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GPS/LORAN in an urban environment-oscillator stability considerations

During the summer of 1994, the Advanced Research Projects Agency and the United States Coast Guard Academy collected extensive radionavigation data in the New York City area. The purpose was to determine the feasibility of using existing radionavigation systems for tracking applications in an urban environment. The data includes the accuracy and availability of the Global Positioning System (GPS) and LORAN, a comparison of electric field (whip) and magnetic field (loop) antennas at both LORAN and Differential GPS frequencies, and the increased availability of both GPS and LORAN by adding a precise clock input to the receivers. The data was collected among the narrow streets and tall buildings in the Wall Street area, among the tall buildings but wider streets of Third Avenue, in the relatively more open streets and smaller buildings of the Bronx, in the vicinity of the large metallic structure of the George Washington Bridge, and under the cover of foliage along the New Jersey side of the Hudson River. Of particular interest in this paper is the study of how the availability of radionavigation fixes can be substantially enhanced in these urban areas by the integration of a precise clock with LORAN and GPS navigation information. More specifically, a clock allows a geographic fix to be obtained from as few as two Time of Arrival (TOA) measurements. In the case of LORAN in NYC, Seneca and Nantucket are 10-12 dB stronger than Carolina Beach, hence a precise clock provides a significantly higher fix availability since only two stations (plus the clock) are used in obtaining the two TOA fix. Similar improvements in fix availability are achieved in the integrated LORAN/GPS and the GPS modes by using a precise clock. Preliminary results of implementing a two satellite fix using an external Cesium reference and a GPS Builder Kit manufactured by GEC Plessey are presented. A secondary advantage of using a precise clock occurs in receiver reacquisition after signal loss in the urban environment. Quite simply, in the case of GPS, one can tolerate longer outages without having the GPS receiver reenter the "search" mode, since a precise oscillator can stay within one chip (one usec) for a longer period of time (like durations corresponding to brief signal outages due to shielding). Should the oscillator drift beyond one chip, acquisition is still quicker since one need only search over a handful of Doppler bins when using a precise clock. By having two GPS Builder Kits side by side, each looking at the same signals, with one referenced to a Cesium and one referenced to its own clock, we examine acquisition and reacquisition improvements in a very controlled way for performance comparisons.