Improving PPP-RTK in urban environment by tightly coupled integration of GNSS and INS

In order to fully exploit the advantages of both precise point positioning (PPP) and real-time kinematic (RTK), a PPP-RTK method has been proposed to achieve centimeter-level positioning by applying the rapid integer ambiguity resolution, which is now widely implemented in some commercial systems such as Trimble RTX-Fast and NavCom StarFire. Nevertheless, the performance of PPP-RTK faces with restrictions under the circumstance of urban environments due to intermittent signal interruptions and unfavorable tracking geometry. Presently, it is increasingly prevalent that the inertial navigation system (INS) is integrated with global navigation satellite system (GNSS) to serve for enhancing the positioning performance. In this contribution, a tightly coupled PPP-RTK/INS integration model is developed, aiming to provide continuous and precise positioning service under the complex urban environments. In the proposed model, the precise atmospheric corrections derived from the multi-GNSS PPP fixed solutions of reference stations are disseminated to users to enable the rapid ambiguity resolution in PPP-RTK/INS. Furthermore, the high-accuracy position information offered by INS is also used to enhance the performance of ambiguity fixing. Experiments in different scenarios of urban roads and overpasses were designed to verify the effectiveness of the proposed method. Results indicate that the solution availability, fixing percentage and positioning accuracy can be significantly improved by PPP-RTK/INS integration. The horizontal positioning accuracy of the tightly coupled PPP-RTK/INS is 1-2 cm in a semi-urban environment and 5-6 cm in a real urban environment with a fixing percentage of 90.7% and 81.2%, respectively. Moreover, INS information also shows capability of bridging the gaps in GNSS data, which enables continuous positioning and fast ambiguity re-fixing under the GNSS-challenged environments. A fast ambiguity recovery within 1-5 s could be achieved for PPP-RTK/INS after outages lasting up to 30 s, while 8-18 s is required for PPP-RTK.