Real-time PPP with Undifferenced Integer Ambiguity Resolution, Experimental Results

D. Laurichesse, F. Mercier, J.P. Berthias

Abstract:	It was shown in 2007 [1] that using pseudo-range information it becomes possible to fix the ambiguities on zero-difference GPS phase measurements for a global network. Theoretically, such a solution has the same performance and observability as a pseudo-range solution, but with measurements errors below 1cm. Thus an important activity has grown on this subject in recent years. The dual frequency pseudo-range information were used to fix the difference of the two ambiguities (one ambiguity for each frequency) using the Widelane (Melbourne-Wubbena) four observables combination [1, 4]. The remaining ambiguity is solved in a zero-difference network solution on the ionosphere free phase combination. The corresponding clocks corrections (GPS satellites and receivers) correspond to 'phase clocks'. These clocks have the property of preserving the integer nature of the ambiguities when processing the phase measurements of an isolated receiver. This allows the precise positioning (PPP) with integer ambiguity fixing. Since November 2009, such constellation clock solutions are available in the CNES/CLS IGS analysis centre solution ('grg' solution). Other approaches were developed in [8], where biases are identified relatively to a reference orbits/clocks solution (for example an igs solution), to allow the preservation of the satellite/satellite single difference ambiguities at user level. Also in [6], Collins presented a unified formulation for these kinds of problems (the 'decoupled clock model'). The application to real time solutions is very promising, because the ambiguity fixing improves drastically the performance of the solutions. This was shown in [1, 3]. The core of the real-time implementation is a Kalman filter working in mixed-mode (with both real- and integer-valued phase ambiguities). The filter produces GPS constellation states (orbits and clocks) with the ‘integer’ property. Algorithms for user receivers are also introduced. In order to improve the user ambiguity fixing, the code/phase biases have been added to the current solution. This allows also being compliant with the parameterization used in the IGS real time pilot project in which CNES is participating since July 2010. The recent improvements achieved at CNES in the real time solution are shown, including user side results. We present several ‘site survey’-type real-time experiments, which demonstrate an achieved horizontal precision close to 1 cm RMS. This is about one order of magnitude better than standard PPP solutions, which rely upon floating ambiguity fixing, and very close to the precision of differential RTK. In this article some properties of the related hardware characteristics are also shown (evolution of the widelane biases and possible variations of the code/phase biases). Future work and long-term objectives of the demonstrator are outlined.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2534 - 2544
Cite this article:	Laurichesse, D., Mercier, F., Berthias, J.P., "Real-time PPP with Undifferenced Integer Ambiguity Resolution, Experimental Results," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2534-2544.
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