GPS Multipath Mitigation by Means of Correlator Reference Waveform Design

Although it is now known how to achieve code pseudoranging accuracy at or near theoretical limits for single-path propagation, it is recognized that multipath remains as a dominant source of error in differential positioning. In recent years several approaches to multipath mitigation have evolved which focus on properties of the cross-correlation function of the received signal with the receiver reference code. Examples of these are the narrow correlator, Multipath Elimination Technology (MET), Multipath Estimating Delay Lock Loop (MEDLL), and very recently, the Strobe Correlator. This paper describes a technique for generating a signal correlation function which has a much narrower width than a standard cross-correlation function, and is therefore capable of mitigating multipath errors over a much wider range of secondary path delays. The new approach reduces systematic range error (i.e., bias) in the presence of multipath by increasing the range resolution of the correlation process at the expense of a moderate decrease in the effective signal-to-noise ratio (SNR). However, in many applications (for example, surveying) there is ample time available to make up for the SNR loss by correlating over a longer period. The narrowing of the correlation function is accomplished by using a specially designed correlator code reference waveform in lieu of the ideal code waveform used in almost all existing receivers. Also treated in this paper is an alternate method of producing a narrowed correlation function which is mathematically equivalent to the use of a special correlator reference waveform. In this case the multipath mitigation is obtained through the use of a pre-correlation signal shaping filter followed by standard correlation. Advantages of this equivalent approach are that the pre-correlation signal shaping filter is common to all satellite channels, and that the standard correlator reference waveform is more easily generated by digital means. Although the approaches described in this paper do not reach theoretical accuracy limits for multipath- induced error previously established by the author, they appear to offer the potential of significant performance improvements over previously published methods, and can be simply implemented at low cost.