## 1 Introduction  
The GPS, as one of Global Navigation Satellite Systems (GNSSs), was first applied for time transfer by [PERSON] and [PERSON] [1] in the 1980s. Afterwards, a technique called Common-View (CV) was employed for International Atomic Time (TAI) comparison. This method provided an opportunity for high-precision (several nanoseconds) time transfer with a low-cost receiver [1]. Furthermore, with development of the precise products released by the International GNSS Service (IGS) [2; 3], precise point positioning (PPP) approach, using phase and code observations, was applied for time and frequency transfer in the time community [4; 5; 6]. PPP has been utilized to compute time links for TAI since September 2009 and is currently used by upon 50% of more than 70 laboratories in the word contributing to TAI and Universal Time Coordinated (UTC) computation [7; 8]. Compared with GNSS code-only techniques, such as CV and All-in-View (AV) [9], better short-term stability in time transfer can be achieve with PPP. The present typical uncertainty of PPP-based frequency comparison is about \(1\times 10^{-15}\) at 1-day average and about \(1\times 10^{-16}\) at 30-day average, where corresponding to type A uncertainty of 0.3 ns for time-links in the Bureau International des Poids et Mesures (BIPM) Circular T. Moreover, the integer-PPP (IPPP) technique implemented by CNES (Centre National d'Etudes Spatiales) was first applied to perform frequency transfer [10]. The results demonstrated that the IPPP technique allowed frequency comparison with \(1\times 10^{-16}\) accuracy in several days and could be readily operated with existing products. With multi-GNSS development, multi-GNSS techniques are widely utilized in timing community. The second GNSS, Russia's GLONASS, has been reinvigorated since October 2011 and is now fully operational with 24 satellites. Furthermore, the number of IGS stations, which can track GLONASS satellites, is increasing [11]. Hence, researchers have begun to investigate the performances of GLONASS positioning [12; 13], ionospheric studies [14; 15], tropospheric studies [16] and time transfer [17]. Unlike GPS, the GLONASS carrier phase and pseudorange observations suffer from different frequencies and inter-frequency biases due to the signal structure of GLONASS, which is based on the frequency division multipath access technique [18]. Many studies have investigated the feature of inter-frequency phase biases (IFPBs) and indicated that a liner function of the frequency number can be employed to model the IFPBs [19; 20]. Moreover, several studies investigated the characteristics of inter-frequency code biases (IFCBs) and demonstrated that IFCBs were dependent on receiver types, antenna types, domes and firmware versions [21; 22]. GLONASS pseudorange observations usually set a very small weight to reduce the effect of IFCBs [12]. However, this assignment will significantly reduce the contribution of pseudorange observations on PPP solutions, especially in the initialization phase. Hence, proper modeling of IFCBs is essential and critical for GLONASS-only PPP-based international time transfer. To fully understand GLONASS IFCBs, [PERSON] et al. [21] estimated the IFCBs using GPS/GLONASS observations about 133 stations. Their study demonstrated that the IFCBs of some receivers showed a linear function of frequency numbers, while other showed a quadratic polynomial function. [PERSON] et al. [23] employed different IFCB handling schemes in GPS/GLONASS satellite clock estimation, giving the conclusion that considering GLONASS IFCBs can achieve better positioning performance of GPS/GLONASS PPP. Furthermore, [PERSON] et al. [13] investigated the influence of GLONASS IFCBs on convergence time and positioning performance of GLONASS-only and GPS/GLONASS PPP, suggesting that the convergence of PPP will be reduced using GLONASS observations by more than 20% when considering IFCBs. To date, only a few studies have focused on combined GPS/GLONASS time transfer [17; 24].