GPS Multipath Mitigation Using a Three Dimensional Phased Array

Digital beam steering with a phased array has previously been demonstrated to improve the Signal/Multipath (S/M) level for precision applications by providing gain in the direction of the GPS satellites. A disadvantage of using horizontal planar antenna arrays is that the beam pattern is symmetric in the Up/Down directions. This means that only the phased array ground plane affects the antenna array Up/Down ratio, which defines the relative gain between the satellite and a horizontally reflected multipath signal. By including both horizontal and vertical antenna element separation in an array design, multipath can be rejected, improving both the Up/Down gain ratio (for horizontal multipath sources) and also rejecting multipath from other directions. In this paper the relative advantages of some alternative 3-D antenna array designs are shown, compared with conventional planar or vertical arrays. Simulation results are shown of the alternative antenna array properties and a 3-D antenna array that has been built and tested using our in-house digital beam-steering GPS receiver is presented. INTRODUCTION The development of advanced digital beam-forming GPS receivers now allows optimized spatial processing algorithms to be applied for GPS anti-jam protection. This same technology can also be extended to improve multipath rejection for high performance applications, such as for the Shipboard Relative GPS Joint Precision Approach and Landing system (SRGPS JPALS). Figure 1 CRPA and High-gain Advanced GSP Receiver (HAGR) Gain Stage on the Carrier Flight Deck Under contract to NAVAIR, NAVSYS has developed an advanced digital beam-forming receiver, the HAGR-A, that uses a software GPS receiver architecture to apply adaptive digital spatial processing for anti-jam and multipath mitigation. This approach allows for high antijam protection and also allows for embedding integrity monitoring features within the GPS receiver to detect RFI, multipath, and antenna electronics failures. The HAGR-A has previously been tested using a conventional Controlled Reception Pattern Antenna (CRPA) installed on an aircraft carrier (Figure 1). This test showed that the carrier environment experienced significantly higher multipath than a fixed installation. The test results also showed that beam steering removed significant portions of the multipath error at all elevations.