field, guiding a vehicle to a precise street address, and guiding an aircraft to a precision instrument landing. In good conditions, positions were repeatable to approximately one metre. A videotape of the Sierra Test System providing a precision instrument approach has been shown at a number of navigation conferences. To test the transmissions at elevation angles similar to those of a real satellite system, the Sierra System was converted in 1984 to become the Airborne Test System. With that version, two light aircraft carried satellite emulators to altitudes of about 3,000 m. A ground station with augmented computing power calculates the changing positions of the emulators by measuring the replies from benchmark transceivers. The new system allowed more realistic testing, although with precision degraded from that of the satellite system by the motion of the emulators in turbulent air (Fig. 3). The Comsat General Corporation is under contract to Geostar for satellite design specification and construction supervision. Another company, Comsat Technology Products, built the prototype U.S. ground station, and is also under contract for overall system review and the functional specification of transceivers. Studies indicate that the first RDSS market will be land transportation, especially truck fleet control. The trucking industry and agencies of the U.S. Federal Government supported Geostar in its requests to the Commission. Outdoor recreational, professional and public safety applications include the small boat industry, fishing, skiing and other sports, forestry, and radio location and two-way communications for cars, taxis, police vehicles and rescue agencies. Aircraft, on a relative basis, do not constitute a large market at this time.
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