## 1 Introduction  
The second-generation of the Beidou Navigation Satellite System (BDS-2) has been providing real-time positioning, navigation, and timing (PNT) services since 27 December 2012. As indispensable information for BDS-2 PNT applications, the satellite orbit and clock correction can be retrived from broadcast navigation messages [1; 2]. The existence of any imperfect navigation message or frequent satellite orbit/clock adjustment would directly deteriorate the performance of BDS-2 broadcast orbit and clock correction [3], which would then bring potential barriers for widespread BDS-2 application. Therefore, it is of great importance to assess the availability, anomalies, and accuracy of BDS-2 broadcast navigation messages. Previous studies mainly focused on BDS-2 satellite anomaly detection and accuracy evaluation through analyses of BDS-2 broadcast navigation messages. In order to maintain their geosynchronousstatus, the BDS-2 Geosynchronous Earth Orbit (GEO) and Inclined Geosynchronous Satellite Orbit (IGSO) satellites would be frequently maneuvered by ground control segments [4]. During the maneuver period, the solar radiation model, phase wind-up model, and antenna phase center offset (PCO) model are different from their nominal statuses [5]. A large orbit error would be induced due to an inaccurate force model, which would certainly degrade the user positioning performance. In addition, some other factors, such as the satellite inbound/outbound transmissions, would cause broadcast orbit and clock anomalies [6; 7]. By computing orbit and clock differences between adjacent broadcast navigation messages, an empirical model was developed to detect satellite anomalies by Jan [8; 9]. One-month BDS-2 broadcast navigation messages in 2013 were utilized to generate the empirical model. Similar to [PERSON]'s method, [PERSON] et al. [10] proposed another empirical model by calculating mutual differences of orbit and clock correction at multiple epochs. By taking the proposed method into the precise orbit determination (POD), the authors correctly detected the C03/C08 maneuver on 5/9 January, 2015. Comparable POD accuracy was obtained for the maneuvered satellites. Another category of BDS-2 satellite anomaly detection based on both broadcast navigation messages and satellite observations has been investigated in recent years. A large error in the broadcast navigation message could enlarge the satellite observation residual. This principle can be applied to satellite anomaly detection by static stations with known positions. The anomaly detection method proposed by [PERSON] et al. [11] was able to locate the exact start and end time of a BDS-2 satellite maneuver. With data collected by one of the International GNSS Service (IGS) Multi-GNSS Experiment (MGEX) stations in 2016, the method was proven effective with a total of 58 GEO and 12 IGSO maneuvers detected. An improved method was proposed afterward by [PERSON] et al. [12] in 2018. With the optimized method, 32/13/1 orbital maneuvers and 102/13/12 non- maneuvering anomalies are detected for the GEO/IGSO/Medium Earth Orbit (MEO) satellites from day-of-year (DOY) 1 to 300 in 2017. As opposed to the single station experiment in [PERSON]'s contribution, [PERSON] et al. [13] applied 13 MGEX stations over the Asia-Pacific region to detect BDS-2 satellite anomalies. A total of 35 BDS-2 anomalies were detected from DOY 305 to 361 in 2015. In addition to the satellite anomaly detection, the orbit accuracy and clock precision of BDS-2 broadcast navigation messages were also intensively studied. [PERSON] et al. [14] assessed BDS-2 broadcast navigation messages from 2013 to 2014 and reported 3.0 and 5.0 m orbit accuracy for IGSO/MEO and GEO, respectively. Better than 8 ns clock precision was also obtained. The influence of broadcast navigation message errors is actually projected onto the line-of-sight (LOS) direction for users. A signal-in-space user range error (SIS URE) is applied for a comprehensive BDS performance assessment.