Automatic Continental Land Mobile Position Fixing via Satellite

Abstract

in a recent series of experiments, the General Electric Corporate Research and Development Center demonstrated effective continental land mobile communications and automatic real—time vehicle position fixing. A station wagon was equipped with a specially designed antenna, a slightly modified commercial VHF transceiver, and a digital tone—code communications bandwidth ranging responder. The General Electric Radio— Optical Observatory near Schenectady, NY served as the major earth terminal; a commercial VHF base station with a satellite antenna deployed first in Washington, DC and later in Tucson, AZ, represented a local control 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. During the six week test schedule across the continental United States, real—time vehicle positions on the order of one quarter mile were achieved. Post experiment data processing yielded vehicle position fixing accuracy on the order of a few hundred feet, under most circumstances. Simultaneous ranging measurements to the local control (ground truth reference) base station provided a measure from which to estimate ionospheric propagation time delays between the vehicle and the satellite. The single test which showed position fixing inaccuracy on the order of one half mile occurred during an ionospheric disturbance; the disagreement in true and measured vehicle positions suggests an ionospheric gradient between the vehicle and the fixed station of approximately 2 × 1017 electrons/m2, over the separation distance of 350 miles. Using a simple model to predict the VHF ionospheric propagation time delay would have resulted in vehicle positions accurate to within one half mile. Future satellite ranging operations at frequencies above 800 MHz would not be affected by propagation delays in the ionosphere. Although the experiments were conducted with an automobile, the technology is most certainly applicable in the maritime and aeronautical arena. Indeed, it is only in the land mobile environment that accurate true vehicle positions can be determined for verifying position fixing accuracy. All measured vehicle positions were compared with true vehicle positions as plotted on U.S. Geological Survey topographical maps.