Analysis of Early Late Phase for Multipath Mitigation

A lot of research has been done to reduce the adverse effects of multipath in order to provide accurate positioning in navigation systems; however it still remains one of the challenges. Most of research in the area of multipath mitigation has been focused on magnitude of correlator outputs (I 2 + Q 2 ), which does not take into account the phase delay between the line of sight (LOS) and reflected signals. Early late phase (ELP) proposed for the multipath detection and estimation exploits this phase difference. ELP is calculated as a phase difference between early and late correlator outputs. This phase difference is dependent on the attenuation of the reflected signal with respect to the LOS and time delay between the two. However, it has been shown in this paper that this phase difference is also dependant on various other controllable and uncontrollable parameters. The effect of these parameters needs to be analyzed in order to minimize false or missed detection of multipath. This provided motivation for the work presented in this paper. As ELP is computed using correlator outputs, these are first analyzed followed by ELP analysis. The effect of thermal noise, Doppler shift in carrier frequency, cross correlation, correlator spacing and ELP averaging period has been analyzed in this paper. The paper also presents mathematical models for theoretical evaluation of ELP in the absence and presence of multipath. Two kinds of mathematical models have been presented, one which is extensive in evaluation but provides exact estimation, and another which is easier to compute but provides a rougher estimate. A comparison of both models and experimental results has also been presented.