It is foreseeable that the BeiDou navigation satellite system with global coverage (BDS-3) and the BeiDou navigation satellite (regional) system (BDS-2) will coexist in the next decade. Care should be taken to minimize the adverse impact of the receiver-related biases, including inter-system biases (ISBs), differential code biases (DCB), and differential phase biases (DPB) on the positioning, navigation, and timing (PNT) provided by global navigation satellite systems (GNSS). Therefore, it is important to ascertain the intrinsic characteristics of receiver-related biases, especially in the context of the combination of BDS-3 and BDS-2, which have some differences in their signal level. We present a method that enables time-wise retrieval of between-receiver ISBs, DCB, and DPB from multi-frequency multi-GNSS observations. With this method, the time-wise estimates of the receiver-related biases between BDS-3 and BDS-2 are determined using all five frequencies available in different receiver pairs. Three major findings are suggested based on our test results. First, code ISBs are significant on the two overlapping frequencies B1II and B2b/B2I between BDS-3 and BDS-2 for a baseline with non-identical receiver pairs, which disrupts the compatibility of the two constellations. Second, epoch-wise DCB estimates of the same type in BDS-3 and BDS-2 can show noticeable differences. Thus, it is unreasonable to treat them as one constellation in PNT applications. Third, the DPB of BDS-3 and BDS-2 may have significant short-term variations, which can be attributed to, on the one hand, receivers composing baselines, and on the other hand, frequencies.

Precise point positioning (PPP) has been a competitive global navigation satellite system (GNSS) technique for time and frequency transfer. However, the classical PPP is usually based on the ionosphere-free combination of dual-frequency observations, which has limited flexibility in the multi-frequency scenario. More importantly, the unknown integer ambiguities are not restored to the integer nature, making the advantage of high-precision carrier phase observations underutilized. In this contribution, using the undifferenced and uncombined (UDUC) observations, we derive the time and frequency transfer model suitable for multi-constellation and multi-frequency scenarios. Notably, in short- and medium-baseline time and frequency transfer, the ionosphere-fixed and ionosphere-weighted UDUC models are derived, respectively, by making full use of the single-differenced (SD) ionospheric constraints. The proposed model can be applied to short-, medium- and long-baseline time and frequency transfer. The ambiguities are solved in a double-differenced (DD) form and can thus be restored to integers. To verify the feasibility of the model, GPS data from several time laboratories were collected, and the performance of the time and frequency transfer were analyzed with different baseline lengths. The results showed that the ionosphere-fixed and ionosphere-weighted UDUC models with integer ambiguity resolution could improve the frequency stability by 25–60% and 9–30% at an averaging time of several tens of seconds to 1 day for short- and medium-baseline, respectively. Concerning the long-baseline, the UDUC model is 10–25% more stable than PPP for averaging time below a few thousands second and over 1 day.

The Chinese new-generation BeiDou navigation satellite system (BDS3) is under construction and has begun to provide satellite-based positioning, navigation and timing solutions for global users. Inter-system biases (ISBs) are critical for the optimal combination of multiple global navigation satellite systems (GNSSs) observations, and their calibration makes it possible to enhance the interoperability among different GNSSs. We formerly presented a method based on the between-receiver single-differenced (SD) multi-GNSS observation equations for estimating the ISBs in scenarios with both overlapping and non-overlapping frequencies. In this paper, we analyze the ISBs between the new-generation BDS3 and other GNSSs, as well as BDS2, using the previously proposed SD method. For the first time, this article presents the ISBs between BDS3 and GPS, Galileo and BDS2 for identical-receiver baselines. Numerical analysis indicates that (1) the new frequencies of BDS3 that overlap those of GPS and Galileo are, rather unexpectedly, free of ISBs, suggesting the existence of interoperability among these three constellations; (2) there are obvious ISBs between BDS3 and BDS2 at non-overlapping frequencies but not at overlapping frequencies; (3) calibrating the ISBs makes it possible to improve the performance of real-time kinematic (RTK) positioning for multiple GNSSs involving BDS3.