Estimation of GLONASS Inter-Frequency Clock Bias Considering the Phase Center Offset Differences on the L3 Signal

GLONASS has been transmitting the code division multiple access navigation signals on the third band L3 for six GLONASS-M + and three GLONASS-K satellites, in addition to the legacy frequency division multiple access signals on L1 and L2. However, the significant inconsistency between the phase bias of different ionospheric-free (IF) combinations for GLONASS-M + satellites, known as the phase inter-frequency clock bias (IFCB), hinders the utilization of GLONASS triple-frequency positioning. We discuss the coupling relationship between GLONASS IFCB and the phase center offset (PCO) and propose a new IFCB estimation model considering the PCO differences between L3 and L2. GLONASS triple-frequency observations from 151 globally distributed IGS stations are employed to validate the proposed IFCB estimation model. The results show that the mean root mean square (RMS) value of IFCB estimates decreases from 0.097 m to 0.028 m when considering PCO differences, suggesting the GLONASS IFCB is ignorable. Meanwhile, the L3 PCO estimates for GLONASS-M + satellites exhibit high stability and consistency, with standard deviations of 52, 113, and 13 mm, in x -, y -, and z -components, respectively. By correcting the estimated L3 PCO instead of the legacy IFCB, the GLONASS-only triple-frequency precise point positioning PPP achieves positioning accuracies of 1.8, 0.9, and 1.5 cm in east, north, and up components, with the improvement of 13%, 3%, and 33%, respectively. Moreover, the RMS value of L3 phase residuals reduces from 10.2 to 5.0 mm. Therefore, we recommend correcting the PCO on L3 for GLONASS-M + satellites and disregarding the IFCB for GLONASS triple-frequency positioning, which can significantly simplify the observation model and achieve higher accuracy.