Integrity Monitoring for Precise Point Positioning

Shaojun Feng, Altti Jokinen and Washington Ochieng

Abstract:	The Precise Point Positioning (PPP) technique uses products from a global network of receivers and carrier phase measurements to enable single receiver users to access centimetre-level accuracy. Unlike conventional Real Time Kinematic positioning (cRTK), PPP does not require local reference stations. Consequently, PPP has the potential to be used in remote areas including oceans where reference stations are either too expensive or impossible to locate. In both techniques, the ambiguity resolution is a crucial step. The level of difficulty in resolving the correct ambiguities depends on the residual errors of the formulated observations. There are a number of challenges in PPPP including formulation of mathematical models, fast resolution of integer ambiguities, ambiguity validation and integrity monitoring. Research to date has focussed on error modelling and ambiguity resolution. The ambiguity validation and integrity monitoring is still to be investigated in detail. Early research on PPP integrity has addressed the transferability of the Carrier phase based Receiver Autonomous Integrity Monitoring (CRAIM) algorithms developed for conventional Real Time Kinematic positioning (cRTK). However, there are significant differences between cRTK and PPP in the characteristics of the corresponding residual errors. For example, the satellite clock errors are removed in cRTK; while there are still satellites clock errors remaining in PPP after the application of correction products. The magnitude of these residual satellite clock errors depends on the quality of the products used. The residual errors in PPP are expected to be bigger than those in cRTK. These errors have significant negative impacts on ambiguity validation and integrity monitoring. This paper addresses these challenges. A two stage integrity monitoring method is adopted from the cRTK CRAIM covering the ambiguity resolution and positioning stage. In the first stage, the state-of-the-art Doubly Non-Central F distribution (DNCF) is used with the popular ratio test for ambiguity validation. In the second stage, the residual errors in the measurements for PPP are characterised taking into account the residual errors in the products and pre-processing. They are subsequently used for the failure detection and the calculation of protection levels. The protection levels are derived taking into account the geometry correlation of pseodorange and carrier phase measurements from each satellite. Tests were carried out using independent correction products and measurements. The products used are from the French National Centre for Space Studies (CNES). The GNSS measurement data used were from the American National Oceanic and Atmospheric Administration (NOAA). This selection is to ensure that any user data are not part of the generation of products. Data from 2 NOAA stations were used for testing. Test results show that the PPP algorithm with the DNCF based ambiguity validation can reach sub-decimeter accuracy. The protection levels calculated overbound the position errors all the time. The relatively low protection levels make the proposed method suitable for use in mission critical high accuracy applications.
Published in:	Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014) September 8 - 12, 2014 Tampa Convention Center Tampa, Florida
Pages:	986 - 1007
Cite this article:	Feng, Shaojun, Jokinen, Altti, Ochieng, Washington, "Integrity Monitoring for Precise Point Positioning," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 986-1007.
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