Understanding the characteristic of GLONASS inter-frequency clock bias using both FDMA and CDMA signals

Currently, four GLONASS-M+ satellites and two GLONASS-K1 satellites transmit CDMA signals on the G3 frequency. It is important to understand the inconsistency between the new G3 and traditional FDMA G1 and G2 signals, wherein inter-frequency clock bias (IFCB) is one of the important indexes to find the difference of triple-frequency carrier phase hardware delays. Using the geometry-free and ionospheric-free (GFIF) phase combinations and an epoch-differenced method, we use 152 globally distributed MGEX stations spanning 30 days to estimate GLONASS IFCB. GLONASS-K1 satellite R09 and GLONASS-M+ satellite R21 are selected for analysis in the experiment owing to enough G3 observations. Results indicate that the magnitudes of the multipath error and SNR on G3 frequency are noticeably smaller than those of the other two frequencies. A satellite-induced multipath error seems to exist on the G3 frequency of R21, whereas R09 has none, which needs to be quantized and modeled further. The intra-day peak-peak amplitudes of R09 and R21 IFCBs are about 0.01 and 0.2 m, while inter-day amplitudes are about 0.03 and 0.3 m, respectively; R21 is even larger than that of GPS Block-IIF satellites (about 0.2 m). The RMS and STD of the IFCB series of R09 and R21 are 0.90, 0.89 cm, and 10.56, 10.53 cm, respectively. Therefore, the IFCB errors must be carefully corrected in GLONASS G3-frequency applications. Fortunately, the IFCBs of R21 present both intra-day and inter-day sine-wave periodic variations, which may be modeled, and even well predicted in the future.