Performance Study of Multi Carrier Ambiguity Resolution Techniques for Galileo and Modernized GPS

S. Schlötzer and S. Martin

Abstract:	This paper presents the results of investigations to determine the ambiguities of carrier-phase measurements on three or even four frequencies simultaneously. Lately developed three carrier ambiguity resolution algorithms are promising means to support (near) real-time positioning solutions. A software simulator which generates biased pseudorange and phase measurements of Galileo and GPS signals has been developed in order to allow for numerical evaluation of the algorithms. The modernization of GPS includes additionally to the enhancements of the current L2 signal the enclosure of a third signal, the L5 signal. Galileo is designed to transmit on even four frequencies: E5a, E5b, E6 and L1. Since more redundant measurements are available simultaneously compared to present dual-frequency GPS, the chance of instantaneous ambiguity resolution is improved. High success rates and enhanced integrity are expected. However, the success rate of ambiguity resolution over medium and long baselines is restricted by the spatial decorrelation of the residual ionospheric error concerning double-differenced observation models. Previous studies have shown that the availability of measurements on a third frequency helps to reduce the impact of the residual ionospheric error on the success rate by introducing more accurate ionosphere modelling. Various ambiguity resolution techniques that have been developed so far, making use of observations acquired at three or four different carrier frequencies simultaneously, are discussed. Geometry-free as well as geometry-based and ionosphere-free as well as ionosphere-weighted models are taken into consideration. Regarding the latest developments of three carrier ambiguity resolution techniques, ionosphere-weighted and geometry-based models predominate. This is explained by the request to extend the baseline length between reference and user receiver and the possibility of combining Galileo and GPS observables. The TCAR (Three Carrier Ambiguity Resolution) algorithm as proposed by Harris (1997) and the Integrated TCAR (ITCAR) algorithm as proposed by Vollath et al. (1998) have been implemented and evaluated with the help of the software simulator. Various simulation environments are supported by the simulator. Special interest has been directed towards the influence of Galileo’s E6 signal, which is meant to be restricted to the Commercial Service, on the overall ambiguity resolution success rate. Furthermore, a comparison between Galileo and GPS signals with regard to TCAR algorithms is drawn in this paper on the basis of the simulation results. It is shown that better performance is obtained for Galileo signal combinations by including the E6 signal than for GPS signal combinations. This is founded in a more convenient frequency spacing between E5a (or E5b), E6 and L1 than between L5, L2 and L1 with regard to algorithms based on forming linear combinations between different frequencies. To conclude, the extension of dual-frequency satellite navigation systems to three or four frequencies shows the tendency to favour Real Time Kinematic (RTK) applications, as the time span for successful ambiguity resolution is likely to be shortened significantly. In addition, it is expected that the range of carrier-phasebased positioning can be extended compared to present applications due to more precise modelling of the ionosphere.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	142 - 151
Cite this article:	Schlötzer, S., Martin, S., "Performance Study of Multi Carrier Ambiguity Resolution Techniques for Galileo and Modernized GPS," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 142-151.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In