user. The ACTS satellite is also unique in that it operates at
frequencies of 30 GHz on the uplink and 20 GHz on the downlink. It is one of the first
systems to demonstrate and test such high frequencies for satellite communications.
The ACTS concept involves a single, rather complicated, and expensive geosynchronous
satellite. An alternative approach is to deploy a "constellation" of low earth
orbiting satellites. By planning the orbits carefully, some number (perhaps as few as 20,
perhaps as many as 250) of satellites could provide continuous contact with the entire
earth, including the poles. By providing relay links between satellites, it would be
possible to provide communications between any two points on earth, even though the user
might only be able to see any one satellite for a few minutes every hour. Obviously, the
success of such a system depends critically on the cost of manufacturing and launching the
satellites. It will be necessary to mass produce communications satellites, so that they
can turned out quickly and cheaply, the way VCRs are manufactured now. This seems a truly
ambitious goal since until now the average communications satellite might require 6 months
to 2 years to manufacture. Nevertheless, at the present time, several companies including
Hughes Electronics, Motorola, and Teledesic, Inc., have indicated their intent to
undertake such a system.
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