Evaluation of GPS Performance in Mobiflex Alarm, a Vehicle Location System

Roger Bengtsson

Abstract:	The leading Swedish telecom- munications operator, Telia, plans during 1994/95 to introduce an automatic vehicle location system for emergency matters, named Mobiflex alarm, using GPS as the position determining device. Simulations, in order to evaluate the ef- fects of signal blocking from buildings and measurements to determine damping from typical vegetation, have been carried out. The results are guidelines for the user where to place the receiver to achieve the best GPS operation. Trimble Navigation Quick Plan software has been used to simulate the effects of ob- structions. Two Magnavox MX4200D have been used to determine the attenuation in several typical rural environments. Conclusions from the measurements and simulations imply that GPS offers very high probability for the receiver to extract a position even under trying conditions - es- pecially if the user knows where to avoid to place the receiver. Good performance are met in different types of vegetation where the satellite signals just undergo damping (and no blocking). No specific type of vegetation was found in southern Sweden where the damping was high enough to obstruct the signals below the receivers sensitivity. However, in metropolitan areas GPS ex- hibits a more random performance. Even if the receiver tracks enough satellites, no po- sition is often given due too high DOP numbers, caused by bad satellite geometry. The direction of an obstacle affects per- formance. For instance, when half the sky is obstructed in Sweden, the best receiving condition is achieved when half the sky to the North-East or half the sky to the North- West is obstructed. The reason why two an- gled half-sky obstacles have better perfor- mance is explained by the 55” inclination of the satellite orbits relative the equator. Then the visible satellites move along the obstacle and are hence in sight for long, continuous periods. The poorer obstacle di- rections exhibit, quite astonishingly, a slightly higher mean value of number visi- ble satellites but the higher variance makes them though less reliable. Maximum and minimum timelimits before it is possible for a receiver to deliver a valid position, with different accuracy’s, at half-sky obstructions are also presented. About 60” is the limit for a half-plane ob- stacle to obtain continuous 2-D position in theory (enough satellites). There is a high correlation between signals received at dif- ferent places within Sweden due to the high satellite orbits. Under line-of-sight condi- tion the satellite signals are very short term stable.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	251 - 258
Cite this article:	Bengtsson, Roger, "Evaluation of GPS Performance in Mobiflex Alarm, a Vehicle Location System," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 251-258.
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