Fast PPP Convergence Using Multi-constellation and Triple-frequency Ambiguity Resolution

D. Laurichesse, A. Blot

Abstract:	PPP is a relatively new but powerful technique for GNSS positioning. The main difference between PPP and standard positioning is the use of the carrier-phase measurements, whose noise is two orders of magnitude lower than the code measurements. It is now widely accepted that PPP techniques can achieve centimeter accuracy globally in real-time, in particular when they are combined with phase integer ambiguity resolution. However, one important drawback of the PPP is the convergence time. Dual-frequency PPP convergence is long, typically half an hour, which makes it impracticable for many applications. However, with the development of the modernized GPS, Galileo and the Beidou constellations, a third frequency is now available on a growing number of satellites. For example, for a user located in the Asia-Pacific region, there are always more than 15 triple-frequency satellites in view nowadays. This number is expected to increase in the next few years thanks to the deployment of the Galileo constellation. In this paper, we will explore the different measurement combination possibilities offered by the new triple-frequency signal capabilities, namely with the availability of “widelanes only” intermediate combination. This is the ionosphere-free combination that is composed of only phase widelanes. The extra-widelane ambiguity can be solved easily thanks to the Melbourne-Wubbena combination. Once the extra-widelane ambiguity is determined, the remaining ambiguity can be easily solved thanks to a loose knowledge of geometry, because the associated wavelength is relatively high (2.40 m for GPS). Once all the widelane ambiguities are solved, this combination is equivalent to a non-ambiguous ionosphere-free measurement. By performing a noise analysis based on actual measurements, we show that the different characteristics of this combination are compatible with a very fast ambiguity resolution, on all the constellations. We then offer a real life experiment of the concept using the tools provided by the IGS Real Time service. In this context, satellite phase biases are computed for the three frequencies thanks to the MGEX network of stations. We characterize these biases in terms of noise and time stability, and show that they can be encapsulated in the uncombined SSR representation offered by the RTCM for phase biases messages. All these computations are carried out in real time conditions. A similar implementation is proposed at user level. We show how the uncombined formulation for phase biases can help partial ambiguity fixing. The improvement of the convergence time compared to the dual-frequency case is demonstrated. In particular, we show that quasi-instantaneous convergence below the 20 cm level is achievable. This concept is implemented in the CNES PPP-Wizard demonstrator. SSR representation adopted by the IGS real time service is compatible with partial ambiguity fixing. The CNES user open-source PPP implementation presented at the ION GNSS 2015 meeting is upgraded to take into account the new biases. Some actual real-time results, in particular in the Asia-Pacific region, are provided.
Published in:	Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016) September 12 - 16, 2016 Oregon Convention Center Portland, Oregon
Pages:	2082 - 2088
Cite this article:	Laurichesse, D., Blot, A., "Fast PPP Convergence Using Multi-constellation and Triple-frequency Ambiguity Resolution," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 2082-2088. https://doi.org/10.33012/2016.14633
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