Modulated Signal Interference in GPS Acquisition

S. Deshpande

Abstract:	The Global Positioning System (GPS) is poised to play a critical role offering commercial opportunities in wireless communications as a result of the Federal Communications Commission's E-911 directive and the expansion of location-based mobile-commerce services (LBS). Successful E-911/LBS products and services will require solutions with features that can implement GPS in mobile phones with low cost, low power consumption, reasonable accuracy, high sensitivity and jamming immunity. Jamming immunity is a measure of the receiver's ability to provide GPS performance despite the presence of interfering signals anywhere else in the frequency spectrum. The ability of a GPS receiver to resist unwanted frequencies is a key measure of its performance. Applications involving cellular handsets provide a guaranteed source of potential jammers namely the cellular frequencies themselves. The aim of this paper is to analyze the effect of modulated signals such as amplitude modulated (AM) and frequency modulated (FM) signal sources on the GPS spectrum during the acquisition process. Interference signals cause distortion in the GPS signal resulting in an incorrect or no correlation peak during acquisition. A GPS simulator (GSS 6560) was used along with a signal generator (E 4431B) and an interference combiner (GSS 4766) to generate the interference signals. The signals were collected using a GPS hardware front end data logger (Signal Tap). A software GPS receiver was developed and used to analyze AM/FM interference effects. The adaptive predetection integration was used to reduce interference effects. Results show that adaptive predetection integration of up to 100 ms is sufficient to mitigate 20 dB relative AM interference power and 30 dB relative FM interference power.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	76 - 86
Cite this article:	Deshpande, S., "Modulated Signal Interference in GPS Acquisition," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 76-86.
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