# Phase Center Corrections for BDS IGSO and MEO Satellites in IGb14 and IGSR3 Frame
## 2 Basic Information  
### Status of BDS Satellites and Tracking Network  
By December 2020, there were 49 BDS satellites in orbit; the status of the BDS constellation can be found at [[http://www.csno-tarc.cn/en/system/constellation](http://www.csno-tarc.cn/en/system/constellation)]([http://www.csno-tarc.cn/en/system/constellation](http://www.csno-tarc.cn/en/system/constellation)) (accessed on 30 December 2020). Among them, five GEO (C01-C05), seven IGSO (C06-C10 and C13), and three MEO (C11, C12, and C14) satellites form the basic constellation of BDS-2. Two MEO (C57 and C58) and two IGSO (C31 and C56) satellites from the BDS experimental constellation (BDS-3s) are set as unhealthy. Moreover, BDS-3 consists of three GEO (C59, C60, and C61), three IGSO (C38, C39, and C40), and twenty-four MEO (the rest) satellites. Unlike all BDS-2 satellites and BDS-3 IGSO/GEO satellites developed entirely by the China Academy of Space Technology (CAST), BDS-3 MEO satellites are built by two different manufacturers, i.e., CAST and Shanghai Engineering Center for Microsatellites (SECM). BDS MEO satellites are distributed in three orbital planes, i.e., Slot-A (C11, C12, C27-C30, C34, C35, C43, C44), Slot-B (C14, C19-C22, C32, C33, C41, C42), and Slot-C (C23-C26, C36,C37, C45, and C46), whereas the BDS IGSO satellites also orbit in three planes, i.e., Plane I (C06, C09, C16, C39), Plane II (C07, C10, C40), and Plane III (C08, C13, C41). Figure 1 shows the variation of elevation angle of the sun above the six orbital planes (\(\beta\) angle) from day of year (DOY) 1 to 365, 2019, which is the selected study period in this contribution. For IGSO satellites, the orbital planes II and III show smaller variations than that of orbital plane I: the satellites will cross eclipse seasons for \(|\beta|<9^{\circ}\). While, for MEO satellites, Slot-A orbital plane has relatively larger variations followed by Slot-C and Slot-B. When \(|\beta|<13.2^{\circ}\), the orbital planes are partially eclipsed. As all BDS-2 satellites have already been tracked by ground receivers with multi-constellation capacity. Hence, we only focus on the tracking status of BDS-3 satellites here. Currently, the ground L-band tracking data of BDS-3 can be obtained from the IGS and International GNSS Monitoring and Assessment System (iGMAS) network. For the total of the 25 stations of the iGMAS network, the B1I, B3I, B1c, and B2a open-service (OS) signals from all deployed BDS-3 satellites can be tracked. With the release of interface control documents (ICDs) for BDS-3 signals [22], the GNSS receivers of IGS network, mainly from Leica, Trimble, Javad, and Septentrio, are gradually upgraded to track BDS-3 signals. Most of them support tracking the backward-compatible B1I and B3I signals, whereas a few stations can track all BDS-3 OS signals. Figure 2 shows the daily number of IGS and iGMAS stations tracking the representative satellites, i.e., C19, C23, C32, C35, C38, and C59, from DOY 1 to 365, 2019. At the beginning of 2019, there were 93 and 19 stations from IGS and iGMAS network with BDS-3 B1I and B3I signal-tracking capacity, and it increased to 171 and 25 at DOY 365, 2019. However, not all deployed BDS-3 satellites can be tracked by each IGS receiver, possibly due to the limitation of tracking channels. In general, the early-launched satellites can be tracked by more receivers. At the beginning of 2019, except for Javad TRE-3 Delta receiver with ability to track all deployed BDS-3 satellites, Trimble receivers can only track the C16-C30 satellites, and Septentrio receivers with latest firmware can only track C19-C22 and C32-C34 satellites. Hence, the number of stations with C19 tracking capacity is the greatest, followed by C32. With the updates of  
Figure 1: Elevation of the Sun (\(\beta\) angle) with respect to the BeiDou navigation satellite system (BDS) orbital planes for inclined geostationary orbit (IGSO) (**up**) and medium earth orbit (MEO) (**bottom**) satellites from day of year (DOY) 1 to 365, 2019. The shaded area indicates the eclipse seasons.