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Multipath Mitigation Performance of Planar GPS Adaptive Antenna Arrays for Precision Landing Ground Stations
September 2000
Jonathan H. Williams, The MITRE Corporation
Robert J. Davis, The MITRE Corporation
Eddie N. Rosario, The MITRE Corporation
ABSTRACT
GPS precision landing systems require significant multipath
mitigation performance from ground station antennas to achieve
the desired accuracy. Recently, multipath-limiting 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 total-power
gain slope at the horizon. The spatial filtering properties of the
antenna array are exploited to reduce 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 ground-station 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.
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