| Precise Point Positioning (PPP) is a positioning technique processing the observations at solely a single receiver on a global scale by fixing precise satellite orbits and clocks. PPP can directly obtain centimeter-level accuracy coordinates for static positioning and decimeter to centimeter-level accuracy coordinates for dynamic modes. PPP has been widely used in the field of deformation monitoring, high-precision navigation and positioning, LEO satellite orbit determination, GNSS meteorology, precise time transferring and so on. Real-time and high precision are two of the main objectives of GNSS researches. With the development of real-time orbit determination and real-time clock estimation technology, PPP has gradually developed into a real-time high-precision positioning means. In the traditional PPP, because of the fractional part of the uncalibrated phase delays (FCBs), ambiguity parameters are estimated as real-valued constants. Scholars both at home and abroad pointed out that the accuracy of PPP at the E direction can still be improved by ambiguity fixing. The research of ambiguity-fixed PPP is a very rewarding work, and it is also a central issue of GNSS research all over the world.Through comprehensive and in-depth study, the author pointed out that ambiguity resolution of PPP based on FCBs can be achieved with existing precise orbit and clock products. Rovers use the corresponding precise orbit, precise clock and FCBs products can achieve ambiguity-fixed PPP. This algorithm can be carried out on the basis of existing clock estimation technology, thus it has a widely foundation and huge development space. This paper comes up with effective solutions on some problems of FCBs estimates and ambiguity resolution of rover stations. A module with functions of both FCBs estimation and PPP with integer ambiguities is developed based on the Wide Area Real-Time Differential GPS software platform. The correctness of the improved algorithm is verified with experiments. In addition, the researches of PPP are all based on precise orbit and clock products currently, this paper has explored and discussed the feasibility of PPP based on broadcast ephemeris and clock products estimated with regional networks, and the precision and certain feasible range is studied.The content of this study mainly includes:1. The theory and technology of ambiguity-fixed PPP are studied systematically. Starts from the basic function model, a detailed analysis of the relationship between parameters which lead the ambiguities to non-integer values and the other parameters are analyzed in detail.2. Based on the difference of the coupling characteristics between the fractional part of the uncalibrated phase delays (FCBs) and the other parameters, systematical classification for PPP ambiguity fixing algorithm both at home and abroad has been done. In addition, a detailed comparative analysis has been made.3. A new method with M Estimator is proposed to estimate the FCBs. In the applications of real-time ambiguity-fixed PPP, the coordinates of reference stations should be fixed when the server estimates the FCBs. Biases of the coordinates can cause biases to narrow-lane ambiguities. Due to the influence of some irresistible factors (such as earthquake), the positions of some reference stations may change, while the server can not update the coordinates promptly. This will cause large deviations in the ambiguity estimation of some reference stations. This paper points out that the float ambiguities with large deviations can be treated as gross errors. The robust estimation theory is introduced into the estimation of FCBs and the influence of harmful observations can be reduced by reducing the weight of harmful observations to improve the robustness of FCBs estimation. Experimental results show that, compared with the original algorithm, this algorithm can reduce the possible influence of gross errors and the problem that reference station coordinates have sudden changes in real-time applications of PPP ambiguity-fixed solution can be solved.4. An improved LAMBDA method was applied for the integer ambiguity resolution of PPP. This method uses bootstrapped probability as an indicator of the performance of LAMBDA decorrelation. The ambiguities are tried to be fixed only when the index of bootstrapping exceed the threshold. The results show that the improved LAMBDA method can improve the success rate of ambiguity resolution of PPP.5. Based on the Wide Area Real-Time Differential GPS software System, which is developed by GNSS Center of Wuhan University for providing real-time precise satellite orbit and clock products, error models which have not been included in current version are added. For example, phase center variations (PCV), horizontal gradients of troposphere delay and etc. Furthermore, the impacts on the narrow-lane ambiguities estimation when neglecting them are analyzed. Based on the improved methods, a module with functions of both FCBs estimation and ambiguity-fixed PPP is added into the Wide Area Real-Time Differential GPS software.6. Currently, the research of ambiguity-float and ambiguity-fixed PPP are all based on precise orbit and precision clock products estimated with observations of globally distributed tracking stations. However, there is hardly any research of PPP with broadcast ephemeris and clock products estimated with regional observations. In this paper, PPP with broadcast ephemeris and clocks estimated with regional networks is put forward. The precision and feasible range of ambiguity-float PPP and ambiguity-fixed PPP are verified both theoretically and experimentally. Preliminary view is that ambiguity-float PPP with broadcast ephemeris and clocks estimated with regional networks can achieve comparable result with PPP based on IGS final products within a region of300km radius. Ambiguity-fixed PPP based on broadcast (brdc) ephemeris and clock products estimated with regional observations is proposed within a region of100km radius.7. Data of Provincial CORS network, Continental Tectonic Environment Monitoring Network Project of China and seismic experiment is used to verify the improved algorithm and the capability of the software. Results based on data of CORS network at the provincial level show that the accuracy of ambiguity-fixed PPP improved about52.6%,23.7%and26.6%in the East, North and Up components respectively than ambiguity-float PPP. The improvements decrease to38.64%,23.88%and15.98%in the East, North and Up components when the large area is used. Results of the earthquake simulation experiment show that the RMS deduce from0.84cm,0.94cm and5.77cm to0.76cm,0.70cm and3.60cm in the East, North and Up components, respectively. PPP with broadcast ephemeris and clocks products estimated with regional networks is verified with Municipal CORS network. The precision of ambiguity-float PPP with broadcast ephemeris and clocks estimated with regional networks is higher than that with IGS final products, the ambiguity-fixed PPP solution has a further increment of22.1%,46.4%,17.1%in N, E and U directions, respectively. |
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