# Characterization of GNSS Signals Tracked by the iGMAS Network Considering Recent BDS-3 Satellites
## 1 Introduction  
With the rapid development of the global navigation satellite system (GNSS), the China Satellite Navigation Office has initiated the international GNSS Monitoring and Assessment System (iGMAS) to monitor and assess the performance and operational status of the Chinese BeiDou Navigation Satellite System (BDS), as well as to promote compatibility and interoperability among different GNSS constellations [1, 2]. As a backbone of iGMAS, a network of multi-GNSS monitoring stations has been set up around the globe to track multi-GNSS satellites. Several research institutions have been selected as analysis centers to process collected GNSS observation data and provide satellite orbits, clocks, and other kinds of information. There are currently 23 stations in the iGMAS network. A key feature of iGMAS stations is the capability to track new BDS signals from recently deployed global system satellites. The development of BDS is divided into three phases: the demonstration system (BDS-1), the regional system (BDS-2), and the global system (BDS-3) [3, 4]. BDS-1 was completed with two geostationary (GEO) satellites and a backup satellite in 2003. BDS-2 consists of five GEO, five inclinedgeostationary orbit (IGSO) satellites, and four medium earth orbit (MEO) satellites, and has been providing official positioning, navigation and timing services over the Asia-Pacific area since December 27, 2012. Each of the BDS-2 satellites transmits signals in quadrature phase-shift keying (QPSK) modulation on three frequency bands, namely B1I at 1561.098 MHz, B2I at 1207.14 MHz, and B3I at 1268.52 MHz. BDS-3 is expected to provide greatly improved services to global users with 3 GEO, 3 IGSO, and 24 MEO satellites by 2020. By February 2016, five experimental BDS-3 satellites (hereafter, abbreviated to BDS-3e), including two IGSO and three MEO satellites, had completed deployment with the task of testing various key technologies for upcoming BDS-3, including new types of navigation signals, inter-satellite links, and onboard frequency standards. During the past two years, many studies have been carried out to investigate various aspects of these BDS-3e satellites, including signal quality analysis [5], precise orbit determination (POD) [6], performance of satellite clocks [7], precise point positioning (PPP) [8], and relative positioning [5]. Since the iGMAS stations support BDS-3 signal tracking, the majority of studies of the BDS-3e satellites were based on the data collected by the iGMAS tracking network. [PERSON] et al. [9] conducted initial POD of BDS-3e satellites using observations of nine iGMAS stations, and the orbit overlap precision for IGSO and MEO satellites is approximately 10 cm and 40 cm, respectively, in the radial component. [PERSON] et al. [10] comprehensively analyzed the satellite-induced code bias for BDS-3e satellites using data from iGMAS stations. [PERSON] et al. [11] estimated the differential code biases (DCBs) of BDS-3e using iGMAS observations and evaluated the performance of both satellite and receiver DCBs, with results indicating that the estimated DCBs of BDS-3e satellites are more stable than those of BDS-2. In terms of satellite signals, [PERSON] et al. [12] analyzed the navigation signals transmitted by the M2-S satellite with the help of a 7.3-m high-gain antenna. [PERSON] et al. [5] performed quality assessment for four BDS-3e satellites using tracking data of only one station, including old B1I and B3I signals and new open service signals, namely B1C, B2a and B2b. In [PERSON] et al. [13], B2a+b and S-band Bs signals transmitted by BDS-3e satellites were also included in quality analysis, which indicated that the B2a+b signal has the best performance. It is worth noting that the BDS-3e satellites would not serve as a part of the BDS-3 constellation, but as a complement to the BDS-2 constellation with the termination of the validation task. At the end of 2017, China launched two BDS-3 satellites into space.