Mitigation of the Phase Wind-up Effect for PPP Ambiguity Resolution using INS and Geometryfree Approach in UAV Experiments

The phase wind-up is a phase change that depends on the relative positional relationship between the transmitting and receiving antennas of the satellite and the receiver, and is one of the error sources affecting PPP/PPP-AR. In the case of PPP/PPP-AR, the attitude of the tilted receiving antenna is not taken into consideration in the conventional phase wind-up model, so that especially there is concern that in mobile PPP-AR positioning, when a sharp turn is involved, it might cause fluctuations in the phase wind-up effects and decrease the success ratio of the ambiguity fixing for PPP-AR. The purpose of this paper is to first confirm the position accuracy and success ratio of PPP-AR under the change of the UAV’s attitude, and secondly, to evaluate the relationship between the attitude of a rover antenna and the variation of the phase wind-up, and finally to propose the method to mitigate the phase wind-up effects using geometry-free linear combinations between L1 and L2 GPS carrier-phase measurements and optional attitude information obtained from an inertial sensor during the change of the attitude. In the UAV experiments, it was confirmed that centimeter-class user positions of fixed solutions with PPP-AR were obtained under three types of flight patterns, however there was no fixed solution at the flight with sways in the Roll direction. Also, it was confirmed that there was the relationship that RMS of the phase wind-up variations was increased from 23.4% to 108% at each satellite and was increased non-linearly in angles of the roll/pitch/yaw direction when the attitude of the UAV changed. By applying the proposed method to reduce the influence of the phase wind-up based on above the relationship, the fix ratio of PPP-AR was improved by 33.3% in this section, and the correct fixed solutions were obtained with the position accuracy of 0.0203 m/0.0186 m/0.0420 m (RMSE) at the East-West/North-South/Up-Down direction. From these results, it was shown that the fluctuations of the phase wind-up effect under the change of the UAV affect the success ratio of ambiguity fixing with PPP-AR.