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• 1. A quality control method for estimating multi-GNSS real-time satellite clock offset is proposed. • 2. The satellite clock offset accuracy of GPS, Galileo, BDS-3 and Galileo is improved. • 3.The precise point positioning accuracy can be improved by using the clock offset. Real-time satellite clock offset plays a prominent role in the Global Navigation Satellite System (GNSS) community. However, the GNSS raw observations are impacted by outliers, resulting in the quality of observations becomes poorer, which will dramatically affect the performance of estimated multi-GNSS satellite clock offset in real-time. In this contribution, an improved Inter Quartile Range (IQR) method is proposed to improve the quality of multi-GNSS satellite clock offset estimation in real-time, in which the improvements include two aspects. Firstly, the empirical threshold C 1 is determined to adapt multi-GNSS satellite clock offset estimation, if one residual is not between (Q 1 - C 1 ∗ I Q R , Q 3 + C 1 ∗ I Q R) , the observation is deleted. More importantly, to guarantee the clock offset of each satellite can be estimated at each epoch, if one satellite corresponding to all observation of stations is deleted, the IQR is conducted again at the current epoch for these observations. Three schemes are designed: 50 stations are used to perform multi-GNSS satellite clock offset estimation in real-time, which are without quality control (S1), with the proposed quality control method in this contribution(S2), and with quality control method using 8.5 medians of residuals (S3). Compared to S1, the satellite clock offset accuracy of GPS, Galileo, BDS-3, and GLONASS for S2 can be significantly improved from 0.082 to 0.055 ns, 0.106 to 0.072 ns, 0.178 to 0.083 ns, and 0.146 to 0.100 ns, respectively. The improvement is 32.93%, 32.07%, 53.37%, and 31.51%, respectively. The clock offset performance of S2 is better than that of S3 in terms of stability and reliability for GPS, BDS-3, and GLONASS. Another 52 stations are applied to perform multi-GNSS kinematic real-time precise point positioning (RT-PPP). For the convergence time and positioning accuracy, the RT-PPP employing the satellite clock offset generated from S2 shows the best performance. The experiment shows that the proposed quality control method can obtain real-time, stable, high-precision, and reliable multi-GNSS real-time satellite clock offset. [ABSTRACT FROM AUTHOR]

[Display omitted] • The long-term performance of GLONASS satellite clocks were revealed for the first time. • The periodic terms existed in the clock are detected. • The comparison between GLONASS and other GNSS satellite clock is conducted. The cesium atomic clock installed on the GLObal NAvigation Satellite System (GLONASS) is a key piece of equipment. However, its performance is easily affected by the external environment and its physical characteristics. Therefore, it is essential to evaluate the long-term performance of the clock. In this study, using eight years of continuous GLONASS precise satellite clock offset products, the long-term performance of GLONASS satellite clocks is revealed for the first time. The results showed that the mean number of phase and frequency jumps for one satellite clock in one year were 0.29 and 0.19, respectively. The fitting precision, which is the standard deviation of the fitting residuals, was 0.88 nanoseconds (ns). Except for the R06 satellite clock, the first or second period was 11.24 h in the clock offset. In terms of clock stability, the short-term clock offset prediction accuracy was 0.52 ns, the frequency stability at 1000 and 10000 s was 4.03 × 10-13 and 1.01 × 10-13, respectively. Both the short-term clock offset prediction accuracy and frequency stability were inferior to those of the rubidium atomic clock of the Global Positioning System (GPS) Block IIF satellites, the second-generation BeiDou Navigation Satellite System (BDS-2), Quasi-Zenith Satellite System (QZSS), and Galileo Passive Hydrogen Maser (PHM). The mean switching number for each GLONASS satellite clock was 0.27 per year. Furthermore, the R15 satellite clock, maybe owing to its hardware noise, delivered the poorest performance among those of all the GLONASS satellite clocks. [ABSTRACT FROM AUTHOR]