A GPS Code Tracking Receiver Design for Multipath Mitigation Using Maximum Likelihood Estimation.
AIR FORCE INST OF TECH WRIGHT-PATTERSONAFB OH
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The NAVSTAR Global Positioning System GPS is currently used in many applications requiring precise positioning data. Improving the precise positioning information requires the removal of errors that perturb the received signals. The errors introduced by multiple propagation channels, termed multipath, are not easily removed. These channels are caused by reflective surfaces near the receiver. As such, multipath is uncorrelated between receivers and, thus, cannot be removed through differencing techniques. This thesis investigates a GPS code tracking loop design which uses maximum likelihood ML estimation to determine amplitude and phase information of the multipath signal which are used to adjust code tracking to account for multipath effects. Analysis of the operations that govern this design for the case of a single reflection shows that it has no steady state tracking error. Results of simulations indicate that the code tracking loop, in conjunction with the MLE, mitigate the effects of multipath and improves code tracking performance over the narrow correlator NCDLL for most scenarios analyzed. Overall results of simulations indicate that the implementation of the maximum likelihood estimator MLE in conjunction with the code tracking loop has the potential to enhance code tracking performance over that offered by the narrow correlator NCDLL in a GPS environment.
- Navigation and Guidance