Font Size: a A A

Research On Real-Time Clock Offset Determination And Real-time Precise Point Positioning

Posted on:2012-05-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:W W SongFull Text:PDF
GTID:1110330344451878Subject:Geodesy and Survey Engineering
Abstract/Summary:PDF Full Text Request
Wide-area real time and precise positioning has become a major trend in the field of GNSS application development. With the progress in real-time orbit and clock offset determination, PPP and other core technologies of DGPS, DGPS system development usher in a new era featuring a decimeter-level global accuracy. The most remarkable examples of DGPS system include the Global Differential GPS system (DGPS) of NASA, StarFire of Navcom and OmniStar of Fugro. A GPS differential augmentation system based on the first generation of BEIDOU satellite navigation system as well as a number of Continuous Operating Reference Systems (CORS) run by provincial and municipal related industry departments have been set up. However, there is still no existing decimeter-level real time service system yet. In recent years, with support from our national High-tech Research and Development Programme (the so called 863 programme), Wuhan University has carried out a relevant research entitled 'A Wide Area Real Time and Precise Positioning Prototype System', aimed at setting up such a prototype system covering the Chinese territory and neighboring countries.'Wide-Area Real-Time Precise Positioning Technology' is a new kind of differential technology generating high accuracy and real time satellite orbit, clock offset as well as ionosphere correction products through innovation of core technologies such as real-time precise orbit and clock offset determination of navigation satellites on the basis of traditional wide-area differential positioning technology. Taking advantage of such products, PPP users with carrier phase measurements anytime and anywhere within a certain wide-area region (or even worldwide) can achieve dual- and single-frequency real-time dynamic positioning with accuracy of decimeter-level and less than one meter respectively. This paper probes into the theories of real-time satellite clock offset estimation methods and real-time PPP on the user terminal and related key issues. On basis of these theoretical methods as well as the PANDA software, an integrated PPP software system, with functioning modules of raw data collection from observation stations, precise clock offset determination, differential products broadcasting and terminal precise positioning. It is demonstrated that accuracy of dual-frequency real-time positioning is about 10 to 20 centimeters.The major contents of this paper are summarized as follows:(1) Combined influence of errors in precise orbit and clock offset information on users is analyzed, pointing out that while the majority of radial orbital error can be eliminated by precise clock offset, the tangential and normal ones remain uncorrected. Further analysis of assessment of precise clock offset products is made and three necessary criteria are suggested, namely, time reference bias, initial satellite clock offset bias and relative satellite clock offset accuracy. Positioning merely using phase measurements can only take account of relative clock offset accuracy. But when pseudorange observation equations are also adopted in positioning resolution, the effect of initial clock offset bias also have to be taken into consideration. While for a differential system, the systematic bias of the time reference should not be too large when compared to the GPS Time. (2) Thorough research on the estimation method of precise satellite clock offset is carried out, especially focusing on aspects such as initial satellite clock offset, computation efficiency and latency of products.The existing problems in current non-differential and difference-between-epoch estimation methods are discussed; initial satellite clock offset and its impact are studied theoretically and further analyzed and verified with real data. Based on these work, a combined differential precise clock offset estimation method is put forward. Using both pseudorange single-difference (between the satellites) and phase double-difference equations (between the satellites as well as the stations), this method adopts simultaneous estimation of all the equations, thus significantly improves the computation efficiency without the need to calculate a huge amount of non-differential ambiguity parameters. Meanwhile, the problem of neglect of initial satellite clock offset in difference-between-epoch estimation is also resolved.Impact of the estimation of troposphere parameters on precise clock offset estimation is studied in depth. A dual-thread method integrating estimation with and without troposphere parameters is proposed. The troposphere parameters resolved second by second in the threads are eliminated.A combined epoch clock offset estimation algorithm is presented to solve the accuracy loss due to latency of real-time products in user's real-time positioning. The extrapolating period of users is reduced to the largest extent by making full use of the data that have already reached the processing center. This can be achieved through converting asynchronous data to a unified observation epoch with clock offset velocity.(3) Some special issues concerning Precise Point Positioning in wide-area real time and precise positioning system are discussed.The impact of phase rotation on PPP is analyzed and it is pointed out that when rotation exists for the receiver's antenna, non-differential PPP will be biased if without proper consideration of the error of phase rotation. Thus single difference between the satellites are suggested as the observation in kinematic positioning when rotation may exists in receivers.The impact of high-accuracy ionosphere model on PPP convergence is also analyzed. With ionosphere correction in certain accuracy level, the convergence of PPP will be accelerated by replacing non-ionophere pseudorange observations with the values after ionospheric correction.Some special preprocessing issues of the real-time user terminal in PPP application is discussed including the impacts of possible rotation of receiver antenna in kinematic state, of single-frequency receivers under complicated kinematic circumstances and of obstacles resulting in loss-of-lock of the majority or even total of satellites. Double-difference preprocessing between epochs is presented, in which the impact of error is reduced with real-time estimated vertical tropospheric delay and slant relative ionospheric delay; in view of the way of double-difference ambiguity determination, it is possible to resort to integral least squares search of ambiguity difference between epochs to achieve rapid recovery and to avoid re-convergence that is needed in PPP when total loss-of-clock occurs in all satellites.(4) Based on the above mentioned theories, methods as well as the PANDA software, a real time precise satellite clock offset determination software and a real-time PPP software are developed. The system proposed in this paper is verified with global real-time data obtained through BKG as well as regional real-time data from observation stations in China. The real-time clock offset products are compared with IGS final products and their accuracies are evaluated and the accuracy of clock offset estimation is better than 0.2ns. Finally, positioning accuracy of the system is tested in static and dynamic PPP with dual-frequency terminal receiver and the system's real-time products. The horizontal and vertical accuracies of dual frequency real time positioning tests are within 10cm and 20cm respectively...
Keywords/Search Tags:wide-area, real-time, PANDA, MASS, real-time clock offset determination, combined epoch estimation, Precise Point Positioning, rapid recovery algorithm
PDF Full Text Request
Related items