Receiver Autonomous Integrity Monitoring for Fixed Ambiguity Precise Point Positioning

There are still many challenges in Precise Point Positioning (PPP) including formulation of mathematical models, fast resolution of integer ambiguities, ambiguity validation and integrity monitoring. Research to date has focused 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 Doubly Non-Central F distribution (DNCF) is justified for the use with popular ratio test for ambiguity validation. The residual errors in the PPP are characterised for the two key processes in RAIM, failure detection and derivation of protection levels. The correction products used for tests were from Centre National d'Etudes Spatiales (CNES). The GNSS measurement data used were from the American National Oceanic and Atmospheric Administration (NOAA). This selection is to ensure that data from same stations used to test the method are not part of the data sets for the generation of correction products. A dataset from 2 NOAA stations was used for testing. Test results show that the PPP algorithm with the DNCF based ambiguity validation can reach sub-decimetre accuracy. The protection levels calculated shown to over-bound the position errors all the time. The relative lower protection levels give the potential for the proposed method to be used in critical high accuracy applications.