Quality Evaluation And PPP Performance Analysis Of GPS/BDS SSR Products

Precise Point Positioning(PPP)has wide application prospects because of its advantage of achieving centimeter-level positioning accuracy worldwide without relying on reference stations.With the revitalization and modernization of multi-system GNSS and the growing demand for real-time and high-precision applications,provides State Space Representation(SSR)provided by the International GNSS Service(IGS)products are becoming more and more abundant.Evaluating GNSS real-time SSR products and their positioning performance in different analysis centers is of great importance for real-time PPP users.The traditional PPP model requires a longe convergence time to achieve centimeter-level positioning accuracy,and the convergence time of nearly 30 minutes constrains the application of PPP in the field of real-time,and the Ambiguity resolution(AR)can improve PPP positioning accuracy and shorten the convergence time to some extent.However,its initialization time still cannot meet the requirements of real-time navigation and positioning.The atmospheric augmentation information extracted based on the regional reference station network can effectively shorten the first PPP initialization time.To this end,this paper evaluates the existing publicly available precise orbit,clock offset and phase bias and their PPP performance,and develops a set of PPP-RTK software for experimental verification.The main research contents and results are as follows:(1)In order to distinguish the accuracy differences of SSR products in different analysis centers,the quality and kinematic PPP positioning performance of SSR products broadcast by six commonly used analysis centers were evaluated.The results show that among the 6 analysis centers,WHU’s GPS satellite has the best 3D orbit accuracy(3D RMS is 3.3cm)and clock error accuracy(STD is 0.05ns).The Signal-In-Space Ranging Errors(SISRE)of GPS satellites from six ACs are in the following order:WHU < GMV < CNE < DLR < GFZ < CAS;BDS orbit and clock accuracy is relatively poor,the SISRE values of MEO satellite precise products of BDS-2 and BDS-3 in WHU analysis center with the best performance were 8.83 cm and 5.91 cm,respectively.In GPS-only kinematic PPP mode,DLR has the slowest convergence speed(38.9 min),while GFZ has the fastest convergence speed(26.7min).After adding BDS satellite,there is no significant difference in positioning accuracy,but it can effectively improve the convergence speed(6%-25%).(2)In order to distinguish the quality difference of different AR products,the quality of AR products published by CNES and GFZ and their AR performance were evaluated.The results show that the wide-lane/narrow-lane ambiguity residuals of GPS,BDS-2 and BDS-3 of CNT and GBM products are approximately subject to normal distribution.As for the residual distribution of wide-lane ambiguity,the percentage of CNT’s three systems,BDS-2 and BDS-3 of GBM were all greater than94% within ±0.25 cycles,while the percentage of GPS wide-lane residual of GBM was relatively low,only 74.11%.As for the residual distribution of narrowlane ambiguity,the percentage of the threesystems in CNT within ±0.25 cycles is close to 90%.Within ±0.25 cycles,the percentage of GPS and BDS-2 in GBM was close to 82.7%,while the percentage of BDS-3 was relatively low(74.73%).The positioning accuracy of single-system GPS PPP fixed solutions of CNT and GBM only differs by mllimeter level,and the 3D RMS values are 3.11 cm and 3.72 cm,respectively.The average first time to fix(TTFF)and fixing rate of CNT were 5.88 min and 96.47%,respectively,which were slightly better than GBM products.(3)The PPP-RTK software is developed and implemented,and the positioning effect of the PPPRTK user terminal is verified.Based on the real-time precise orbit,clock offset and phase bias products provided by CNES,the slant path ionospheric and zenith tropospheric modeling products of the service region were generated by using Shaanxi CORS data.The results show that the modeling error of the zenith tropospheric delay is better than 2cm,and the ionospheric modeling error of each satellite is less than 5cm.When atmospheric products were added for constraint,the time to first fix(TTFF)of static station was significantly shortened,and 55% of the session could achieve fixed ambiguity in a single epoch.The PPP positioning results of the vehicle-borne observation are roughly in decimeter level.Adding the external atmosphere as a constraint can significantly improve the fixing rate of ambiguity and positioning accuracy,and the positioning accuracy of PPP-RTK is within 15 cm.