Instrumental Biases in Ionospheric Measurements Derived from GPS Data

Brian D. Wilson and Anthony J. Mannucci

Abstract:	Line-of-sight ionosphere measurements derived from differencing dual-frequency Global Positioning System (GPS) range 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. The line- of-sight differential delay can be modeled as the sum of a receiver bias, a satellite transmitter bias, and the line-of- sight ionospheric delay or TEC (total electron content). While the receiver bias can be calibrated directly for some types of receivers, the satellite biases must be estimated from the GPS data itself by using a model of the ionosphere. Ignoring the satellite (receiver) biases when computing TEC measurements from GPS will result in an error of 59 (k30) TECU (1 TEC unit = 1016 electrons/mete+. Using a global ionospheric shell model to fit GPS-based ionospheric delay data from a world-wide network of 30- 40 receivers, we can estimate, with a single fit, satellite biases for the entire GPS constellation and receiver biases for all the uncalibrated receivers. Current studies indicate that the estimated receiver biases agree with the hardware calibrations at the level of 1 nanosecond (ns) and the day- to-day scatter of the estimated satellite biases ranges from 0.3 to 0.5 ns. Preliminary results show our estimated satellite biases agree with other reported values only at the level of 0.7 ns (RMS difference over all satellites). Further investigation will be required to reconcile these differences. If the true accuracy is 0.5 ns, as derived from day-to-day scatter, then the total uncertainty in line-of- sight TEC measurements derived from GPS is 0.6 ns or 1.8 TECU (1 ns corresponds to 2.85 TECU).
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	1343 - 1351
Cite this article:	Wilson, Brian D., Mannucci, Anthony J., "Instrumental Biases in Ionospheric Measurements Derived from GPS Data," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 1343-1351.
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