Multipath Rejection Through Spatial Processing

The development of low cost, small size antenna arrays and digital antenna electronics has resulted in small, affordable GPS phased arrays and digital receivers that are suitable for commercial applications. NAVSYS Highgain Advanced GPS Receiver (HAGR) product includes the capability to spatially process signals from up to 16antenna elements. Previous papers have presented the HAGR’s capability to perform beamforming, in both static and mobile environments, providing over 10 dB of additional signal gain on each satellite tracked. This capability results in highly accurate code and carrier tracking providing superior performance for high accuracy applications, such as kinematic GPS. In this paper, a further enhancement to the HAGR digital signal processing is described. A spatial signal processing algorithm has been developed that allows multipath signals to be detected and an adaptive antenna array pattern is then used to minimize their received signal power while still applying gain in the direction of the direct GPS signals. Test data is included showing the benefit of this method in reducing the multipath errors on both the pseudo-range and carrier-phase observations. INTRODUCTION Multipath errors are caused by the receiver tracking a composite of the direct GPS signals and GPS signals reflected from nearby objects. The resulting phase error is a function of the phase offset between the direct and multipath signals and the relative signal strength. For a fixed installation, these errors appear as biases, changing only as the line-of-sight to the satellite changes due to the satellite motion. In a mobile application, the multipath errors will also change due to the motion of the antenna. GPS receivers commonly use broad antenna gain patterns so that all of the satellites above the horizon can be tracked. Unfortunately, this increases the susceptibility of GPS receivers to multipath reflections of the GPS signals from nearby objects. Multipath signals from below the receiver antenna can be removed using antennas with good low elevation signal rejection, such as a choke ring, but signals arriving from elevations above the antenna cannot be rejected without also eliminating the satellite signals that are needed to obtain high accuracy GPS solutions. If the GPS receiver cannot be located above all potential multipath sources, signals arriving from buildings or nearby vehicles can cause significant multipath errors that will not be rejected by the antenna pattern Signal processing techniques have also previously been developed for reducing the effect of multipath errors on the tracking loops. These techniques include methods that apply temporal filtering of the multipath errors, multipath correction from site calibration or correlation shape correction using multiple digital correlators. While many of these methods have proven successful in reducing the effect of multipath on the code tracking loops, temporal filtering and multi-correlator signal processing have no effect on the multipath carrier phase errors. The spatial processing design described in this paper minimizes the multipath effect on both the code and carrier tracking loops within the GPS receiver. Until recently, however, only military GPS receivers have taken advantage of spatial signal processing to improve the robustness of the GPS signal through the use of Controlled Reception Pattern Antennas (CRPAs). NAVSYS pioneered the first commercial receiver to include spatial signal processing, the High-gain Advanced GPS Receiver (HAGR). This receiver uses digital spatial processing to combine the signals from up to 16 antenna elements, which allows over 10 dB of gain to be applied to each GPS satellite tracked, when operating in a beamforming mode [,,]. The reprogrammable digital spatial processing approach adopted by the HAGR also enables adaptive beam and null-forming to be applied which can be used to reduce the effect of multipath errors. GPS MULTIPATH ERRORS Multipath errors are caused by the receiver tracking a composite of the direct GPS signals and reflected GPS signals from nearby objects such as the ground or nearby buildings. GPS RECEIVER DIRECT SV SIGNALS MULTIPATH SV SIGNALS ANTENNA MULTIPATH SV SIGNALS Figure 1 GPS Multipath Errors The resulting phase error is a function of the phase offset between the direct and multipath signals and the relative signal strength. For a fixed installation, these errors appear as biases, changing only as the line-of-sight to the satellite changes due to the satellite motion. ) ~ sin( ) ~ ( ) sin( ) ˆ ( ) sin( ) ˆ ( ) ( ˆ ) ( ) sin( ) ( ) sin( ) ( ) ( θ θ ω θ ω τ τ θ ω τ τ θ ω τ θ ω τ + + + = + − + + − = + + + + + = t A A t R A t AR t s t s t t C A t t AC t s