| Satellite signals at low elevation angles are severely affected by atmospheric delay,multipath effects,and observation noise.Positioning often discards low elevation angle data by setting the cut-off elevation angle.The 15° cut-off elevation angle in traditional positioning may make low-angle satellite utilization under certain special conditions not high,such as satellite data with low elevation angle and high signal-to-noise ratio.The BeiDou navigation system is currently in the construction phase,and the number of satellites available to users is limited.In some specific regions,most BeiDou satellites are low-elevation satellites.Therefore,how to use low-angle BeiDou satellite positioning is a hot issue that needs to be studied.At the same time,since the BeiDou satellite’s constellation and signal frequency have its own characteristics,the difference between the navigation signals and other satellite systems needs to be considered.Relevant studies have shown that there are certain systematic deviations in BeiDou satellite navigation signals,and the source and impact of this deviation need to be further studied.In response to the above problems,the main research contents and conclusions of this paper are as follows:1.The characteristics of atmospheric delay and multipath effect in satellite signals with low elevation angles are analyzed,and the influence of geometrical intensity and vertical direction accuracy in positioning is researched.Through the datas from several observation stations,the atmospheric delay,geometry intensity,and vertical direction location accuracy under different cut-off elevation angles were analyzed.2.The systematic deviations in the measured data of the BeiDou satellites are analyzed.The results show that the deviations are related to the types of satellites,signal frequencies,and satellite elevation angles.Aiming at the bias of IGSO and MEO satellites,two modified models,piecewise weighted polynomial and neural network,are proposed.Extracting the smoothed value of the previous period of observation data by "Sliding Window Mean Recursive Method" to correct the GEO satellite systematic deviations of the current period.Using the measured data,the changes of the MW combination sequence and PPP positioning accuracy before and after the correction of the model were compared,and the effectiveness of the algorithm was proved.3.A method for solving low-angle satellite ambiguities between BeiDou network RTK reference stations was proposed.The common-view satellites between the reference stations are divided into high and low angle satellites.First,the ambiguities of the high-angle satellites are fixed.Using the fixed high-elevation satellite data,the constraints of tropospheric delay and ionospheric delay between low-elevation satellite reference stations are constructed to assist in solving the integer ambiguities of low-elevation satellites.The BeiDou observation data provided by the Hong Kong CORS station verified the effectiveness of the above method.4.The low-angle satellite positioning strategy for different positioning modes was studied.In the PPP,a “self-adaptive sliding window threshold method” is proposed for the defect that the fixed threshold of the traditional cycle slip method is insensitive to the special cycle slip in low-angle satellite data.For the tropospheric delay correction model,the performance of three tropospheric mapping functions at low cut-off elevation angles is compared,and the optimal selection of mapping functions at low elevation angles is analyzed.Aiming at the defects of single random model under different elevation angles,a joint stochastic model is proposed to improve the positioning accuracy at low elevation angles.In the network RTK,using the proposed low-angle satellite ambiguity resolution method,combined with "error synthesis linear interpolation method",the positioning accuracy of the rover is analyzed to verify the effectiveness of the strategy. |
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