Analysis of Multipath Mitigation Techniques in GNSS for Pseudorange Error Reduction

In satellite based navigation system the location accuracy depends on how accurately the pseudorange is measured. The pseudorange, the distance from satellite to receiver and is measured by the time taken for the signal to travel from satellite to receiver multiplied by speed of light. There are several factors affecting the measured delay, like ionospheric and tropospheric delays, clock errors, ephemeris error and multipath error, etc. These errors (in terms of delays) are directly reflected in pseudorange error. Among these errors multipath is the type of error which depends on the receiver antenna position. The received signal consists of Line Of Sight (LOS) signal along with reflected signals. The accurate measure of the delay corresponding to LOS is crucial, when the received signal is corrupted with multipath reflected signals. To reduce this error in pseudorange measurements there are several methods in use, starting from simple antenna based methods to sophisticated adaptive filtering based LOS signal estimation methods. The conventional methods follow Early Minus Late delay lock loop approach to measure the code phase by detecting correlation peak of the received signal with locally generated code sequence. Correlator spacing between early and late is limited by the front end bandwidth of the receiver. The advanced multipath mitigation methods use adaptive filtering techniques to estimate the fundamental LOS signal parameters viz delay, amplitude and phase. The approximation of LOS correlation function from the received signal is based on some measurement metrics of the signal. These LOS correlation function estimation based methods are capable of mitigating multipath error in closely spaced multipath scenarios. In this paper the signal model of the received signal, correlation function representation when the three reflected paths present along with the LOS signal and the study of different multipath mitigation methods are discussed.