Considering inter-frequency clock bias for GLONASS FDMA plus CDMA precise point positioning

Nowadays, GLONASS is providing CDMA signals on the third G3 frequency of two GLONASS-K1 and four GLONASS-M + satellites, making it possible for the joint use of GLONASS FDMA and CDMA signals for precise point positioning (PPP). However, there are two main obstacles to GLONASS triple-frequency PPP. First, a triple-frequency PPP model that simultaneously uses GLONASS CDMA and FDMA signals is currently available. Second, significant IFCB errors are noticed, defined as the difference between satellite clocks computed with different ionospheric-free carrier phase combinations. Therefore, this contribution presents a new GLONASS FDMA + CDMA PPP model considering IFCB errors. A total of 135 globally distributed MGEX stations with 150-day datasets are utilized to estimate GLONASS IFCBs, and another six stations are selected to validate GLONASS triple-frequency PPP. Results indicate that GLONASS IFCBs are satellite dependent and exhibit periodic signals. Peak-to-peak amplitudes of the 150-day IFCB series are in meters: (- 0.53, - 0.36, R04), (- 0.42, - 0.52, R05), (- 0.04, - 0.04, R09), (- 0.68, - 0.52, R12), (- 0.62, - 0.50, R21), and (- 1.68, - 1.16, R26). Unlike GLONASS-M + satellites, no obvious IFCB errors of GLONASS-K1 satellite R09 can be observed. This difference in IFCBs may originate from GLONASS satellite types. Besides, the average normalized cross-correlation values of satellite R05, R21, and R26 between IFCB series of two days with an interval of eight days are about 0.88, 0.96, and 0.93, respectively, which can be expected to be modeled even predicted future. With precise IFCB products, triple-frequency PPP can be performed. After employing IFCB corrections, the average positioning accuracy of GLONASS triple-frequency PPP is improved from (5.8, 11.4, 11.3, 16.9) mm to (3.6, 5.4, 7.7, 10.1) mm in the north, east, up and 3D components, respectively. However, the additional third CDMA frequency has only a marginal contribution to improving positioning accuracy compared with dual-frequency solutions.