TCAR and MCAR Options with Galileo and GPS

W. Werner, J. Winkel

Abstract:	The big number of available Galileo and GPS-IIF signals and frequencies available at the end of this decade makes the use of three-carrier ambiguity resolution (TCAR) or multiple carrier ambiguity resolution (MCAR) approaches interesting. However, not all possibilities perform equally. Due to the different wavelengths that are involved different success rates for a correct fixing of any type of wide-lane ambiguity can be given. All relevant different options of signal combinations have been analysed systematically with respect to their success rates and are presented in the paper. The current Galileo baseline signal structures and the modernized GPS-IIF signal structures have been investigated. The important point in selecting a well performing signal and frequency combination for TCAR is that in the cascading steps, the noise of the measurement of one step is low with respect to the virtual wavelength of the next steps ambiguity. Having this in mind and having available a certain number of frequencies, this leads to the MCAR approach, where the TCAR steps can be sequentially ordered according to their virtual wavelength to maximise the success rate for correct ambiguity fixing. In literature, the term "gap-bridging" concept is used for the steps of TCAR, where the gap between code pseudorange and finally the base carrier phase measurement accuracy is bridged. Analogously, the MCAR approach presented here makes use of a concept that can be called "fine-gap-bridging" concept. By making use of all available signals the risk for a wrong fixing in any of the steps is minimised. Furthermore, comparisons to the state-of-the-art ambiguity resolution approaches - sometimes called real-time kinematic (RTK) - have been made. With more than two frequencies available both approaches yield very good results. However, one major difference between both approaches is that in the TCAR (or MCAR) case no geometry information is exploited. This leads to the severe disadvantage that there might be wrong ambiguity fixings. As these wrong fixes can even happen in the early steps of TCAR or MCAR, huge range errors result. If this is the case, then the remaining steps generate a random lower-level range error as a result. Of course, there are several possibilities to perform post-TCAR plausibility checks, but there is no guarantee that could lead to a strong statement about integrity. This paper reviews the TCAR approach, lists all assumptions that have been made and presents the results of the TCAR/MCAR analyses that have been made. Comparisons to RTK approaches are also discussed. Finally, recommendations based on these results are given. The main results can be summarised as follows: 1. The code accuracy of Galileo AltBOC (15,120) on E5ab does allow for a (very) safe correct fixing of any super-widelane ambiguity. 2.The signal combinations that do not make use of the E5ab code range for the first step, have a high success rate for the remaining two fixing steps (widelane and base frequency ambiguity fixes). 3. The combination with E5ab, E6-E5ab, L1-E5ab, E5ab seems to be the best under the used assumptions. 4. Compared to GPS TCAR, in Galileo there are combinations with higher success rates, based on the relationships of the available signal frequencies. 5. Using MCAR, the risk of wrong ambiguity fixings is below the 10-5 level under the assumptions that have been made here.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	790 - 800
Cite this article:	Werner, W., Winkel, J., "TCAR and MCAR Options with Galileo and GPS," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 790-800.
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