GPS Antenna Group Delay Variation Induced Errors in a GNSS Based Precision Approach and Landing Systems

Tim Murphy, Preston Geren, and Tom Pankaskie

Abstract: RTCA recently updated the Minimum Operational Performance Specification (MOPS) for GNSS antennas intended for use in aviation applications. During the development of the specification it was discovered that GPS antennas commonly used by commercial air transport airplanes exhibited unexpectedly large variations in group delay when referenced to the group delay performance at the antenna boresight. The group delay variation in some cases was several nanoseconds. This raised a concern that perhaps a significant error source had not been accounted for in the development of differential GPS based precision approach and landing systems. Consequently, an investigation has been conducted to quantify the effect of the group delay variations on system performance. Pattern data were obtained in a Boeing compact range test on two off-the-shelf GPS antennas. The frequency range was + 16 MHz, centered at L1. The resulting transfer function data were used to calculate pseudorange error for wide and narrow correlators, for C/A code. The results were then processed to provide an elevation plot of bias and standard deviation. The standard deviations represent the variation of error with observation angle, which is believed to be due primarily to the 90-degree hybrid and the non-ideal axial ratio of the antenna. The measured pattern data and resulting pseudorange error elevation plots are compared with calculations from an electromagnetic model of the antenna, finite ground plane, and 90-degree hybrids. The comparison provides insight into the mechanisms that produce the error inherent to the antenna and circuitry. Simulation and analysis concerning the impact of errors induced on differential GPS accuracy are included. The implications of these results for GBAS, SBAS and the RTCA Antenna MOPS are clarified in the conclusions.
Published in: Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007)
September 25 - 28, 2007
Fort Worth Convention Center
Fort Worth, TX
Pages: 2974 - 2989
Cite this article: Murphy, Tim, Geren, Preston, Pankaskie, Tom, "GPS Antenna Group Delay Variation Induced Errors in a GNSS Based Precision Approach and Landing Systems," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2974-2989.
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