Software-based Multipath Mitigation: Sampling for Multipath Invariance

Multipath potentially introduces a significant bias error into GPS code phase positioning solutions. That error is dependent on the amplitude, delay, phase, and phase rate of the multipath with respect to the line-of-sight. Many current mitigation techniques attempt to eliminate or reduce the impact of this error source. These techniques vary as widely in hardware and software complexity as they do in overall mitigation performance. All, however, perform very differently for different multipath conditions. They exhibit a characteristic dependence on the multipath parameters. For example, most will easily mitigate long-delay multipath, however, few or none perform as well for multipath at short relative delays. In this paper, a new software-based mitigation approach is introduced. It is asserted that since the C/A codes are not orthogonal, for each satellite autocorrelation function, a different location which is off the main lobe will remain virtually unaffected by multipath. A method is given for sampling at and around the autocorrelation function peak and/or sidelobes with fidelity, such that the tracking errors may become essentially multipath invariant-independent of the multipath parameters. Theoretical performance plots are shown for validation. It is asserted that since the remaining error bias is essentially constant, it may subsequently be modeled, calibrated, and/or estimated away. An algorithm for finding this point is provided. It was implemented in software on a Mitel Semiconductor receiver. Preliminary analyses and results are given from data taken in real-time with both a GPS signal simulator and actual satellite data. The experimental data supports the claim that there are multipath invariant points corresponding to each PRN code and that with some iteration and smoothing, they can be found. It is believed that with more refinement and proper implementation, this approach may provide superior mitigation performance to many techniques currently in use.