Use of the Downlink Ionospheric Model to Enhance Dual-Frequency Ionospheric Measurements

David M. Chandler and Steven L. Patton

Abstract:	With the full GPS constellation in place, performance enhancements that lead to even small reductions in the GPS error budget have become more significant. In this paper, we present a new technique for reducing one of the largest contributors to user position error -- ionospheric delay. Compensation for ionospheric delay error has previously been made using dual- frequency measurements or the downlink ionospheric model. Because dual-frequency measurements are the most accurate compensation technique, dual-frequency receivers usually do not incorporate the downlink model. However, by combining dual-frequency measurements with downlink model data in an enhanced filter, it is possible to significantly reduce ionospheric delay errors due to pseudorange measurement noise. This technique is especially of interest in a jammed environment, where dual-frequency measurements were not previously possible due to increased measurement noise under jamming. Test results show that it is now possible to achieve unjammed ionospheric delay accuracy in a jammed environment. This greatly reduces position error for jammed receivers. Rockwell’s Miniaturized Airborne GPS Receiver (MAGR) uses a single-state Kalman filter to estimate ionospheric delay from dual-frequency measurements. Statistical analysis shows that simple averaging could improve accuracy under some conditions, but too much averaging would cause filter response to be sluggish in the afternoon, when the ionospheric delay is changing most rapidly. A two-state filter provides the optimum amount of “averaging” for the expected rate of change. Using a measurement and the downlink model to initialize the filter results in faster stabilization.
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:	1383 - 1386
Cite this article:	Chandler, David M., Patton, Steven L., "Use of the Downlink Ionospheric Model to Enhance Dual-Frequency Ionospheric Measurements," 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. 1383-1386.
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