Extracting Ionospheric Measurements from GPS in the Presence of Anti-Spoofing

Brian Wilson and Anthony Mannucci

Abstract:	Line-of-sight ionospheric measurements derived from differencing dual-frequency Global Positioning System (GPS) pseudorange data are corrupted by instrumental biases in both the receiver and GPS satellite transmitters due to hardware delays in the Ll and L2 signal paths. Hardware calibration of the instrumental delay is possible for some types of receivers (e.g., Allen Osborne Associate’s Rogue SNR-8), but the satellite biases must be estimated using a remote technique. Ignoring the satellite (receiver) biases when computing line-of-sight TEC measurements from GPS observables may result in an error of +-9 (+-30) TECU (1 TEC unit = 1016 electrons/meter2 = 0.35 nanoseconds of differential delay). Using a global ionospheric shell model to fit GPS-based ionospheric delay data from a world-wide network of 50+ receivers, we can simultaneously estimate a global ionospheric map, satellite biases for the entire GPS constellation, and receiver biases for all the uncalibrated receivers. The uncertainty in the resulting estimates of the satellite biases is a dominant error source in extracting line-of-sight TEC measurements from GPS observables. We present the resuits of daily fits of satellite biases over a period spanning 20 months from Jan. 1993 through Aug. 1994. Seven separate time periods each consisting of IO- 12 consecutive days were processed. The day-to-day variability of the estimates has been computed in an effort to assess their precision. Before anti-spoofing (AS) encryption of the GPS ranging code, the estimated satellite biases exhibited a day-to-day standard deviation of 0.6 nanoseconds or 1.7 TECU. With the advent of AS encryption, the pseudorange observable exhibits a lower signal-to-noise ratio, but the day-to-day reproducibility of the biases has not changed significantly. This preliminary study indicates that AS has not had a significant impact on our estimates of the satellite biases. The current satellite bias estimates (with AS on) are consistent with the pre-AS values to a precision of 0.6 ns or 1.7 TECU.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	1599 - 1608
Cite this article:	Wilson, Brian, Mannucci, Anthony, "Extracting Ionospheric Measurements from GPS in the Presence of Anti-Spoofing," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 1599-1608.
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