| The constellation deployment of the four global GNSS systems has been completed step by step,and the modernization process is also in rapid progress.Several regional satellite systems also provide rich positioning resources for users in the coverage area.At present,using multi frequency and multi system GNSS in the field of satellite navigation and positioning has become the future development trend.On the one hand,the development of Multi-GNSS has greatly increased the number of available satellites,providing users with a better satellite space geometry.On the other hand,in the process of modernization of various systems,many new signal frequency have been added,and there are many signals of the same frequency,so the compatibility and interoperability of various systems are enhanced.Making full use of Multi-GNSS to provide positioning and navigation services for users in complex environment(such as city)is the current research hotspot.For the complex environment of city,there are few available satellites of a single system because of the shelter of tall buildings and trees.At this time,Multi-GNSS relative positioning can effectively solve the problem of insufficient number of satellites of single system,enhance the observation model and improve the positioning reliability.Moreover,the tight combination(or inter-system double difference)between satellite systems can maximize the advantages of increasing multi system satellites.This paper introduces the main similarities and differences of each system and the unified method around the Multi-GNSS tight combination relative positioning technology,gives the mathematical model of tight combination relative positioning,calculates and studies the ISB characteristics of GPS/Galileo/BDS-2/BDS-3/QZSS systems on the receiver side of each common frequency signal,and verifies the performance of the tight combination relative positioning algorithm through the urban vehicle experiment.The main contents and research results are as follows:1.This paper introduces the difference and unified method of space-time datum of GPS/Galileo/BDS-2/BDS-3/QZSS satellite systems in detail,and introduces the satellite position calculation method of each type of satellite in each system,and discusses the premise and foundation for relative positioning of combined multi system.2.The mathematical model of relative location of Multi-GNSS loose combination and tight combination is derived in detail,and its differences,advantages and disadvantages are pointed out;the specific form of ISB is given,and three commonly used estimation and extraction methods are compared.3.Through two sets of zero baseline experiments,the ISB between different receiver pairs is calculated.For all the frequency of Multi-GNSS with the same frequency signal,the ISPB and ISCB of each baseline are displayed and analyzed one by one.The observation noise and ISB characteristics of each frequency are studied.The measurement data show that the ISB is stable in the time domain,but it varies with the receiver manufacturer and firmware version,so it needs to be considered in the tight combination positioning.4.For BDS system,we treat BDS as two independent satellite systems according to the difference of signal system between BDS-2 and BDS-3.Firstly,BDS-2 satellite-induced code pseudorange variations is verified by the analysis of experimental data,which needs to be corrected.Then the zero-baseline ISB experiment results show that between the Trimble Alloy receiver and Septentrio receiver involved in this study,there is no ISPB between BDS-2 and BDS-3,but there are ISCBs of 1.057 m and 0.149 m on B1 I and B3 I,respectively.5.The urban vehicle experiments were carried out to compare the relative positioning performance of tight and loose combinations,and the definitions of positioning availability and other indicators were given.The experimental results show that in a complex environment with a small number of satellites,the tight combination can bring significant improvements in positioning accuracy,ambiguity resolution rate,and positioning availability compared to the loose combination.Taking GPS/Galileo dual system single frequency positioning as an example,when the cut-off height angle is 30 °,the ambiguity fixation rate is increased by 22.3%,and the positioning availability is increased by 19.3%.When the number of satellites is sufficient,the performance is well-matched between the loose and tight combination.So the advantage of the tight combination is to solve the positioning problem in complex environments,which is of great significance for application scenarios such as autonomous driving,navigation in urban area,and positioning in canyons.6.Using the urban vehicle experimental data,comparison and analysis of whether or not that ISB correction is used in the combined positioning of BDS-2 and BDS-3 were carried out.The results show that the positioning performance of BDS system can be improved significantly after ISB is corrected in both DGPS and RTK modes.Taking the positioning availability of RTK as an example,when the cut-off height angle is 10 ° ? 30 °,it can be increased by about 15% ? 38%.Therefore,it is suggested that it is necessary to correct ISB in order to obtain better BDS service when using two generation system for navigation and positioning. |
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