Pseudorange Multipath Mitigation By Means of Multipath Monitoring and De-Weighting

The management of pseudorange multipath in GPS software processing ranges from total ignorance of the phenomenon to complex schemes for the estimation of the multipath signal. The former can cause significant parameter estimation degradation and the latter cannot necessarily provide accurate estimates. Therefore an alternative method is proposed here, which includes endeavouring to monitor the multipath signal and deweight the affected observations based on a sentinel observable. The objectives of this approach are the removal of pseudorange multipath-induced position outliers and the reduction of positioning noise. A modified version of the pseudorange minus carrierphase observable has been identified in the literature as a possible monitoring observable, and this linear combination is used in our technique. A straightforward, analytical de-weighting function based on this observable is introduced. The relationships between the observable and other weighting criteria such as carrier-to-noise density ratio and satellite elevation angle are also discussed. Initial static pseudorange testing results indicate that the technique shows great promise, providing 30% to 50% improvement in position estimates in some cases. Similar improvements are shown with carrier-phase smoothed pseudoranges. INTRODUCTION The multipath problem has received attention from researchers since the early days of GPS positioning technology. In recent years this interest has peaked due to the great strides that have been made in reducing other components of the GPS error budget. The research performed and reported in this paper was initiated by the need to reduce the effect of this phenomenon on static and kinematic pseudorange-based positioning. Multipath occurs when signals travelling from a transmitter to a receiver propagate via multiple paths due to reflection and diffraction. The underlying theory of GPS multipath is described in many GPS texts, e.g., Braasch [1996], Kaplan [1996], Langley [1998], etc., and will therefore not be described here. However, multipath compensation strategies will be reviewed, and conceptual explanations will be given where required. Multipath Mitigation Philosophies There are four classes of multipath alleviation techniques: judicious antenna siting, hardware solutions, software solutions, and hybrid solutions. All four have advantages, flaws and limitations. The selection of low-multipath locations for antenna placement is a simple and effective method for reducing