Applying Multipath Template Technique to Improve the Real-Time Ionospheric Delay Estimation Accuracy

Lao-Sheng Lin

Abstract:	When radio waves such as the GPS signals propagate through the ionosphere, they experience an extra time delay. The ionospheric delay can be eliminated (to the first order) through a linear combination of L1 and L2 observations from dual-frequency GPS receivers. Taking advantage of this dispersive principle, one or more dual-frequency GPS receivers can be used to determine a model of the ionospheric delay across a region of interest and, if implemented in real-time, can support single-frequency GPS positioning and navigation applications. There are, however, several factors that may affect the accuracy of real-time ionospheric delay estimation using dual-frequency receivers. One of them is the multipath effect on pseudo-range measurements. The multipath effect on GPS measurements depends on the physical environment and the receiver-satellite geometry. As the GPS satellites are in nearly circular orbits, at an approximate altitude of 20200 km, they will again be over the same position on the surface of the earth at the end of a sidereal day (approximately 23 hours 56 minutes in length). Thus, the viewing geometry is the same each day, but with a shift of about four minutes per day. When the physical environment about a GPS receiver remains unchanged from day to day, then the multipath effect will mostly be constant. Based on this assumption, the multipath template technique has been developed and used to mitigate GPS pseudo-range multipath at static GPS reference stations. This paper describes the algorithm that generates the so-called multi-day multipath template using the previous days’ GPS dual-frequency measurements. In order to improve the accuracy of real-time ionospheric delay estimation using dual-frequency receivers, the generated multipath template can be applied to successive days data. The algorithm permits the pseudo-range multipath error for a specific day to be corrected using this multipath template, on an epoch- by-epoch basis. Data from National Chengchi University (NCCU), in Taiwan, over 18 consecutive days was used to test on the application of the multipath template in the context of real-time ionospheric delay estimation. Preliminary results indicate that: (1) the performance of the multi-day multipath template is superior to that of the single-day multipath template, (2) the performance of the various 4-day multipath templates generated from the GPS data of the previous days is better than that of other multi-day multipath templates, and (3) the multipath template technique is an effective method to mitigate GPS pseudo-range multipath and should be implemented at reference stations within LADGPS, WADGPS and WAAS networks, GPS deformation monitoring networks, etc.
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:	1872 - 1880
Cite this article:	Lin, Lao-Sheng, "Applying Multipath Template Technique to Improve the Real-Time Ionospheric Delay Estimation Accuracy," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1872-1880.
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