For the ground segment, stations will be located in secure sites and provided with independent emergency power. Within each computer complex, redundant systems will be provided in all components including data bases. For each ground station, a second system, fully redundant, should be built several thousand kilometers from the first. In the event of a major disaster, such as a tornado that destroys the antennas communicating with the satellite relays, the alternate site could take over the system load immediately. SYSTEM CAPACITY CONSIDERABLE The traffic capacity of Geostar-like radio determination systems can grow without limit, as long as successive generations of satellite relays are equipped with larger antennas which cover a given geographical area with larger numbers of spot beams. This can be seen most easily by considering a single spot beam covering a given geographical area. When a second generation satellite relay of twice the antenna diameter is emplaced, covering the same area with four spot beams (Fig. 5), the ground station can send the messages to a particular user through just one of the four beams, because the ground station “knows” the approximate location of the user. The ground station can therefore transmit to four users simultaneously, one in each of the four spot beams, quadrupling the outbound capacity compared to a single beam. For the inbound direction, the four spot beams allow four users to transmit data simultaneously. By the same reasoning, the total power required of the satellite does not increase as successive generations of satellites are emplaced. A certain signal strength must reach the user from a satellite to support the flow of outbound data. That signal strength is a power divided by a total geographical area. The power required for each of four beams feeding the same total geographical area is therefore just one-fourth the power required for a single beam, and the total power required of the satellite stays the same, although the total capacity has increased fourfold. In a practical system, these numbers are altered somwhat by beam overlap and by the concentration of users near cities. Bearing in mind that in the long term Geostar-like systems do not become saturated, it is useful to compare the maximum imaginable system loading from aviation with the capacity of one beam of a Geostar-type RDSS. Taking the example of the United States, there are some 2,600 airliners, 11,000 nonairline turbine aircraft, and
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