Standalone Real-time Navigation Algorithm for Single-frequency Ionosphere-free Positioning Based on Dynamic Ambiguities (DARTS-SF)

A. Simsky

Abstract:	This paper demonstrates that compensation of ionosphere delays can be achieved in a purely standalone real-time mode, even if only one frequency is available and no additional information is used. The algorithm presented in this paper is based on the combination of two ideas: the use of an iono-free single-frequency range observable (code + phase)/2 and the concept of phase processing with dynamic ambiguities. The concept of DARTS (Dynamic Ambiguities Real-Time Standalone) was described in detail in a previous paper presented at this conference in 2003 [1]. DARTS is based on joint processing of code with phase by the Kalman filter, where floating phase ambiguities are modeled not as constants, but as slowly changing values. The original DARTS was a dual-frequency algorithm, where ionosphere delays were fully compensated due to the use of iono-free code and iono-free phase observables. The main result achieved with DARTS was the reduction of the positional errors owing to system biases by a factor of about two. Direct application of the DARTS processing to the single frequency case is feasible, but a straightforward single frequency version of DARTS, which is based on the combined processing of the C/A code and L1 phase, suffers from uncompensated ionosphere delays. The central idea of the new algorithm, the DARTS-SF (DARTS-Single Frequency), is to use an iono-free singlefrequency combination, (code+phase)/2 as iono-free phase. This combination has the same geometrical behaviour as phase. Due to a well-known fact that the contributions of ionosphere delays to phase and code measurements have the same values but an opposite sign, this combination contains no ionosphere delays. In DARTS-SF, the (code+phase)/2 observable is used in the Kalman filter in the place of iono-free phase. The measurement noise model must be modified appropriately in order to take into account the higher noise of this “iono-free quasi-phase” observable as compared to normal phase measurement. The error budget and positional performance of DARTSSF are in fact similar to these of the original dualfrequency DARTS (DARTS-DF). The DARTS-SF shows almost complete compensation of ionosphere delays and about the same reduction of system biases as DARTS-DF. The multipath noise is reduced by a factor of 2 as compared to standard C/A code processing. In benign ionospheric conditions, the standard deviation of computed heights with DARTS-SF is around 1.5 m, which is even lower than the typical accuracy level of standard dual-frequency positioning and is comparable to the performance of dual-frequency DARTS. DARTS-SF shows its best when high ionospheric activity is present. For the data collected during the well-known storm of November 20 2003, the gains in positional accuracy are by a factor of 5-10 as compared to standard singlefrequency code-based processing. DARTS-SF is designed having in mind its application in single-frequency GNSS receivers. It uses only the data available to a standalone L1-only receiver in real time.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	301 - 308
Cite this article:	Simsky, A., "Standalone Real-time Navigation Algorithm for Single-frequency Ionosphere-free Positioning Based on Dynamic Ambiguities (DARTS-SF)," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 301-308.
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