Land mobile communications and position fixing using satellites

In a recent series of experiments, the General Electric Corporate Research and Development Center demonstrated effective satellite-aided land mobile communications, analog and digital data relay, and automatic real-time vehicle position fixing. In one sequence of tests a station wagon was equipped with a specially designed antenna, a slightly modified commercial VHF transceiver, and a digital tone-code ranging responder that operated within the communications bandwidth. The General Electric Radio-Optical Observatory near Schenectady, NY, was the major earth terminal. A commercial VHF base station with a satellite antenna deployed first in an office building in Washington, DC, and later in Tucson, AZ, represented a headquarters or a sector office ground station. Communications were relayed by NASA's ATS-3 geosynchronous satellite. Both ATS-1 and ATS-3 were used for position fixing the vehicle. Voice, slow scan television, audio test tones, prerecorded intrusion sensor data, and telephone patches were relayed by the satellite to and from the vehicle under a variety of conditions in greater Washington, DC, and in the southwestern United States. The experiment demonstrated continent-wide communication of a quality comparable to fringe area reception of present local VHF mobile communications but with a notable lack of multipath flutter. Due to the high angle of signalling path from the vehicle to the satellite, solid structures such as buildings, mountains, bridges, or tunnels degraded communications only occasionally. An absolutely clear line-of-sight signalling path was not required. Trees directly in the signal path seldom interrupted communications. Noisy radio environments such as power lines and vehicle ignitions degraded signals received in the vehicle but did not affect vehicle transmissions. Vehicle positions to within one quarter mile were achieved in real time and within several hundred feet after post experiment recalibration and analysis. In another sequence of tests, similar equipments plus biomedical sensors and a medical telemetry unit were installed in an ambulance. NASA's ATS-3 satellite relayed two-way voice communications between a hospital and the ambulance and electrocardiograms from the ambulance to the hospital. Signals were received with excellent quality from various points within the United States, all well beyond the range of conventional line-of-sight communications. The future of operational systems depends not only on technology but on the need to define user requirements, international frequency allocations, and a commitment to support the initial hardware investment. One key technology that would have to be developed is a multibeam spaceborne antenna with low sidelobes. It has yet to be shown that a large space structure will be low enough in cost to attract the large number of subscribers needed to make it an attractive business venture. It appears likely, but not certain, that the cost of a satellite system to serve large, thinly populated areas may be less than a network of terrestrial repeaters that serve the same area. The likelihood is sufficient to justify further studies.