| Since the development of Global Navigation Satellite System (GNSS), it has been widely applied in many fields, such as satellite navigation, surveying and positioning, deformation monitoring, together with atmospheric detection, thanks to its global coverage, all-weather, high precision and efficiency as well as strong security.At present, dual-frequency receivers have been commonly used in GNSS precise navigation and positioning mainly to eliminate the ionospheric error. When carrying out researches on regional deformation monitoring and atmospheric probing based on GNSS technology, massive GNSS receivers have to be laid out in order to collect information with high spatial and temporal resolution. Under this circumstance, using GNSS dual-frequency receivers for all stations would no doubt be expensive, which will definitely limit the future development and application of GNSS technology greatly in these fields. Because of this, how to eliminate the ionospheric error and realize high-precision positioning using relatively cheaper single-frequency receivers in a wide area has become one of the research hot spot in the present satellite geodesy at home and abroad. In addition, how to realize the unification of existing GNSS technology that:the unification of PPP and network RTK technology, the unification of regional and global data-processing, the unification of single-and dual-frequency data-processing strategy are also the currently difficult problems to be solved. This dissertation paper focuses on investigating and solving the problems, mainly include performing in-depth research on the method of precise positioning using single frequency receivers, breaking through its key technologies, promoting the unification of existing GNSS technology and developing practical software for real application.Firstly, thorough investigations have been done on various kinds of model construction method for current regional error correction. Some improvement has been made to the Satellite-specific Epoch-differenced Ionospheric Delay model (SEID), which then has been applied successfully in the GNSS precise positioning using single-frequency receivers for large-scale reference network, yielding very good results. Secondly, the author has proposed a new network RTK approach based on undifferenced observation corrections. Although both methods could be used effectively in various GNSS single-frequency precise positioning services for post processing, real time or near real time applications, this new network RTK approach based on undifferenced observation corrections exhibits more general characteristics, which is a unification of various regional error fusion technologies. Integrating with advantages of PPP and network RTK, this new method realizes a technological unification and seamless joint service between global PPP and regional RTK by using the existing various kinds of satellite orbit and clock error products. Users inside and outside network could obtain precise positioning service with different accuracy requirements under the same data processing mode (that is PPP mode). Those who could receive regional error correction information inside the network would achieve rapid precise positioning results equivalent with network RTK, while others who haven't receive error correction from this region or outside the network would achieve PPP positioning accuracy.Finally, based on the existing PANDA software and the above methods, this paper has realized an epoch by epoch processing function for single-frequency data by using fortran programming language, and carried out effective simulation analysis towards the future prospect of this new network RTK approach based on undifferenced observation corrections to be used in real time PPP-mode precise positioning application. Main contributions and innovative achievements of this dissertation paper include:1) Current status of GNSS precise positioning at home and abroad has been thoroughly reviewed from aspects of GNSS mathematical model, error correction technology, precise positioning method as well as software development. Aiming at the key contents of this paper, deep discussion and summarization have been made on the GNSS precise positioning using single-frequency receivers together with model construction for regional error correction;2) Detailed investigation has been done on the theory of GNSS precise positioning, including various spatial-temporal frames involved in precise positioning and their transformation, observation equation and error correction, as well as least square parameter estimation algorithm;3) Various regional error correction model construction methods have been illustrated and analyzed systematically, which could be demonstrated as a comprehensive summary towards this kind of technology;4) Theoretical consistency between network RTK and the HiRIM method has been proved, pointing out that there is only little difference in concrete implementation between them. This paper has conducted theoretical description for network RTK and the HiRIM method, and focused on the comparison of their error correction effects. The equivalence and difference between these two kinds of method have been demonstrated clearly. Then some beneficial conclusions have been drawn;5) The SEID method has been developed, and a complete set of data processing strategy for GNSS precise positioning using single-frequency receivers has been proposed. As for the reference network ranging smaller than 90km, its daily static measuring accuracy could almost achieve the precision obtained by dual-frequency data based on the SEID method by only using single-frequency data inside the network, which could satisfy the requirement of mm-level daily static precise positioning at the vertical component for single-frequency receivers;6) A new network RTK approach based on undifferenced observation corrections has been put forward in this paper, which exhibits a unification of the current regional error fusion technologies, and belongs to a more general model construction method. By effectively eliminating the influences of ionospheric delay, tropospheric delay, receiver hardware delay, as well as satellite orbit and clock error, this method realizes a technological unification and seamless joint service between global PPP and regional RTK;7) Mathematical models for epoch-differenced ionospheric delay correction and network RTK approach based on undifferenced observation corrections have been rigorously deduced. Two sets of software for regional error correction model construction have been developed; 8) Module composition for GNSS single-frequency data processing software has been investigated. Principle and formula for data quality control, ambiguity resolution, as well as least square parameter estimation method has been illustrated. Also the realization flow for data processing algorithm from single-frequency receivers has been given in this paper. Under these backgrounds, the author has participated in the development of GNSS single-frequency data processing software, verified the resolving ability and precision level of the new method as well as the software by using massive numerical examples, and in particular carried out effective simulation analysis towards the future prospect of this network RTK approach based on undifferenced observation corrections to be used in real time PPP-mode precise positioning application.
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