The long-term characteristics of GNSS signal distortion biases and their empirical corrections

Global Navigation Satellite System (GNSS) precise data processing depends on the accurate processing of various errors. However, the distorted GNSS live signals will result in systematic biases in pseudorange observations, namely, signal distortion bias (SDB). Studies have shown that GNSS SDBs are stable over a short time and can be well modeled according to receiver brands and models. Thus, we focus on analyzing the long-term characteristics of SDB and establishing their empirical corrections based on many GNSS observations from 2017 to 2019. The results show that the SDBs differ among different satellite system. For example, the SDBs are within ± 1 ns for all the signals of GPS and BDS-2 while they are within ± 0.5 ns for Galileo and QZSS. Also, most of the SDBs remained very stable even as the firmware version of the receiver was being upgraded during the 3 years. The portion of SDB STDs over 3-year series and within 0.1 ns are 93.2, 99.9, and 86.7% for GPS, Galileo, and QZSS, respectively. As for BDS-2, the SDB STDs within 0.1 ns and 0.2 ns are 70.0% and 96.8% due to the poor quality of pseudorange observations. Thus, the estimated GNSS SDBs are given as constant values for each satellite-receiver-group pair and signals in SINEX BIAS format. The validations show that both pseudorange residuals and STEC extraction of zero-baselines show about 1 ns systematic biases without SDB corrections. However, the RMS of pseudorange residuals decreases by 28.26 to 51.18% while the RMS of double-differenced STEC decreases by 12.5 to 49.2% for different satellite systems with SDB corrections. The results from 2017 to 2019 validate that most GNSS SDBs could be treated as constants if there is no update of satellite and receiver hardware. Also, it should be noted that SDB corrections may not be applicable for individual satellites and receivers when there are satellite signal fault or flex power. Therefore, it should be routine to calibrate the GNSS SDBs with the replacement of satellite and update of receiver brands and models.