Integration of multiple-constellation GNSS for long-range real-time kinematic positioning

Abstract

Network Real Time Kinematic (NRTK) positioning with instantaneous ambiguity resolution (AR), Precise Point Positioning (PPP), and PPP-RTK are currently popular techniques for real-time precise positioning using carrier phase observations. These techniques have been widely used in civilian and military fields. Both NRTK and PPP-RTK provides centimeter-level positioning services with a relatively short initialization time. However, they are limited to the local or regional area due to the atmospheric effect over long-range baselines. In contrast, a reliable real-time PPP service can provide precise positioning on a global scale. However, PPP needs a comparatively long initialization time of about 20 minutes. In addition, PPP is less precise compared to NRTK. In this dissertation, the work on long-range RTK positioning with multiple Global navigation satellite systems (GNSS) constellations and multiple-frequency signals to realize the real time centimeter-level precise positioning service on a global scale is reported. To achieve the purpose, several key issues have to be solved: fast estimation of the atmospheric effects, reliable ambiguity validation and efficient ambiguity resolution (AR) for long-range baselines. In addition, BeiDou Navigation Satellite System (BDS) is a new GNSS system that broadcasts triple-frequency signals. The navigation performance of BDS and GPS and BDS integration (GPS/BDS) is assessed and compared. First of all, the navigation performance of BDS, GPS and GPS/BDS has been evaluated. This study demonstrates that the absolute and relative positioning precisions of BDS are very similar to that of GPS. The availability, integrity and positioning accuracy of GPS/BDS are significantly improved due to more satellites available. In addition, experimental results show the multiple-constellations and multiple-frequency signals significantly enhance the effectiveness of RTK. Ambiguity validation is crucial for any carrier phase based positioning. This study proposes a new geometry-based ambiguity validation (GBAV) method, which ensures that different ambiguity candidates are both geometrically separable and the miss fixing (mis-fixing) probability controllable. Experiments demonstrate the reliability of the GBAV method is significantly better than that of the ratio test. To reduce the atmospheric effects, a new ionospheric delay search method is proposed to eliminate the effect of ionosphere in the ambiguity search process in this study. In addition, the performance of the Numerical Weather Prediction (NWP) model from Shanghai Astronomical Observatory which is applied to predict the tropospheric delay is assessed. Experiments show that the accuracy of the tropospheric delay is high enough for any carrier phase-based positioning services. Based on the methods mentioned above, a new atmosphere-free AR (AFAR) method for long-range baseline RTK using the multiple-constellations and multiple-frequency signals is proposed. Experiment results based on the GPS/BDS observations show that AR can be achieved within 6.5 min and 16 min for 60 km and 600-700 km baselines using the AFAR method with GPS/BDS observations. Long-range RTK with AFAR performs better than PPP. It can provide a centimeter-level of accurate trajectories with a short initialization time on a global scale.