Methods Of GNSS Short Multipath Error Suppression

Multipath is the dominant source of the error in GNSS receivers. The multipath canaffect both code tracking loop and carrier tracking loop in GNSS receivers. Generally, thecode tracking error due to multipath is more significant than carrier tracking error, usuallysmaller than1cm. Moreover, the current multipath mitigation methods are inefficient tocope with short multipath (smaller than0.5chip). Therefore, this paper focus on thereduction of short multipath error using modified narrow correlator and multipathparameter estimation method based on maximum likelihood principle. The contributionof the thesis is listed as follows:(1) Analyze and compare the major multipath mitigation methods. These methodscan be categorized into two groups: modified delay lock loop (DLL) methods andmultipath parameter estimation methods. This paper derives the multipath errorenvelope of narrow correlator, ELS and Strobe methods respectively. In the parameterestimation methods, analyze and simulate the performance of fast iterative maximumlikelihood algorithm (FIMLA) based on multipath estimation delay lock loop (MEDLL) usingMATLAB.(2) Propose a new short multipath mitigation method based on modified narrowcorrelator through adding an additional correlator in the traditional narrow correlator toestimate and then eliminate the short-delay multipath error in real-time. The additionalcorrelator doesn’t involve the code tracking process, so the error estimation process willnot affect the code tracking and the hardware doesn’t need to modified, which is veryeasy to implement.(3) Propose a multipath mitigation method based on maximum likelihood principle.This method can avoid local convergence encountered in most iterative methods usingoffline grid search and online iteration, which can greatly improve the estimation accuracy.Also, this method can reduce the online computation of grid search to meet therequirement of real-time positioning. Moreover, considering the piece-wise linearity ofauto-correlation function (ACF), derive a new iteration method which greatly simplifiesthe iteration procedure and enhances the convergence speed.