Research On Multi-frequency GNSS/INS/OD Fusion High Precision Positioning Technology In Urban Environment

As a typical application scenario of car navigation system,urban environment has become the focus and difficulty of car navigation technology due to its frequent occlusion and significant multipath effect.The GNSS/INS combined system is the core component of the car navigation system.The existing methods mostly use single-frequency GNSS signals.It is difficult to fix the ambiguity,easy to appear floating point solutions,and difficult to guarantee the positioning accuracy and availability.The multi-frequency GNSS signal has the advantage that the ambiguity can be fixed in a single epoch,and the positioning error can be quickly eliminated after the satellite signal is restored,which is suitable for frequent occlusion scenes such as urban environments.On the other hand,the positioning performance of the GNSS/INS combined system when the satellite loses lock depends on the INS performance index.The addition of odometer(OD)is expected to improve this situation.Therefore,this paper deeply studies the multi-frequency GNSS/INS/OD fusion high-precision positioning method.The main research work of the paper is as follows:(1)The basic theory of multi-frequency GNSS/INS/OD fusion positioning is introduced.First,the basic theory of GNSS positioning is introduced,and the basic observation equations of GNSS,the linear combination of GNSS multi-frequency observations and the doppler velocity measurement equation are introduced respectively,and the optimal dual-frequency and three-frequency combination coefficients of each system are selected;then,the inertial navigation solution and error equation are derived;finally,the INS/OD combined update algorithm and error model are introduced.(2)The GNSS/INS/OD time synchronization data acquisition embedded platform based on GNSS board PPS+stm32 is developed.Taking the PPS signal of the GNSS board as the reference,the PPS signal is used as the sampling clock of OD and INS after pulse width adjustment and two-stage phase-locking frequency multiplication.Compared with the pure software time synchronization method,this method can effectively suppress the continuous drift of the sampling clock and improve the time synchronization accuracy.Using DMA to output sampled data,compared to blocking transmission,it can avoid the main program from blocking,improve the response speed of the program,and reduce the data output delay.(3)The INS-assisted dual-frequency/triple-frequency fusion GNSS ambiguity fixation method is studied.Use INS to calculate pseudoranges to replace the original observation pseudoranges,and use the three-frequency EWL/WL ambiguity that can be fixed in a single epoch to assist in fixing the dual-frequency WL ambiguity,and then assist in fixing the NL ambiguity.Compared with the existing single-frequency ambiguity fixation method,it can significantly increase the ambiguity fixation speed,avoid the problem of too long initialization time for the fixation of the single-frequency ambiguity,and improve the usability of satellite positioning in frequent occlusion scenarios such as urban environments.In Example 1,the existing NL ambiguity fixation method has a fixed ambiguity rate of 90.1%,and this method increases the ambiguity fixation rate of a NL to 97.9%.In terms of positioning accuracy,the overall positioning accuracy of the dual-frequency/triple-frequency fusion GNSS/INS is improved by 53.7% compared with the single-frequency GNSS/INS combination,and the speed measurement accuracy is improved by 35.5%.(4)The multi-frequency GNSS/INS/OD fusion positioning method is studied.An INS/OD aided multi-frequency GNSS cycle slip detection and repair method is introduced.Case 1 is used for testing,and the simulated cycle slips can be detected and repaired correctly.The addition of OD effectively reduces the satellite-to-ground distance error during satellite lockout period,and reduces the requirement of cycle slip detection for INS device performance indicators.On the basis of multi-frequency GNSS/INS,incremental OD measurement is added,the measurement equation of the fusion system is expanded,and the divergence of positioning error when the satellite is out of lock is suppressed.In case 1,this method is based on multifrequency GNSS/INS,the overall 3D positioning accuracy is increased by 21.7%,and the speed measurement accuracy is increased by 19.6%;in case 2,the addition of OD assistance significantly reduces the system during the loss of lock of the satellite Positioning error.