## 1 Introduction
Currently, most studies only focus on tightly combined relative positioning using observations from the overlapping frequencies between GPS, Galileo, QZSS, and IRNSS (i.e., L1-E1 and L5-E5a). Regarding BDS, existing contributions either focus on the overlapping frequency of BDS-2 B2I and Galileo E5b signals [19; 24] or the non-overlapping frequencies between BDS-2 and GPS (i.e., B1I-L1 and B2I-L2) [26; 27; 28; 29; 30]. None have focused on the model and performance evaluation of tightly combined precise relative positioning of BDS-3 (especially operational satellites) with other GNSS systems such as GPS, Galileo, etc. As an aspect of evaluating the interoperability of BDS-3 operational satellites and their new navigation signals with other systems, for the first time this contribution will investigate the feasibility of conducting tightly combined real-time kinematic (RTK) positioning between BDS-3 B1C/B2a signals and GPS/Galileo L1-E1/L5-E5a signals, and how well it performs. The results could provide us with new insights into the interoperability of China's BDS-3 with modernized GPS and Galileo. In this contribution, we will provide initial assessment of tightly combined RTK with GPS, Galileo, and BDS-3 operational satellites using observations from their overlapping frequencies of B1C-L1-E1/B2a-L5-E5a. We first evaluate the characteristics of B1C/B2a signals from BDS-3 operational satellites in comparison to GPS/Galileo L1-E1/L5-E5a signals in terms of observed carrier-to-noise density ratio, pseudorange multipath and noise, together with double-differenced carrier phase and \begin{table}
\begin{tabular}{c c c c c} \hline
**Band** & **Frequency (MHZ)** & **Signal Component** & **Modulation** & **Code Rate (Mcps)** \\ \hline B1 & 1575.42 & B1C\({}_{\rm D}\) & 1 BOC (1,1) & 1.023 \\  & & B1C\({}_{\rm P}\) & 2 QMBOC (6,1,4/33) & 1.023 \\  & 1561.098 & B1I & 3 BPSK (2) & 2.046 \\ B2 & 1176.45 & B2a\({}_{\rm D}\) & 4 QPSK (10) & 10.23 \\  & & B2a\({}_{\rm P}\) & & 10.23 \\  & 1207.14 & B2b\({}_{\rm I}\) & QPSK (10) & 10.23 \\  & & B2b\({}_{\rm Q}\) & & 10.23 \\ B3 & 1268.52 & B3I & BPSK (10) & 10.23 \\ \hline \end{tabular}
\end{table} 
Table 2: Available open service signals transmitted by BDS-3 operational satellites [1; 4; 6]. code residuals using data collected with scientific geodetic iGMAS and commercial M300 Pro receivers. Then, we investigate the size and stability of phase and code differential ISBs between BDS-3/GPS/Galileo B1C-L1-E1/B2a-L5-E5a signals using short baseline data collected with both identical and different receiver types. Finally, the performance of single-epoch tightly combined BDS-3/GPS/Galileo RTK with single-frequency (B1C-L1-E1) and dual-frequency (B1C-L1-E1/B2a-L5-E5a) observations is evaluated by a formal as well as an empirical analysis, consisting of ambiguity dilution of precision (ADOP), ratio values, empirical ambiguity resolution success rate and the positioning accuracy. The emphasis is on instantaneous positioning since this is obviously the most challenging for integer ambiguity resolution. This approach has the advantage that the results will be insensitive to the occurrences of troublesome carrier phase cycle slips. The remainder of this contribution is organized as follows. Section 2 overviews the GNSS data collection. Section 3 evaluates the characteristics of BDS-3 B1C/B2a signals in comparison with GPS L1/L5 and Galileo E1/E5a signals. In Section 4, we first briefly introduce the observation model; and then investigate the size and stability of phase and code differential ISBs between BDS-3/GPS/Galileo B1C-L1-E1/B2a-L5-E5a signals. After that, we present performance assessment of single-epoch tightly combined BDS-3/GPS/Galileo RTK with single-frequency and dual-frequency observations. Finally, some conclusions are given in Section 5.