## 1 Introduction  
Global navigation satellite systems (GNSSs) have experienced rapid development in recent years. Currently, both GPS and GLONASS are in full operation. As for BDS, the official declaration of positioning, navigation, and timing (PNT) services by regional BDS (BDS-2) for the users over the Asia-Pacific regions and by global BDS (BDS-3) for global users was made on 27 December 2012 and on 27 December 2018, respectively. The deployment of Galileo constellation is also progressing rapidly and will be finished in the next few years. As of August 2021, there are 31 GPS satellites, 26 GLONASS satellites, 15 BDS-2 satellites, 34 BDS-3 satellites (including four experimental satellites), and 26 Galileo satellites in orbits. The multisystem combination has been an emerging trend, as it can significantly enhance the solutions of satellite-based positioning technology, such as real-time kinematic (RTK) positioning, precise point positioning (PPP) and single point positioning (SPP). This benefits from the increased number and the improved geometry of the visible satellites. Among the satellite-based position services, SPP belongs to the open position service and is widely used in satellite navigation and low-accuracy positioning due to its flexibility, low cost, and simplicity. Despite the many benefits of multisystem combination, there are still many issues to be solved in the multisystem integrated data processing, especially in the aspect of compatibility among different GNSSs. A bias between observations from different GNSSs can be caused by the distinct time scales employed by them. In addition, the receiver-specific hardware delays are also distinct for different GNSSs. To achieve rigorous multi-GNSS position solutions, these intersystem biases (ISBs) must be properly handled. The introduction of additional parameters to estimate the ISBs is a widely used approach. However, this approach will reduce the redundancy of observations. The observations from at least two satellites for each satellite system can theoretically contribute to the solutions of multi-GNSS integrated positioning, which will limit its performance in the extremely constrained visibility environments. When only a satellite for a specific satellite system is available in the multi-GNSS combined processing, the observation residuals of this satellite will equal to zero and the ISB estimates will encounter a deviation from the true value [1]. Many researchers focused on the mitigation of ISB in precise relative positioning. The GPS/Galileo intersystem double-differenced model based on overlapping frequencies L1/E1 and L5/E5a was derived in [PERSON] and [PERSON] [2]. The results indicated that the between-receiver ISB could be ignored for the baseline with two receivers of identical types, while it was not the case for that characterized by different receive types. When the differential ISB was a priori corrected to recover the integer nature of double-differenced ambiguities between different satellite systems, the success rate of short-baseline ambiguity resolution (AR) with different receiver types could be comparable to the case with the same receiver types. [PERSON] and [PERSON] [3] also demonstrated that the obvious ISB could not be ignored for the baselines with receivers of different types in GPS/Galileo tightly combined precise relative positioning, and they also validated the benefits for phase AR from the introduction of the known ISBs. In addition, the results indicated that the single-epoch solutions could be achieved at a noise level of 1-2 mm and decimeters in terms of phase and code ISB, respectively. [PERSON] et al. [4] conducted the ISB estimation for the GPS/BDS-2/Galileo tightly combined RTK. For the code- and phase-specific ISB, the influencing factors included the adopted overlapping frequencies, the firmware versions, and the receiver types.