Combining L1 L2 Carrier Differences with Iono-sphere Model Estimates for an Improved Iono-sphere Error Corrector

David Dodd and Sunil Bisnath

Abstract:	The two remaining obstacles to long-range, high-accuracy (LRHA) GPS positioning are the errors associated with the ionosphere and the troposphere. Several models and methods have been developed to address both of these obstacles. This paper describes a method for combining L1/L2 differences with ionosphere estimates to create an improved ionosphere corrector. Traditionally, the effects of the ionosphere on LRHA positioning have been mitigated through L1/L2 ionospherefree combinations. This method almost completely removes the effect of the ionosphere; however, the resulting combined observations have an increased noise level and the integer nature of the initial carrier ambiguity is lost. The result being that the L1 and L2 integer ambiguities can not be estimated, making ambiguity fixed solutions impossible. Several ionosphere models exist that are used to estimate ionosphere errors, and can be applied to both L1 and L2 observations. In this situation the integer nature of the ambiguity is preserved, making fixed ambiguity solutions theoretically possible. However, tests have shown that the existing models tend to smooth out the high frequency localized ionosphere effects, leaving residual errors that are too high for reliable ambiguity fixing. Combining L1/L2 differences with the model error estimates provides a method for introducing the local high frequency effects into the models to reduce uncertainty. Previous test have shown that a post-processed version of the combined method gives results very comparable to the ionosphere-free solution [Dodd and Bisnath, 2006]. The tests conducted in conjunction with this paper show that a real-time version of the combined method produces results almost as good as the ionosphere-free solution, which makes it, theoretically, good enough for ambiguity resolution. The tests also show that the combined solution, regardless of the model used to produce the initial ionosphere error estimates, produces better results than those determined from any of the models alone.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2291 - 2301
Cite this article:	Dodd, David, Bisnath, Sunil, "Combining L1 L2 Carrier Differences with Iono-sphere Model Estimates for an Improved Iono-sphere Error Corrector," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2291-2301.
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