| In recent years, flourishing development of GNSS promotes profound revolution in the field of navigation and positioning, leading to increasingly widespread applications, particularly in geodesy, spatial science, geophysics and so forth. Facing the expanding GNSS applications of high-precision and real-time, challenging requirements on innovating algorithms and developing software systems are coming.For PPP, the long initialization time and real-time acquisition of satellite constellation orbit and clock offset are the two development bottlenecks. Several common problems encountered in PPP are studied and the algorithms are optimized in this thesis, the main research topics include:1. Optimizing the Model and Method of PPP Observation EquationsThe PPP basic theories, including function model and stochastic model, are systematically studied. The development of PPP model is analyzed and summarized. In order to improve the convergence rate and positioning precision, some new models are proposed. Including:(1) A combined PPP model of difference between satellites is proposed. This procedure can eliminate part of the unknown parameters and improve the efficiency of computation. The initial values of the unknown and covariance matrix are more appropriate. Besides, the method can avoid introducing the correlation between different observations. The results show that the convergence time is about 30% shorter than the undifferenced model.(2) Considering the effect of carrier to noise ratio (C/No) to the random error of observations, on the basis of the elevation dependent stochastic model, C/No dependent scaling factor is introduced. This procedure can reflects the system state accurately and compensate the influence of random error on parameter estimation. The results show that the required time of achieving decimeter accuracy is about 36% shorter than the elevation dependent stochastic model.2. Refining the Empirical Model of Troposphere DelayTo improve the meteorological parameters, zenith delays and mapping functions, some modifications are made to propose a new global troposphere model called SEUTROP. Compared with the original models, the advantages of SEUTROP embody:(1) By applying the global grid meteorological parameters, SEUTROP can reflect the change of troposphere delay more accurately in the time sacle and spatial scale. Compared with the results from standard meteorological parameters, the accuracy is improved by an order of magnitude.(2) Compared with existing models (take UNB3m as an example), SEUTROP acquires finer and more uniform precision all over the world. The advantage of SEUTROP model is obvious in the southern hemisphere (especially in Antarctica). The performance in the northern hemisphere is better in winter.3. Verifying the Accuracy and Reliability of IGS Real-time ProductsThe availability of real-time products, provided by IGS, is overall analyzed. There are two types of products available for real-time PPP:products based on IGU prediction and products transferred via Ntrip protocol.(1) IGU prediction orbit and clock offset performs differently in precision. Since the satellite moves continuously and the constellation orbit is derivable, extrapolation algorithm cannot cause great loss of orbit accuracy. The orbit error is less than 5cm after 9 hours. However, the discrete and random characters of satellite clock offset led to great bias by using extrapolation algorithm. The error is about 10ns, which is much more than the performance index (3ns) from IGS. Therefore, it is not available for real-time PPP.(2) IGS Real-time Service calculates the precise orbit and clock offset and broadcasts to the user vir Ntrip protocol. The error of precise orbit varies from 2cm to 4cm, which is better than the accuracy indication from IGS. When it turns to the satellite clock, the error of 0.3ns is significantly improved compared with IGU prediction clock offset. Besides, the data integrity rate of the orbit and clock product is better than 95%, and it is available for real-time PPP.4. Improving the Theory and Application of PPP based on Regional CORS AugmentationThe feasibility of augmentation from regional CORS is verified by theoretical deduction and experimental analysis. The service system of regional CORS is improved and the potential value is exploited. The augmentations include:(1) Based on the sparse regional CORS network, fast estimation of real-time satellite clock offset is achieved. By fixing satellite orbit, it is achievable to estimate satellite clock offset using a single station in theory. The algorithm of joint weighted estimation can enhance the reliability and stability. In order to obtain satellite clock offset with high sample rate, double difference between satellites and epochs algorithm is applied to estimate the clock variation between adjacent epochs, and the results are accumulated to the satellite clock offset calculated from difference between the satellites. The precision of satellite clock estimation is the same with IGS precise product.(2) ZTD of each reference station is extracted and modeling ZTD in a regional area is achieved. By applying static PPP algorithm, ZTDs of regional CORS reference stations can be calculated with millimeter accuracy. Considering the effect of station elevation, at least 4 stations are required for modeling. Spatial regression model can provide ZTDs to the rovers in or nearby the network with millimeter accuracy.(3) UPD can be separated based on regional CORS, and then PPP ambiguity resolution is achieved. For the reference stations, a three-step approach named "wide line-iono free-narrow line" is applied to calculate FCBs. For the rovers, the integer ambiguity can be fixed by receiving satellite UPD. It helps to make the positioning result more stable.The rovers achieve fast PPP calculation by receiving augmentation information from regional CORS. The convergence time is about 30% shorter than the original.5. Presenting Broadcast Ephemeris based Real-time PPP ConceptApplying the regional CORS based satellite clock fast estimation method, and taking broadcast ephemeris instead of precise ephemeris, an independent PPP algorithm is proposed. This algorithm can extend the range of precise positioning service out 500km from CORS peripheral area. The main innovations are summarized as follows:(1) Theoretical derivation and experiment result demonstrate that, there is coupling relationship between satellite orbit and clock offset, and they can be coupled as one parameter for estimation purposes. For PPP rovers in the network, the effect on the OMC vectors of observation equations caused by the coupling estimation is only a few millimeters.(2) The positioning service provided by traditional CORS is limited in the network. Hence, it is difficult to deal with the cases such as rovers from one network elements to another or directly out of the network (eg. offshore area). The broadcast ephemeris based real-time PPP algorithm not only achieves independent processing, but also performs better than traditional PPP algorithm. The position accuracy of the stations inside and outside the network improves by 38.8% and 36.1%, respectively, and the convergence speed improves by 61.4% and 65.9%, respectively. It could be an alternative solution for regional positioning service before global PPP service comes into operation.(3) Without the requirement of BDS precise ephemeris, precise point positioning based on BDS is achieved. The results show that, the horizontal accuracy of single day resolution is about 1cm, which is the same with PPP based on GPS. However, when it turns to the vertical direction, the accuracy is poorer, which decreases to 10cm to 20cm. The main reason is the uncertain antenna parameters of satellites and receivers for BDS. |
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