Concepts for Undifferenced GLONASS Ambiguity Resolution

S. Banville, P. Collins, F. Lahaye

Abstract:	Precise point positioning (PPP) allows single-receiver accurate positioning through the use of precise satellite orbit and clock products. Recently, fixing of GPS carrier-phase ambiguities in PPP has become possible through proper handling of satellite and receiver equipment delays, leading to reduced convergence time and improved stability of the position estimates. Combining GPS and GLONASS has been shown to be beneficial as a result of increased redundancy and enhanced geometry, however GLONASS ambiguity resolution is problematic because of the nature of frequency division multiple access (FDMA). Even in differential mode, processing of GLONASS data from mixed receiver types is impacted by receiver design. It was recently demonstrated that a misalignment between code and phase observables results in apparent inter-frequency carrier-phase biases which need to be calibrated in order to perform ambiguity resolution. Code observations are also affected by frequency-dependent biases and prevent unbiased estimates of slant ionospheric delays. As a consequence, GLONASS ambiguity resolution is currently limited to baseline lengths over which the ionosphere cancels out or can be predicted using external information. To achieve undifferenced GLONASS ambiguity resolution, it is imperative to adequately model both inter-frequency carrier-phase and code biases. We first demonstrate that the apparent linear frequency response associated with carrier phases can be rigorously modelled by selecting two reference satellites with adjacent frequency channels. This condition allows explicit estimation of the frequency response, while preserving the integer properties of all ambiguity parameters. As a result, receiver-dependent external calibration values are not required for ambiguities to naturally converge to integers. An analysis of inter-frequency code biases reveals that, for most receiver types, these biases have a quasi-linear response to frequency. To account for this characteristic, we model receiver code clocks using a linear model in the adjustment process, i.e. by explicitly estimating an offset and a slope for each signal. However, since the frequency response is typically not perfectly linear, additional measures are required for proper identification of the integer candidates. For this purpose, we test the quasi-ionosphere-free (QIF) method of ambiguity resolution which is less sensitive to misspecifications in the stochastic model than integer least-squares theory. We also further explore the benefits of estimating a station-specific ionospheric model to provide additional constraints in the ambiguity resolution process. The performance of our approach is assessed by differential processing of mixed receiver types over long baselines, which shares similar challenges for ambiguity resolution as precise point positioning.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1186 - 1197
Cite this article:	Banville, S., Collins, P., Lahaye, F., "Concepts for Undifferenced GLONASS Ambiguity Resolution," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1186-1197.
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