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

Modern mobile devices are now equipped with GPS-receivers to provideE911 and other location based services. Working indoors, the GPS receiveris likely to experience a situation where some of the received signals arestronger than others. These strong signals can prevent the acquisition of adesired weak signal or lead to false acquisition, due to high cross correlationbetween the strong and weak signals. The C/A Gold code used in GPS L1signal has an inherent correlation protection of about 24 dB. This becomesinadequate for weak signal acquisition in the presence of multiple strongsignals. Various cross correlation mitigation techniques are used to deal withthis problem. The impact of cross correlation on weak signal acquisitionlargely depends on the relative power level and carrier offset of the desiredweak signal with reference to the strong interfering signals. This paper firstpresents an analysis of the L1-C/A cross correlation in the presence ofmultiple strong signals under different conditions of relative Doppler offsetand dwell time. Acquisition of weak signals with different SNR levels isthen performed under these conditions, using the ¿Tong¿ search algorithmand the results of acquisition are presented. The results indicate that, forcertain relative Doppler offsets, extended dwell times assist in combating thecross correlation noise hence improving the detection probability