Use of Signal-To-Noise Ratio for Multipath Error Correction in GPS Differential Phase Measurements: Methodology and Experimental Results

Carrier phase multipath is currently the limiting er- ror source for high precision GPS applications such as attitude determination and short baseline survey- ing. Multipath reflections affect both the carrier phase measured by the phase lock loop (PLL) and the sig- nal amplitude (SNR - ratio of signal to noise power within the tracking loop bandwidth). In most GPS data processing schemes, the SNR information is dis- carded, or used only for measurement screening - i.e. too low an SNR implies an unreliable signal. This paper describes a technique which uses the SNR in- formation to correct multipath errors in differential phase observations. The potential of the technique to reduce multipath to almost the level of the receiver noise was demonstrated in simulations, the multipath error being reduced from 10.4 to 3.3 millimeters. The effectiveness on real data was demonstrated with con- trolled static experiments, where the correction tech- nique reduced multipath errors from from 7.3 to 5.6 millimeters. The remaining error was predominantly high frequency multipath produced by distant reflec- tors. The low frequency multipath from nearby reflec- tors was essentially eliminated.