Research On Ambiguity Resolution In Multi-GNSS Precise Point Positioning

Ambiguity fixed efficiency has always been the bottleneck which restricts the development of PPP,the positioning accuracy and convergence speed of PPP are also affected by many factors such as the geometry layout of the visual satellite and the decreasing degree of systematic error.In recent years,GNSS has shown a pattern of multimode and multi frequency development.It is possible to have multiple system fusion the PPP because the number of visual satellites and the types of observations of PPP users in the same location and the same epoch are increasing.Multi-GNSS can not only optimize the geometry layout of satellites,but also provide more alternative approaches for the processing of systematic errors,which contribute to fast fixing of ambiguity parameters and improve PPP positioning accuracy.However,due to the difference between kinds of satellite systems,Multi-GNSS is not just a simple combination of systems,and there are still many problems to be solved.On the basis of analyzing the related theories of Multi-GNSS PPP,this paper makes a deep research on the systematic error handling strategy,mathematical model and parameter estimation method,adaptive weight determination and fast fixing of ambiguity parameters of Multi-GNSS PPP.The aim of this paper is to improve the convergence speed and positioning accuracy of PPP.And the main contents of this paper are as follows:(1)The unified method of space-time datum for Multi-GNSS PPP and the processing strategy of systematic error are analyzed in detail.(2)The mathematical model of Multi-GNSS PPP and the process of parameter estimation are introduced.The performance difference of single/multi-GNSS ambiguity-float PPP is analyzed,too.(3)Due to the difference of range accuracy during the different observation types in the Multi-GNSS PPP,the traditional weighting method is no longer reliable,the Helmert variance component estimation is introduced into the Kalman filter to adjust the weight ratio of the various kinds of observations in the Multi-GNSS PPP.Meanwhile,because of the difference in unit-weight standard error between prediction weight ratio and observations weight ratio,the Kalman filter prediction value is taken as a quasi observation which is adjusted with observations by using Helmert variance component estimation.The experiments show that: on the basis of determining each observation weights of current epoch reasonably,this algorithm can balance the contribution to the parameter estimation efficiently between the observation information and prediction information,and it can improve the positioning performance of Multi-GNSS PPP effectively.(4)Based on Ionosphere-free combination model,the reason that the traditional PPP ambiguity can not be fixed is analyzed.In addition,the fixed characteristic recovery method of GPS single difference ambiguity based on the integer phase clock is realized.Moreover,a cascaded ambiguity fixing strategy considering quality control is given.The experiment shows that when the ambiguity is fixed,the positioning accuracy of PPP can be rapidly increased from the decimeter level to the centimeter level,and the positioning accuracy and stability of PPP can be improved obviously.(5)Considering PPP ambiguity fixing needs high-precision ambiguity-float,this paper constructs an algorithm of ambiguity fast fixing for GPS PPP by employing multi-GNSS observations which is based on the characteristics of fast convergence and high precision of Multi-GNSS ambiguity-float PPP.Experimental results prove that Multi-GNSS PPP can promote GPS ambiguity converge to higher precision quickly,and reduce the first fixed time of ambiguity parameter.