## 2 Data Collection for BeiDou-3 Satellites POD  
The BeiDou-3 satellites retain the navigation-signal-transmitting modulation system in deployment phase 1 (i.e., 1561.098 MHz (B1), 1207.140 MHz (B2), and 1268.520 MHz (B3)). Furthermore, BeiDou-3 satellites will include the migration of its civil BeiDou B1 signal from 1561.098 MHz to a frequency centered at 1575.42 MHz, which is identical to the GPS L1 and Galileo E1 civil signals, and its transformation from a quadrature phase shift keying (QPSK) modulation to a multiplexed binary offset carrier (MBOC) modulation similar to the future GPS L1C and Galileo's E1 [17]. Two frequencies of B2 (1176.45 (B2a) and 1207.14 (B2b)) and two frequencies of B3 (1278.75 (B3c) and 1268.52 (B3c)) will be candidates for the migration of civil BeiDou B2 and B3 signals. Because the new signals of BeiDou-3 satellites remain in the internal test stage, observations of old B1, B2, and B3 frequencies were selected for the BeiDou-3 satellite POD in this study.  
China is developing the International GNSS Monitoring and Assessment Service (iGMAS) to monitor the nation's BeiDou satellites and other global navigation satellites to ensure the precise-orbit products of the systems in order to improve the signal availability, reliability, precision, and accuracy. The iGMAS consists of three operational Data Centers (DCs), 12 Analysis Centers (ACs), 18 Tracking Stations (eight domestic, eight overseas, one in the North Pole, and one in the South Pole), one Monitoring and Analysis Center, one Operation Control and Management Center, and one Analysis Center Coordinator. The three DCs are located in Wuhan University, National University of Defense Technology, and the National Time Service Center. The twelve ACs are located in the Institute of Geodesy and Geophysics (IGG), Wuhan University (WHU), Shanghai Astronomical Observatory (SHA), Xi'an Satellite Control Center (XSC), Chang'an University (CHD), Beijing Space Information Relay and Transmission Technology Research Center (TAC), Chinese Academy of Surveying and Mapping (CGS), The PLA Information Engineering University (LSN), Xi'an Research Institute of Surveying and Mapping (XRS), China University of Mining and Technology (CUM), Beijing Aerospace Control Center (BAC), and the National Time Service Center (NTS). The Monitoring and Analysis Center is located in Xi'an Research Institute of Surveying and Mapping. The Operation Control and Management Center and Analysis Center Coordinator are located in Beijing. A continuous tracking network for BeiDou and other GNSS satellites for scientific and engineering applications has been deployed by iGMAS. By August 2016, there are nine iGMAS stations that can track the BeiDou-3 satellites, each with B1 and B3 observations. The Multi-GNSS Experiment (MGEX) was established by the International GNSS Service (IGS) to track, collate, and analyze all available GNSS signals, which include signals from the BeiDou, Galileo, Quasi-Zenith Satellite System (QZSS), Navigation with Indian Constellation/IRNSS (NAVIC) systems, modernized GPS and GLONASS satellites, and any space-based augmentation system (SBAS) of interest. There are only four MGEX stations, roap, stj3, ttsg, and yel2 that can track the BeiDou-3 satellites, although each currently has only B1 observations.  
In this study, 9 iGMAS stations (cf. Figure 1) were selected for the BeiDou-3 satellite POD, and the station information is listed in Table 2. For comparison, 52 MGEX stations (cf. Figure 1) were selected for BeiDou-2 satellites POD. The observation data of iGMAS and MGEX from 16 July to 14 August 2016 were processed for BeiDou-3 and BeiDou-2 satellite precise-orbit determination, respectively.