Research On Tightly Combined BDS/GNSS Multi-frequency RTK Positioning Method

BeiDou Navigation Satellite System(BDS)is the infrastructure of the national Position Navigation and Timing(PNT),as well as the important member of Global Navigation Satellite System(GNSS).The technology of Real Time Kinematic(RTK)is widely used in the field of GNSS to provide centimeter level positioning accuracy.Compared with the single system method,the traditional multi-system RTK increases the number of available satellites.The ambiguity resolution success rate and the positioning performance can be improved simultaneously.However,under the urban or canyon environment,the satellite signals can be seriously blocked,which leads insufficient satellite signals for RTK.Meanwhile,the elevation can be high for the available satellites,and the performance of traditional multi-system combination RTK positioning mode will have large positioning errors or even positioning continuity interruption,which severely restricts the application of satellite navigation.Therefore,the tightly combined BDS/GNSS multi-frequency RTK method is proposed to achieve high precision positioning and solve the issue of limited available satellites.Building the tightly combined observation model through choses the same GPS pivot satellite with BDS,so that the redundancy of observation can be improved.The single difference ambiguity of pivot satellite is calculated to establish the estimation model of inter system bias(ISB).Based on the stable features of ISB,the impact of ISB on the integer ambiguity resolution of tightly combined method is eliminated by calibrating the estimated ISB.The tightly combined method with dual-systems and multi-frequency can fix the integer ambiguity instantaneously.The tightly combined RTK positioning performance is limited by the low precision of pseudo-range observation.By introducing the MW tightly combined method,the performance of positioning performance of the tight combination can be further improved.By considering the wide lane ISB,the MW tightly combined model is constructed to replace the due-frequency pseudo range observation of multi-frequency tight combination.The influence of ISB of MW wide lane on the fixed ambiguity of tight combination is calculated and eliminated.Finally,the ambiguity of double-frequency narrow combination is fixed.Compared with multi-frequency tight combination,introducing MW tightly combined observation can further improve the positioning performance of tight combination RTK.In order to verify the performance of the tightly combined method,static and dynamic tests were carried out to simulate the constraints of receiving satellites by increasing the cut-off altitude angle of satellites.The stable features of ISB has less than 0.1 cycle influence for fixing integer ambiguity,and the method of tightly combined can be used.When the number of satellites is large,the due-system and due-frequency tight combination,MW tightly combination and loose combination methods studied in this paper all have good positioning performance;with the decrease of satellite number,the fixed rate of ambiguity of loose combination decreases rapidly,while the fixed rate of ambiguity of tight combination and MW tight combination can still maintain a high fixed rate of ambiguity,and the fixed rate of ambiguity of MW tight combination is higher than that of tight combination.Among them,in the static test of 50° cut-off altitude of satellite,the fixed rate of tight combination is increased by 115%,the fixed rate of MW tight combination is 0.1% higher than that of tight combination.In the dynamic test of 60° cut-off altitude of satellite,the fixed rate of tight combination is increased by 91%,the fixed rate of MW tight combination is 9.4% higher than that of multi-frequency tight combination and 105% higher than that of loose combination.The results of static and dynamic tests show that when the number of satellites is insufficient and the occlusion is serious,the positioning performance of the multi-frequency tightly combined RTK positioning method is obviously improved compared with the traditional loose combination mode;compared with the MW tightly combined RTK positioning method and the multi-frequency tight combination,the positioning performance is improved,the positioning accuracy of the RTK is effectively improved,and the RTK method effectively expands its applications.