234,000 piston-engine aircraft registered. A total of 49.5 million hours are flown per year. At peak, there are about 12,400 aeroplanes in the air nationwide, of which 1,800 are airliners, 1,200 are nonairline turbine aircraft, and 9,400 are piston-engine powered. Data on the number of landings indicates a typical flight time of 90 minutes. With control of the Geostar update rate from the ground computer (overridden at any time if the pilot wants more information), a reasonable Geostar usage per flight is shown in Table 2. The update rate on position fixes is increased from its cruise value during climb and descent, and further increases below 600 meters altitude. Below 150 m, the fix rate increases to one fix per 0.5 sec. (That rate is considerably faster than the update rate provided by the conventional military GCA precision landing system.) Whenever the ground station detects a potential collision threat, the computer programme also automatically increases the fix rate. The computer programme at the ground station, of course, also give the pilot collision warnings. To give each pilot a situation display, the Geostar transceiver in the cockpit can read the downlink data channel and so display to the pilot, if the aeroplane is equipped with an appropriate display unit, the positions and velocities of all Geostar-equipped aircraft in his vicinity. An upper limit of system load is taken by the extreme assumption that every aircraft is Geostar equipped, and that every flight ends with a Geostar instrument approach and landing. (We have not proposed that the system be mandatory.) From Table 2, for the typical flight profile, the time average per aircraft is 520 fixes/90 min, or about 1 fix every 10 sec. For all aircraft combined, the peak total is about 1,200 fixes/sec. The system could handle that number even if all the 12,400 aircraft in the air simultaneously (peak instantaneous air count) over the contiguous United States were to be in the same spot beam. Within the dimensions of the antenna of the ATS-6* satellite, launched successfully in 1974, the number of spot beams in a Geostar satellite is such that the maximum aviation demand, as estimated above, would be only a few percent of system capacity. *Applications Technology Satellite of the U.S. National Aeronautics and Space Administration.
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