Research On The Key Technologies Of GNSS RT-PPP&RTK And Their Integrated Service Method

With the development of Global navigation satellite system(GNSS)and positioning technology,the GNSS users’ demand for positioning availability,reliability and accuracy is increasing,the meter-level positioning results can not be satisfied by customers.Currently,the main positioning technology used in GNSS real-time high-precision applications are RTK and PPP technology.The RTK technology is most widely used in the market,however,as the baseline distance increases,the correlation of atmospheric errors will decrease.The reliable PPP fixed solution depends on high-precision orbit,float clock/integer clock or UPD products.In the actual application,we should pay attention to the orbit error which will influence the integer clock/UPD and PPP-AR solution.In addition,with the development of regional reference station network,we can provide fast high-precision service for the users by using atmosphere information extracted from CORS network.However,the service methods are independent for the PPP and RTK users.Also,we should consider the impact of communication latency in real-time high-precision positioning.In view of the above problems,this paper mainly focuses on GNSS medium-long baseline RTK positioning,GNSS real-time integer clock/UPD estimation and PPP-AR positioning,real-time fast and precise positioning service method for RT-PPP and RTK users,and GNSS precise positioning considering the communication latency.The main work and contribution of this paper are as follows:1)The factors affecting the BDS medium-long baseline RTK positioning are analyzed and a troposphere constrained RTK model is proposed.For medium-long baseline RTK positioning,the troposphere parameter is introduced along with coordinates,and the model is ill-conditioned due to its strong correlation with the height parameter.For the BDS system,due to a lack of variation for BDS GEO satellites and nonhomogeneous satellite distribution,the height component was more difficult to distinguish from the troposphere zenith delay compared with the GPS.As a result,the performance of BDS medium-long baseline RTK was worse than that of GPS,particularly for the height direction.Even for the GPS/BDS combined RTK,although the geometric structure is evidently improved,due to the lack of variation in geometry structure,the convergence time of ambiguities for the BDS GEO satellite was significantly longer than that of satellites with other orbit types,which may result in lower precision in height direction in the initial period.The troposphere constrained model can alleviate the ill-conditioned problem of medium-long RTK model.In the case of 15 baselines,the results show that the GPT2-constrained model can improve the performance of medium-long baseline RTK positioning.The TTFF(time to first fix)of BDS and GPS/BDS combined RTK was decreased by 30.9%and 33%when the GPT2-constrained RTK model was used.The STD of the height component was reduced by an average of 52.4%and 34.0%for BDS-only and GPS/BDS combined situations,respectively.In terms of RMS error,the average performance was improved by 40.0%and 19.8%for the BDS and GPS/BDS combined situations,respectively.It is difficult to obtain accurate troposphere information under tricky troposphere conditions,as a result,a larger variance is usually proposed to ensure reliability.2)The method that estimating the integer clock by the regional reference station is proposed to improve the accuracy and reliability of PPP fixed solution.Although96% of the orbital radial error can be compensated by the satellite clock,the PPP user will be affected by the orbit residual error when global orbits and integer clock/UPD products are used.The GPS satellite residual orbit error of CNES products is usually under 5 cm,the orbital residual error for BDS satellite is larger,and the orbit residual error of the IGSO and MEO satellites may exceed 10 cm.Using the regional integer clock product can effectively compensate the orbit residual error,thereby improving the positioning accuracy and initialization speed for the PPP users.In order to validate the performance of regional integer clock product,three rover stations within the coverage of the regional reference station and the two rover stations outside the coverage area(about 150 km and 600 km)are used to conduct PPP dynamic positioning.Compared to the CNES global product,the horizontal accuracy of GPS PPP fixed solution is improved by 53% on average,and the vertical accuracy of the fixed solution is improved by 44%.For GPS/BDS combined situation,horizontal accuracy of the PPP fixed solution is improved by 65% on average,and the vertical accuracy of PPP fixed solution is improved by 50%.The position accuracy of PPP fixed solution can be comparable to the RTK solution when regional integer clock is used,and the horizontal accuracy can reach to 0.5 cm.Compared to the CNES global product,the initialization time of PPP positioning is significantly shortened when regional integer clock products are used.The TTFF of GPS PPP is shortened from 22 minutes to 14 minutes,whereas the TTFF of GPS/BDS combined PPP is shortened from 18 minutes to 13 minutes.3)On the basis of regional integer clock,the method of providing fast and precise positioning services for both PPP and RTK users are studied.First,the UPD products that support the ambiguity resolution(AR)for both ionosphere-free(IF)and undifferenced and uncombined(UDUC)PPP are studied.The new UPD products are generated from regional integer clocks,wide-lane UPD,global floating clock and DCB products.When the new generated UPD products are used for the IF and UDUC PPP,the positioning accuracy of the fixed solution is comparable to the positioning results using the regional integer clock,and the horizontal accuracy can be about 0.5cm.The initialization time of UDUC PPP mode is faster than IF PPP mode for the reason that the narrow lane ambiguity parameters can be updated by the fixed WL ambiguity,however,it still takes more than 10 minutes to obtain the fixed solution for UDUC PPP.Based on the atmosphere information extracted from reference stations after the undifferenced ambiguity is fixed,the method of generating VRS observations and PPP augmented atmosphere information for RTK and PPP user is studied.The atmosphere information extracted from the reference stations(the inter-station distance is about 100 km)is used to speed the initialization time of RTK and PPP users.The rover data is initialized every 10 minutes,the results show that 60% of sessions can get a fixed solution by single epoch,and 96.4% of sessions can get a fixed solution within 4 epochs(the sampling rate is 30s)for PPP users.For RTK users with VRS observations,87.7% of sessions can get a fixed solution by single epoch,and 95% of sessions can get a fixed solution within two epochs.It should be noted that the RTK positioning based on the VRS observation may introduce atmospheric modeling errors,and the positioning accuracy will be reduced when the atmospheric modeling accuracy is poor.In the PPP-RTK mode,the accuracy of the fixed solution will not decrease when the stochastic model for the external atmospheric delay is set properly.4)The time delay related error terms in ARTK model and their impacts on RTK results are studied.In the asynchronous RTK mode,the degradation of position RMS precision can reach a few centimeters,but the accuracy degradation to a different degree by different systems.BDS and Galileo ARTK usually perform better than GPS and GLONASS due to the smaller variation of broadcast ephemeris error.Based on selective precise ephemeris products,the time variation of broadcast ephemeris error is assessed.The average RMS statistics of broadcast orbit error variation over the latency of 15 s are 1.6,9.3,2.6,and 1.4 mm for GPS,GLONASS,BDS,and Galileo.The average RMS statistics for the variation of broadcast clock offset error are 10.6,14.0,6.3,and 2.6 mm for GPS,GLONASS,BDS,and Galileo,respectively.Overall,the clock offset error influence appears more serious for the ARTK model.The performance of ARTK decreases due to the influence of ionosphere error variation.The variation of ionosphere error for some satellites over 15 seconds can reach a few centimeters under higher ionosphere activity sessions.In addition,compared with the other systems,BDS ARTK shows an advantage under higher ionosphere activity levels.The possible reason is that the variation of ionosphere error for five BDS GEO satellites is usually small because of their geometry structure almost staying the same.The performance of multi-GNSS combined ARTK solutions can be improved by considering the different characteristics of broadcast ephemeris and ionosphere error variation analyzed in this study.