Time Diferencing and Filtering Technique for Mitigating Errors in GPS Carrier Phase and Pseudorange Observations

John A. Momoh

Abstract:	Motivated by the need to improve precise point positioning, this paper unveils a new technique for mitigating the level of error in GPS carrier phase and code pseudorange observations. The technique which involves time differencing, filtering and smoothing (DFS) of generated filterable observables of all observed satellites at any epoch, aims at mitigating the observation error composed of the errors generated by multipath, inaccurate satellite orbital position, receiver clock drifts, unmodelled errors and model inaccuracies. To improve performance under challenging conditions, the processing algorithm of the technique also provides robustness to short duration observation gaps that often result in cycle slips occurences. The algorithm also generates smoothed time series range residuals suitable for resolving carrier phase ambiguities, with or without observation gaps or observation discontinuities (temporary loss of an observed satellite). The inherent observation error in carrier phase observation often hinders ambiguity resolution or resolution to the correct value. Moreover, existing code pseudorange carrier-smoothing techniques, such as the Hatch filter, do not only absorb the error on the carrier phase observation used to smooth the pseudorage, but also become hampered and degrades in performance in the presence of cycle slip and observation gap occurence. As often the case, real-time positioning relies on the broadcast satellite ephemerides which usually degrades in accuracy with time. Furthermore, receiver clock error originating from significant oscillator drifts, which are not always eliminated prior to positioning, tend to reflect as error in the positioning solution. Hence, to improve precise point positioning and enhance positioning for high accuracy and precision demanding applications, the error levels on carrier phase and pseudorange observations, as well as errors due to inaccurate satellite ephemeris and receiver clock error among others, have to be mitigated if not eliminated. Upon implementing the DFS technique aimed at mitigating the observation error levels in all carrier phase and pseudorange observations, the obtained initial test results from static mode dual-frequency positioning indicate that the DFS is capable of achieving its aim, and enhance autonomous receiver code positioning accuracy and precision. On comparison of these initial results, the DFS smoothed pseudorange positioning solution showed 53% improvement over the raw (unsmoothed) pseudorang positioning solutions; and on average, achieved up to 9% improvement over the Hatch filtered pseudorange positioning solutions. More significantly, the DFS achieved considerable improvement over the Hatch filter technique in the vertical position component.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2216 - 2230
Cite this article:	Momoh, John A., "Time Diferencing and Filtering Technique for Mitigating Errors in GPS Carrier Phase and Pseudorange Observations," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2216-2230.
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