Considerations for Characterizing Antenna Induced Range Errors

Karl Shallberg and Joe Grabowski

Abstract:	Recently, there has been considerable focus in both the Local and Wide Area Augmentation System programs (LAAS and WAAS, respectively) on range biases introduced by the reference station antennas. These biases result from antenna element group delay variation and can vary with respect to both satellite elevation and azimuth. The presence of such biases is problematic for system performance, but more so for their impact on the tortuous integrity analysis required for these differential systems. Characterization of such antenna biases is critical to system design and the analysis which must properly account for these errors. This paper describes two techniques for measurement of spatially dependent antenna biases. The first technique uses a system configuration to emulate reference station measurement performance. Here, a GPS signal is transmitted in an anechoic chamber and received using the reference station antenna and GPS receiver. The second technique also is conducted in an anechoic chamber but is less complex and uses a network analyzer to directly measure group delay across the GPS frequency bands. Group delay is a natural measurement of this instrument and indicative of range error introduced by the antenna. The antenna itself is rotated in these tests so that range errors can be mapped in both elevation and azimuth. This paper compares pseudorange and group delay measurements for several orientations and demonstrates that centimeter level agreement can be achieved. This is an important finding since measuring group delay is considerably less complex than performing system tests. The antenna tests described in this paper focused on the WAAS reference station antenna, although it is anticipated that these techniques will be of interest to others attempting to characterize antenna induced range errors. In particular, significant attention is given to the practical aspects of this investigation such as instrument settings, measurement processing and lessons learned.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	809 - 815
Cite this article:	Shallberg, Karl, Grabowski, Joe, "Considerations for Characterizing Antenna Induced Range Errors," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 809-815.
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