Multipath Mitigation Performance of Planar GPS Adaptive Antenna Arrays for Precision Landing Ground Stations

GPS precision landing systems require significant multipath mitigation performance from ground station antennas to achieve the desired accuracy. Recently, multipathlimiting antennas have been designed specifically to meet requirements of the Local Area Augmentation Systems (LAAS) Ground Facility (LGF) specification. The most promising designs incorporate a linear-vertical array of antennas weighted to form a fixed beam with a steep totalpower gain slope at the horizon. The spatial filtering properties of the antenna array are exploited to reduce the amplitude of ground-based multipath, which arrives from angles that are below the antenna horizon. Military GPS antenna arrays, however, are currently configured as planar arrays to mitigate hostile interference through adaptive spatial filtering. These systems have yet to address the issue of multipath mitigation. In this paper, the multipath mitigation performance of a planar GPS adaptive antenna array is presented. Several techniques for improving the performance are implemented on the array and compared quantitatively using figures of merit proposed for groundstation environments. An implementation of fixed pattern shaping is demonstrated by mounting the antenna array on a prototype resistivity tapered groundplane. Multipath mitigation performance is compared to the performance of the same antenna array mounted on a highly conductive groundplane. Adaptive pattern shaping and fixed pattern filtering are simulated from the antenna array patterns measured on both types of groundplanes. The figures of merit are calculated and presented for each technique and for the combined techniques. Recommendations are given for areas of further improvements in multipath mitigation performance given the limitations of the techniques found in this study.