| Large-scale and global high-precision positioning technology plays an important role in the fields of crustal deformation monitoring,earth reference frame maintenance,and time synchronization.As a traditional high-precision positioning method,Global Navigation Satellite System(GNSS)relative positioning has been widely used in related fields such as geodesy and engineering construction in the past decade.Compared with the current Precise Point Positioning(PPP)technology,the relative positioning technology has low requirements on the accuracy and self-consistency of satellite orbits and clock offsets products,and has strong dynamic adaptability.It can quickly resolution integer ambiguity and obtain high-precision positioning results.However,for a long time,due to the influence of satellite broadcast ephemeris error and atmospheric delay error,the error correction model is not fine enough,and the communication technology between stations are limited,the difficulty of carrier phase integer ambiguity resolution increases,and the distance between stations in real-time relative positioning is limited.With the construction and improvement of multiple global navigation satellite systems such as GPS,GLONASS,BDS and Galileo,more and more satellites and observations can be used by users.With the increasing number of ground tracking stations and the advancement of data processing technology,the broadcast ephemeris accuracy of satellite navigation systems has been greatly improved.In addition,the accuracy of ultra-rapid and real-time satellite ephemerides and clock products provided by IGS,MGEX and iGMAS is getting higher and higher,and the development of wireless network communication technology is also relatively mature,which provides new opportunities for the development of GNSS long baseline precise real-time relative positioning.Focusing on the topic of GNSS long baseline precision real-time relative positioning technology,this paper mainly studies the mathematical model and error influence,ambiguity resolution and real-time data flow quality control in long baseline precision relative positioning.The main work and achievements of this paper are as follows:(1)The current development status of long-baseline precise real-time relative positioning technology is summarized,and the feasibility of long-baseline precise real-time relative positioning is discussed.Based on the original observation equation,the characteristics and applicable scenarios of combined observations commonly used in GNSS relative positioning are systematically summarized.Finally,the basic theory and method of parameter estimation are introduced.(2)The mathematical model of GNSS long baseline precise relative positioning is expounded.The influence of GNSS broadcast ephemeris orbit error,ionospheric delay error,tropospheric delay error and tidal correction on single difference observation are analyzed.The variation of these errors with baseline length are also analyzed.The influence of single/multi-system on the common-view satellites number and the influence of multi-system combination on the positioning accuracy are analyzed.Analysis results show that when the baseline length is less than 500 km,the influence of broadcast ephemeris orbit error of GPS and BDS(non-GEO)satellites on baseline is less than 2 cm,and the influence of broadcast ephemeris orbit error of Galileo satellite on baseline is less than 1 cm.The GEO satellite in BDS will cause systematic deviation to the single-difference observations between stations,up to 5.2 cm.The accuracy of tropospheric delay can only reach decimeter level by using Saastamoinen+GPT2 model correction method,and the accuracy is better than 1 cm by using parameter estimation method.For the baseline within 1000km,the effect of tidal correction on the horizontal direction is less than 2 cm and the effect on the elevation direction is more than 5 cm.The positioning accuracy of two long baselines(266 km and 456 km)is analyzed based on broadcast ephemeris.Compared with the single system,the multi-system combination has obvious advantages in improving the number of common visible satellites and positioning accuracy.The horizontal accuracy of the three-system combination(floating ambiguity)is better than 2 cm,and the elevation accuracy is better than 4 cm.(3)The integer ambiguity resolution algorithm in the long baseline is studied.The ambiguity of ionosphere-free combination does not have integer characteristics.It can be decomposed into the combination of wide-lane ambiguity and narrow-lane ambiguity.In this paper,the fixing rates of wide lane and narrow lane ambiguity in different systems are compared,and the influence of ambiguity fixing on the positioning results is analyzed.The differences of positioning accuracy and convergence times between PPP and relative positioning are compared and analyzed.The results show that the MW combination is only affected by the pseudorange observation error,and the fixation rate can reach more than 90%after 2～3 minutes of smoothing.The system deviation related to the height angle of BDS satellites will lead to the decrease of the fixation rate of wide lane ambiguity.The LAMBDA method needs a certain convergence time to fix the ambiguity of narrow lane.The test results show that the multi-system combination can significantly shorten the time to first fix(TTFF)to an average level of 13 minutes.The integer ambiguity resolution accuracy of GPS,BDS and GPS+GAL+BDS mode is 25.2%,14.0%and 18.5%higher than that of floating ambiguity solution,respectively.The positioning accuracy of relative positioning is better than PPP in single system.In multi-system combined positioning,the accuracy of relative positioning in the elevation direction is better than that of PPP,and the convergence time is less than that of PPP.Combined with damping LAMBDA algorithm and partial ambiguity fixing algorithm,the generalized damping LAMBDA algorithm is proposed.Compared with the classical LAMBDA algorithm,the fixed rate of the improved algorithm is increased by 28.8%.(4)The method of obtaining and decoding IGS real-time data stream based on NTRIP protocol is introduced.The algorithm of real-time precision orbit recovery is discussed.Aiming at the problem of real-time product interruption,a polynomial fitting method of real-time orbit correction number is proposed in the update period of the broadcast ephemeris,and the fitting accuracy is better than 0.01m.Designed and implemented a set of long baseline precision real-time positioning software.The real-time data streams of 4 stations in the Australian CORS network were selected to analyze positioning effect of 4 long baselines(256km～996km).The experimental results show that:using the real-time precise ephemeris+partial ambiguity resolution strategy,the GPS+GAL+BDS combination mode can achieve a positioning accuracy of better than 2 cm in the horizontal direction and 4 cm in the elevation direction in a baseline with a length of less than 1000km.The average time to first fixing is 16.75 minutes,the averaged fixing rate is 78.8%. |
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