Research On The Key Technologies Of GPS/BDS Long-Range Network RTK

The Global Positioning System (Global Positioning System, GPS) of U.S. was built and put into use in 1994, from then on, GNSS has become one of the most important method of positioning and timing in daily work and life, that played a huge rold in military, agriculture, transportation, infrastructure construction, disaster monitoring, meteorology, earth science research and etc. In 2004, China officially started to build the second generation BDS, which currently has the capacity to provide navigation service to China and the surrounding areas, and the global BDS will be fullfilled before 2020. As the completion of BDS regional system, the CORSs (Continuously Operational Reference Stations System, CORS) in China is being upgraded or will be upgraded in the near future. Network RTK software is the soul for CORS, therefore researching on the theory and technology of GPS/BDS long-range network RTK is of important practical significance.The key technologies of GPS/BDS long-range network RTK mainly include three aspects:Firstly, ambiguity resolution for reference stations; Secondly, three-carrier ambiguity resolution for BDS; Thirdly, ambiguity resolution for rover stations.The procedure of ambiguity resolution for reference stations can divided into two steps:the first step, estimating float ambiguity solutions; the second one, searching for integer ambiguity solutions and followed with ambiguity validation. The ambiguity resolution speed and reliability is determined by the accuracy and convergence speed of float ambiguity solutions essentially. Furthermore, the accuracy of float ambiguity solutions is related to the observation model, the baseline solution mode and the atmosphere delay processing strategy. The single baseline and multi baseline solution mode are the two main baseline solution mode for reference stations ambiguity resolution, however, the correlation between the estimated parameters of different baselines is not taken into consideration in the single baseline solution mode, for the latter, only a group of independent baselines are solved, owing to the selection of independent baselines is not unique, it can not guarantee that the selected independent baselines have the highest success rate of ambiguity resolution all the time. Network Solutions mode is generally used in post GNSS data processing, independent ambiguities with the highest success rate is selected to be fixed, however, it is not applied in reference stions ambiguity resolution currentlly.Atmospheric delay is the largest error source which blocks ambiguity resolution for reference stions. Atmospheric delay mainly contains ionospheric and tropospheric delay. For the ionospheric delay, using the ionosphere weighted method to restrict the double differential ionospheric delay based on a priori information, can significantly improve the ambiguity resolution speed. What's more, since the double difference ionospheric delay has a strong temporal correlation during short time, using ionospheric delay prediction method can solve the problems of signal interruptions and cycle slips. However, the existing ambiguity resolutions methods for reference stations do not take into account the spatial correlation between the double difference ionospheric delays of different baselines, moreover, the tropospheric delay parameters are not restricted generally.BDS is the first satellite navigation system that all the satellites have three carrier signals in the world, which makes multi-system and multi-frequency network RTK positioning become possible. Looking for the best linear combinations is the key for three-carrier ambiguity resolution, with resonable linear combinations, not only the ambiguity resolution speed would be accelerated, but also the reliability of integer ambiguity solutions would be improved. Currently, the observations linear combination method is the mainly approach to looking for the optimal linear combinations, that the noise, the wavelength and the ionosphere effect of the combined observations should be considered clearly. This approach is very complex, if the satellite navigation system has four or more carrier signals, it would be very hard to find out the optimal combinations. What's worse, it lacks the only definite target to quantify the linear combinations, so the selected optimal combinations might be subjective.In terms of rover stations ambiguity resolution, the existing rover RTK positioning mode can be called forward mode:the rover upload its approximate coordinates, then the data processing center generates a virtual reference station based on the approximate coordinates and broadcast it to the rover users, finally, RTK positioning is completed by the rover receiver. The disadvantages of this mode include:it is difficult to develop the client software for rover receiver, limits the upgrading of network RTK algorithms, the data processing center is lack of usefull information from the rover users. Some scholars proposed the reverse RTK mode, in which the data processing procedure is inverted:the rover stations upload the original observation data to the data processing center who complete the RTK positioning aferwards, finally, the positioning results is returned to the users. In addition to solving the problems of the forward RTK mode, the reverse RTK mode hase some other advantages:it has better algorithm consistency and higher reliability, moreover, it is easy to process the rover stations'observations in post. However, study on the reverse RTK mode is not mature enough now, there are further research space.This paper focuses on the aspects of fast ambiguity resolution for reference stations, BDS three-carrier ambiguity resolution and rover stions ambiguity resolution, the main work and contribution of this paper is as follows:(1)The single baseline solution mode, multi baseline solution mode and network solution mode are compared, the linear correlations between the double diference ambiguities, double diference ionospheric delays and zenith tropospheric delays are taken into consideration in estimating float ambiguity solutions, network solution is achived by mapping and adding normal equations. Consequently, the initialization speed is improved and the right rate of integer ambiguity solutions increases to some extent.(2)A zenith wet tropospheric delay model is studied, the correlation between the zenith wet tropospheric delay and the heights of stations is analysed. The tropospheric delay parameters are restricted based on the heights of stations, which improve the accuracy of relative zenith wet tropospheric delay, and reduce the influence of tropospheric delay on the float ambiguity solutions. As a result, the initialization speed is accelerated and the right rate of the integer ambiguity solutions increase to some extent.(3)The spatial correlation between the double difference ionospheric delays of different baselines is taken into account in estimating float ambiguity soltuions, the float ambituity solutions based on ionosphere model constrainting method is achievd by mapping and adding normal equations. The initialization speed is improved notablely, and the reliability of integer ambiguity solutions is improved as well.(4)The advantages of reverse RTK mode are analysed, some mixed networking strategy with both reference and rover stations are compared. Both the correlation between the double difference ambiguities of reference and rover baselines and the spatial correlation between the atomospheric delays between different baselines are taken into consideration in estimating ambiguities, and network solutions in reverse RTK mode are achieved by mapping and adding normal equations. The fixing rate with single epoch data is improved relative to forward RTK mode, the reliability is improved as well. The result with multi epoch data in reverse RTK mode is also improved to some extent.(5)The concept of Ambiguity Precision Factor (APF) is proposed, which is used as the only indicator of the performance of linear combinations. The APF value of every linear combination ambiguity can be calculated with non-combined double difference observation model, so it is easy to find out the optimal linear combinations. In terms of long baseline, the rules of the optimal combinations are summarized. The short baseline is also studied, the residual ionospheric delay is taken into account, and linear combinations which is insensitive to residual ionospheric delay are found out.(6)GPS/BDS three-carrier long-range network RTK is realized, and its performance is analysed. In terms of reference stations ambiguity resolution, the initialization speed is improved with GPS/BDS dual-system relative to GPS single-system. In temes of rover RTK positioning, comparison on the fixing rate, availability rate and positioning accuracy of GPS/BDS dual-system and GPS single-system with difference cut-off elevation angles are carried out, which indicates that the availability of GPS/BDS dual-system is better, and the dual-system's ascendancy is very outstanding with a high cut-off elevation angle.