Impact of interference on a generic GPS receiver and assessment of mitigation techniques

Over the past few years, many projects using code division multiple access (CDMA) techniques have being investigated. These systems are known to have many advantages over FDMA and TDMA one being its robustness against interference. The GPS system is a well-known application of CDMA techniques. Nevertheless, it appears in many papers and reports that GPS receivers may be disturbed by several types of interference. Mitigation techniques to combat these interferences and to improve the pre-correlation signal-to-noise ratio are to be developed to enhance the GPS receiver robustness. This paper starts with an overview of potential pre-correlation mitigation solutions for the GPS receiver. A potential mitigation technique for RF interference rejection on the GPS C/A-code of a receiver for aeronautical applications is investigated. The principle of the implementation of the adaptive transversal filter solution is shown. In order to evaluate the anti-jamming capability for civil use, a GPS receiver simulator has been developed using the communication systems simulation software COSSAP. It appear that using a notch filter in DSP just after the analogue-to-digital converter (ADC) and in front of the digital loops (DPLL and DDLL) provides significant improvements. Attenuation of the continuous wave interference (CWI) from 30 to 55 dB have been measured. This paper is not restricted to the GPS system but also gives an overview of notch filtering which is well suited for CDMA as a potential mitigation technique.