GPS Receiver Structures for the Urban Canyon

Benjamin Peterson, Richard Hartnett, and Geffrey Ottman

Abstract:	Conventional GPS receivers typically have limited capabilities for continuous tracking of satellite signals in deep urban canyons. In such environments we find that receivers obtain brief glimpses of satellites, but rarely see three or more satellites simultaneously, resulting in poor fix availability. This would suggest that a GPS receiver that can obtain useful pseudorange information from brief glimpses of satellites, or one which can obtain fixes from as few as two satellites, may enhance fix availability in urban canyons. Here we present interim results of an ongoing effort to address these issues. More specifically, we present several software and hardware modifications to GPS BuilderTM, a 12 channel GPS development system from GEC Plessy Semiconductors. The hardware has been modified to use either a Cesium or ovenized quartz oscillator as an external frequency reference, thereby allowing a position solution from as few as two satellites. As the number of tracked satellites decreases from three to two, the clock is assumed to drift at its last known frequency, a pseudolite is placed at the earths center, and the fix is calculated. Results on effects of clock accuracy on initial acquisition times and the ability of a receiver to coast through periods when a satellite is in the shadow of a structure are presented. We also present results on obtaining fixes based on as little as one millisecond of data. The cross correlation of “Doppler shifted PRN templates” and one millisecond of digitized RF are calculated using Fast Fourier Transform (FFT) techniques. The peaks in absolute values of these cross correlation functions are then further processed to obtain a Time of Arrival (TOA) resolution of 10 ns. These TOA’s (modulo 1 millisecond) have typical one sigma code phase noise of 30-50 ns. Finally, we present an interesting method whereby these TOA’s can be processed to obtain a fix, even if time is only known to several minutes (and hence satellite positions are not known).
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	1323 - 1332
Cite this article:	Peterson, Benjamin, Hartnett, Richard, Ottman, Geffrey, "GPS Receiver Structures for the Urban Canyon," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 1323-1332.
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