Mitigation of the Code-carrier Divergence Effect on Single Frequency user Receivers for Enhanced Availability and Accuracy under Ionospheric Degraded Conditions

Jon I. Caudepon, Teodoro Bernardino, Esther Sardon

Abstract:	It is well-known that satellite code and carrier measurements are affected differently by the ionosphere: a positive delay in the code and an advance for the carrier, both equal in absolute value. Therefore the code and carrier measurements diverge from each other at twice the rate of ionospheric changes along every user-satellite line of sight (this is the so called code-carrier divergence effect). At SBAS user level it is assumed that the broadcast corrections eliminate the ionospheric delay from the smoothed pseudorange measurements in order to obtain a IONO-free navigation solution. Therefore, the user smoothing filter does not consider the code-carrier divergence effect, denoting intrinsic limitations of the standards at user level. The smoothing filter of the latest standards [1] mixes code and carrier measurements without any special care about this effect, assuming that the rate of change of the ionosphere over a few minutes is small. If this is the case, as it is usual for receivers at mid-latitudes, the impact of this effect in the resulting error from the smoothing process is small. However, the presence of significant temporal gradients in the ionosphere is not actually rare out of mid latitudes, being especially relevant in equatorial regions. This paper develops a strategy to cope with the codecarrier divergence effect produced in the user smoothing process by removing the ionosphere contribution of satellite measurements prior to the filter. The results of the suggested implementation will be evaluated through the software based GPS receiver performance analysis tool Teresa (Testing Receiver for EGNOS using Software Algorithms). The tool is able to acquire real time and post processed single frequency data in order to compute and provide a wide range of performance statistics, including availability, accuracy and integrity information. It will be shown that degradation of the accuracy due to ionospheric gradients is considerably mitigated and that, contrary to standard filters, the proposed implementation can gain robustness against scintillation by applying longer smoothing times with no practical loss in accuracy.
Published in:	Proceedings of the 2010 International Technical Meeting of The Institute of Navigation January 25 - 27, 2010 Catamaran Resort Hotel San Diego, CA
Pages:	784 - 798
Cite this article:	Caudepon, Jon I., Bernardino, Teodoro, Sardon, Esther, "Mitigation of the Code-carrier Divergence Effect on Single Frequency user Receivers for Enhanced Availability and Accuracy under Ionospheric Degraded Conditions," Proceedings of the 2010 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2010, pp. 784-798.
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