Field Test Results of an Enhanced GPS Coverage Through Map Aiding in Urban and Canyon Environments

James LaMance and Alison Brown

Abstract:	The development of an effective motorist Mayday system for travelers in rural and urban parts of the United States and Canada has been hampered by the lack of an infrastructure to receive and process emergency requests and by the inability of most motorists to precisely define the location of their vehicle. The development of intelligent vehicle-highway systems and an expanded network of computer-switched radio systems will greatly improve the ability of highway agencies, both public and private, to respond to emergency requests. The missing element is still an inexpensive and reliable means of determining the location of vehicles in distress. The requirements for a viable motorist Mayday service are: low cost, reliable response, and automatic linking to a two- way radio service. GPS has been proposed as a potential method of providing location in support of a Mayday service. However, the expense of a conventional GPS receiver will prohibit widespread installation on vehicles. One market source has indicated that the total cost to the user must be less than $100 before the system would be widely used. Also, the time-to-first-fix of a GPS receiver is unacceptable for use in an emergency situation unless the receiver is continually operating. Finally, the inability of a receiver to compute a navigation fLu except when four satellites are in view makes this a poor system design for operation in mountainous or urban regions where there is poor satellite visibility. NAVSYS has developed a low cost GPS sensor, the TIDGETTM, which solves these problems for a GPS Mayday service. The TIDGET sensor does not track the GPS signals, but instead captures a brief “snapshot” of raw GPS sampled data. This “snapshot” is transmitted to the emergency dispatch facility where it is processed to compute the location of the motorist. The processing also includes aiding data from a map database to allow the vehicle’s location to be determined when fewer than four satellites are in view of the sensor. In large quantities, the TIDGET sensor cost is reduced to $50 a unit (not including the communications link). The TIDGET requires no initialization or warm up time, since the data processing functions are not performed at the vehicle. The sophisticated software used to process this message at the dispatch center allows a three-dimensional position solution to be derived in many cases when only two GPS satellites are in view. This paper includes a description of the TIDGET Mayday system and presents test results demonstrating the ability to locate a stranded motorist even in areas of decreased GPS coverage.
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:	1317 - 1322
Cite this article:	LaMance, James, Brown, Alison, "Field Test Results of an Enhanced GPS Coverage Through Map Aiding in Urban and Canyon Environments," 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. 1317-1322.
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