## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**SVN** & **RRN** & **Common Name** & **COSPAR ID** & **NORAD ID** & **Launch Time** & **Orbit** \\ \hline C201 & C19 1 & BDS-3 MEO-1 & 2017-069A & 43001 & 5.11.2017 & MEO \\ C202 & C20 & BDS-3 MEO-2 & 2017-069B & 43002 & 5.11.2017 & MEO \\ C203 & C27 & BDS-3 MEO-7 & 2018-003A & 43107 & 11.01.2018 & MEO \\ C204 & C28 2 & BDS-3 MEO-8 & 2018-003B & 43108 & 11.01.2018 & MEO \\ C205 & C22 & BDS-3 MEO-4 & 2018-018A & 43207 & 12.02.2018 & MEO \\ C206 & C21 & BDS-3 MEO-3 & 2018-018B & 43208 & 12.02.2018 & MEO \\ C207 & C29 & BDS-3 MEO-9 & 2018-029A & 43245 & 29.03.2018 & MEO \\ C208 & C30 & BDS-3 MEO-10 & 2018-029B & 43246 & 29.03,2018 & MEO \\ C209 & C23 & BDS-3 MEO-5 & 2018-062A & 43581 & 29.07.2018 & MEO \\ C210 & C24 & BDS-3 MEO-6 & 2018-062B & 43582 & 29.07.2018 & MEO \\ C211 & C26 & BDS-3 MEO-11 & 2018-067A & 43602 & 24.08.2018 & MEO \\ C212 & C25 & BDS-3 MEO-12 & 2018-067B & 43603 & 24.08.2018 & MEO \\ C213 & C32 & BDS-3 MEO-13 & 2018-072A & 43622 & 19.09.2018 & MEO \\ C214 & C33 & BDS-3 MEO-14 & 2018-072B & 43623 & 19.09.2018 & MEO \\ C215 & C35 & BDS-3 MEO-16 & 2018-078A & 43647 & 15.10.2018 & MEO \\ C216 & C34 & BDS-3 MEO-15 & 2018-078B & 43648 & 15.10.2018 & MEO \\ C217 & C59 & BDS-3 GEO-1 & 2018-085A & 43683 & 1.11.2018 & GEO \\ C218 & C36 & BDS-3 MEO-17 & 2018-093A & 43706 & 18.11.2018 & MEO \\ C219 & C37 & BDS-3 MEO-18 & 2018-093B & 43707 & 18.11.2018 & MEO \\ C220 & C38 & BDS-3 IGSO-1 & 2019-023A & 44204 & 20.04.2019 & IGSO \\ \hline \hline \end{tabular}  
* 1.2 PRN C19 used until 12.06.2018 and since 15.11.2018. PRN C28 used until 11.06.2018 and since 20.12.2018. As no official space vehicle numbers (SVNs) are available, SVNs are assigned based on the satellite generation (C27 with monotonically increasing number for BDS-3). The SVN used here follows NORAD ID. In case of dual launches, the smaller NORAD ID is assigned the smaller SVN and vice versa. Information on BDS-3 is preliminary and might be subject to change.  
\end{table} 
Table 1: The status of launched BDS-3 operational satellites (early May 2019) [5]. The availability of overlapping frequencies between BDS-3 and GPS/Galileo brings opportunity for the tightly combined processing or inter-system double-differencing of observations from GPS/Galileo, and BDS-3 operational satellites. Different from the classical loosely combined double-differencing of observations from each system independently (i.e., choosing a single-reference satellite for each system separately), the tightly combined observational model utilizes a single-reference satellite for observations from all the involved Global Navigation Satellite Systems (GNSSs). As a result, additional double-differenced observations can be created, and the model strength is then improved [17; 18]. However, when inter-system observations are created, additional between-receivers differential inter-system biases (ISBs), which originate from difference in the receiver hardware delays that affecting signals from different GNSS systems, are also introduced and should be carefully considered [17]. The phase and code differential ISBs are frequency dependent, which means that they may present with different values for different frequencies [17; 18; 19]. Previous studies have also revealed that the phase and code ISBs between overlapping frequencies that are shared by GPS, Galileo, QZSS, and IRNSS (i.e., L1-E1 and L5-E5a) are receiver-type dependent. They are close to zero when identical types of receivers are utilized at both ends of a baseline, but generally present with non-zero values for baselines with different receiver types. Fortunately, since the ISBs are stable in the time domain, they can be easily calibrated and corrected in advance [17; 18; 19; 20]. If these ISBs are a-priori calibrated and introduced as corrections, the performance of precise relative positioning can be remarkably improved with respect to the loosely combined model in terms of ambiguity resolution and positioning accuracy [21; 22; 23; 24; 25].