An analysis of L1-C/A cross correlation and acquisition effort in weak signal environments

Abstract

Modern mobile devices are now equipped with GPS-receivers to provide E911 and other location based services. Working indoors, the GPS receiver is likely to experience a situation where some of the received signals are stronger than others. These strong signals can prevent the acquisition of a desired weak signal or lead to false acquisition, due to high cross correlation between the strong and weak signals. The C/A Gold code used in GPS L1 signal has an inherent correlation protection of about 24 dB. This becomes inadequate for weak signal acquisition in the presence of multiple strong signals. Various cross correlation mitigation techniques are used to deal with this problem. The impact of cross correlation on weak signal acquisition largely depends on the relative power level and carrier offset of the desired weak signal with reference to the strong interfering signals. This paper first presents an analysis of the L1-C/A cross correlation in the presence of multiple strong signals under different conditions of relative Doppler offset and dwell time. Acquisition of weak signals with different SNR levels is then performed under these conditions, using the ¿Tong¿ search algorithm and the results of acquisition are presented. The results indicate that, for certain relative Doppler offsets, extended dwell times assist in combating the cross correlation noise hence improving the detection probability.