GPS Receiver Responses to Satellite Anomalies

John W. Lavrakas and Daniel Knezha

Abstract:	Over the past five years, Overlook Systems has been monitoring the performance of the Global Positioning System at both the classified and unclassified level. In the course of this monitoring we have observed numerous anomalies caused by short-term disruptions in the spacecraft’s broadcast navigation message. We have observed these anomalies on a variety of GPS receivers. This paper examines the effect of these anomalies on a number of receiver types, including single and dual frequency receivers, both high end (survey grade) and low end (vehicle location). The various ways in which receivers respond to the anomalies are of keen interest to safety of life of users, especially if anomalies occur during such critical activities, such as aircraft landing or missile tracking. The GPS space segment was designed with a number of safeguards to protect users from satellite anomalies. These include satellite health settings, L-band parameter settings, the use of non-standard or untrackable PRN codes, default navigation data, and parity. Our data and analysis indicates that various types of GPS receivers take advantage of all these safeguards in different ways and provide different levels of protection to the end user. In our research we have observed markedly different behavior between GPS receivers in responding to these anomalies. The Doppler values output by receivers and the receiver’s ability to acquire and maintain lock on a signal are all affected. We have observed that some of the receivers output spurious instantaneous values that exceed Standard Positioning Service (SPS) Signal Specification levels in range rate and range acceleration. This paper characterizes the nature, frequency, duration, and possible causes of the anomalies for each of the receiver types. Previous papers have discussed the occurrence of these outages, but have not looked at their effect on various receiver types. Although, this research is not exhaustive in its selection of GPS receivers to be reviewed, we believe its results will be of interest to those who seek to understand some of the effects GPS satellite anomalies have on various receiver types.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	621 - 626
Cite this article:	Lavrakas, John W., Knezha, Daniel, "GPS Receiver Responses to Satellite Anomalies," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 621-626.
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