Research On GNSS Integer Ambiguity Estimation Methods

As the construction and modern development of Global Navigation Satellite System(GNSS), GNSS has been widely used in various fields around the world. And the applications are more and more high to the requirement of the accuracy and reliability of the GNSS, even some of which need near real-time or the real-time positioning. GNSS integer ambiguity resolution is the key to fast and high-precision GNSS positioning. Therefore, this dissertation mainly focuses on the methods of ambiguity estimation in GNSS positioning and carries on detailed analysis and research for the ambiguity search space and search strategy in the process of integer ambiguity resolution. Moreover, GNSS ambiguity estimation is also known as the closest vector problem(CVP) in lattice theory, so the search algorithm for solving the CVP is introduced into ambiguity resolution. And the method is carried on the improvement and optimization to try to solve the the fast ambiguity resolution. In addition, precise point positioning(PPP) ambiguity resolution is discussed and the performance of PPP solutions and application based on float ambiguity is analysized detailedly. The calculation of ionospheric total electron content is also preliminarily concerned and studied. The main research contents and results are as follows:(1) The size of the ambiguity search space plays a critical role in ambiguity resolution. In view of conservatives in traditional methods for determining ambiguity search space, a method is presented based on least-square ambiguity decorrelation adjustment method. Firstly, the mixed integer least squares method for ambiguity resolution is introduced, and then the three classic methods of determination of the ambiguity search space are given a detailed introduction and in-depth analysis and comparison, and their merits and demerits are evaluated. On this basis, an impact factor η is defined, and a modified formula is presented based on LAMBDA method. Experiments using modified formula based on the simulated and measured data, show that the modified method can guarantee at least the requested candidates but get fewer integer point number inside the ambiguity search space. And more than 90% of actual number of integer candidates is close to expectation.(2) In ambiguity searching domain, integer bootstrapping method calculating efficiency is high, but the success rate is low. Integer least-squares method is rigorous in theory, but the calculating efficiency is relatively low. In order to guarantee the both the computational efficiency and success rate of ambiguity resolution at the same time, a new estimated method, depending on the R-Ratio test and Bootstrapping success rate, is put forward and only needs partial searching process, which is a combination of integer bootstrapping method and integer least-squares method. The numerical results show that the proposed method, compared with other methods, has a good performance on computational efficiency and success rate. It is indicated that the presented methods can be used in high dimensional ambiguity resolution.(3) Ambiguity estimation is a problem of mixed integer least-squares, but also closest vector problem(CVP) in lattice theory. According to the fact that the current methods for ambiguity estimation do not meet with the demand of fast positioning, M-VB method is introduced for ambiguity estimation after presentation of searching methods for CVP. And M-VB method is improved from two aspects. One is optimizing the method with updating the upper bounds, and the other is determining the radius with bootstrapped estimator. The improved M-VB method is compared to LAMBDA and MLAMBDA methods based on simulated data and real observations on the condition of with and without decorrelation. The results indicate that the improved M-VB method can fix integers faster than LAMBDA and MLAMBDA methods.(4) The ambiguity resolution of precise point positioning is introduced detailedly based on the ambiguity fixed method with corrected fractional cycle bias(FCBs). And the advantages and disadvantages of PPP based on fixed ambiguity is also analysed detailedly. Moreover, the accuracy and convergence time of PPP on the basis of float ambiguity are analyzed and evaluated by using Bernese 5.0 and Canadian Spatial Reference System(CSRS) PPP online tool. At last, an approach of using precise point positioning combined with precise orbit and precise clock products to establish control points is presented, considering that establishing near mine points can be a costly business and precise inconsistent using traditional methods. The results show that accuracy is better than 5cm. This study indicates that static PPP had a good performance in surveying near mine points.(5) The total electron content(TEC) of ionosphere is one of the major error resources for the accuracy of satellite navigation positioning. After the introduction of GNSS models and positioning resources, a simple method is presented for computing ionospheric TEC. To verify the computational precision of the presented method, a test is conducted with the International GNSS Se.rvice(IGS) TEC data sample(low latitude, mid-latitude and high latitude within four periods of time) in lower and higher solar activity, repectively. The results show that the TEC residuals, computed by the presented method and the IGS values, are small. Specifically, 90% of TEC calculation residuals are less than ± 3TECU, and the performance of the presented method in lower solar activity is better than that in higher solar activity. Moreover, the presented method is also superior to interpolating method for computing TEC.