Performance Assessment and Integrity Support Information Estimation of CAS Real-time Orbits and Clocks Products

Objectives: The accuracy and integrity of real-time orbits and clocks are essential for achieving high-precision and assured positioning within the Global Navigation Satellite System (GNSS). Current assessments mainly focus on the short-term accuracy of real-time products from various analysis centers, there is a lack of extensive research on the integrity of real-time orbits and clocks. Methods: This study researches the real-time orbits and clocks of the Chinese Academy of Sciences (CAS) in 2022 as a case study. Evaluating the performance of real-time orbit and clock products from the perspectives of long-term products accuracy and dynamic precise point positioning (PPP) accuracy. Assessing the signal-in-space user range error (SIS URE) distribution characteristics of real-time orbit and clock products by analyzing the instantaneous signal-in-space user range error (IURE) calculated by 100 uniformly distributed stations worldwide. Finally, the integrity support information for the CAS real-time orbit and clock products is computed. Results: By comparing the CAS real-time orbits and clocks with post-processed precision products provided by GFZ, it is observed that the orbit accuracy of GPS and Galileo is better than 5 cm, with a standard deviation of clock errors better than 0.08 ns. However, the orbit accuracy of BDS and GLONASS is better than 15 cm, with a standard deviation of clock errors better than 0.3 ns. The kinematic precise point positioning using the CAS real-time products is conducted with observations collected from 28 globally distributed stations. The positioning accuracy of PPP with the combined four GNSS systems is better than 4 cm, and the corresponding convergence time is better than 11 min. The IURE RMS (Root Mean Squared) for GPS and Galileo is better than 4 cm, and the distribution of IURE can be accepted as a Gaussian distribution. GLONASS exhibits an IURE RMS within 10 cm, with noticeable differences among satellites, leading to an obvious leptokurtic and fat-tailed distribution; The BDS demonstrates an IURE RMS better than 11 cm, but the IURE distribution differs among different types of satellites, and the BDS-3 SECM satellites display distinct bimodal characteristics. Regarding the prior probability of constellation fault and satellite fault, the prior probability of GPS constellation fault is the smallest, which is 5.2×10^-5, the prior probability of constellation fault of other systems is less than 1.0×10^-3. The prior probability of GLONASS satellite fault reaches 2.7×10^-3, the prior probability of Galileo satellite fault is the smallest, which is 8.7×10^-4. Regarding the standard deviation of the SIS URE envelope σ_URE and the standard deviation of the SIS URE σ_URE , the difference between σ_URE and σ_URE of GPS and Galileo is within 4 cm; the difference between σ_URE and σ_URE of GLONASS is basically greater than 5 cm, compared with σ_URE , the difference of σ_URE between BDS-2 and BDS-3 is larger. Conclusions: Therefore, it can be concluded that the accuracy of the BDS and GLONASS real-time orbits and clocks products provided by CAS needs to be further improved to support high-precision and assured positioning.