GNSS PPP-RTK: integrity monitoring method considering wrong ambiguity fixing

In previous studies on multiple hypothesis solution separations (MHSS) advanced autonomous integrity monitoring (ARAIM) in phase-based GNSS precise point positioning (PPP)-real-time kinematic (RTK), the risk of wrong (or incorrect) ambiguity fixing is generally considered to be negligible after ambiguity validation. However, this unrealistic assumption can introduce risks to PPP-RTK positioning solutions that have strict requirements for accuracy and integrity. In this study, a new integrity monitoring method considering the risks of wrong ambiguity fixing is proposed. The proposed method extends the scope of ARAIM to accommodate a type of pseudo-measurements for integer ambiguity fixing. Then, wrong ambiguity fixing can be treated as a type of fault in the MHSS ARAIM scheme. We test the proposed integrity monitoring method with both simulated and real-world PPP-RTK data sets, demonstrating that this method is conservative and can properly bound the positioning errors. Additionally, we analyze the effect of the probability of wrong ambiguity fixing on PPP-RTK positioning protection levels (PLs). The numerical results show that when the probability of wrong ambiguity fixing is smaller than the magnitude of the integrity budget, the effects on PLs are usually very small. On the contrary, due to the impact of wrong integer ambiguity fixing risks, it can increase the PLs levels of ambiguity-fixed solutions, which however will still be smaller than the PLs of ambiguity-float solutions.