## 1 Introduction  
The BeiDou demonstration system (BDS-1), the regional service system (BDS-2), and the global service system (BDS-3) have been developed by a \"three-step\" strategy [1]. BDS-1 consists of geosynchronous orbit (GEO) satellites launched from 2000 to 2003. On 27 December 2012, the BeiDou system, which has a space constellation of five GEO, five inclined geosynchronous orbit (IGSO) satellites, and four medium Earth orbit (MEO) satellites, began to provide services to the Asia Pacific region. BeiDou started to evolve from a regional service capability to a global service capability with the launch of the new-generation BeiDou experimental satellite (BeiDou, 11-S) into orbit in March 2015. Five BDS-3 experimental satellites (C31-C35) and two BDS-3 satellites (C19 and C20) were in orbit by the end of November 2017 [2]. Moreover, the new-generation BeiDou system plans to achieve a 30 satellites network by 2020 (three GEO, 24 MEO, and three IGSO) providing global navigation, positioning, and timing services [3]. However, the orbit accuracy of BeiDou is currently one of the major challenges with the expanding application of BeiDou in the field of navigation and positioning. Researchers have assessed the BDS-2 orbit [4; 5; 6] well to improve the accuracy and relevant algorithms of BeiDou orbit determination. The results showed that the three-dimensional root-mean-square error (3D RMS) of BDS-2 one-day overlapping arc for MEO (and IGSO) and GEO were improved from 0.5 m and 3.0 m to 0.2 m and 1.0 m, respectively. Furthermore, the orbit models of BDS-2 were refined in recent studies, which included the introduction of a radial constant acceleration of GEO [7; 8], yaw attitude model [9; 10], hardware delay [11], and inter-frequency bias (IFB) [12] to further improve the orbit accuracy. Recent assessment studies suggested that the satellite laser range (SLR) residuals from different data analysis centers for BDS-2 orbits are better than 0.2 m for MEO (and IGSO) and 0.5 m for GEO [11], respectively. In general, BDS-2 orbits have been improved and refined over the years. For the new-generation BeiDou experimental satellites (hereafter called BDS-3) orbits, in [13; 14], four BDS-3 orbits (C31-C34) were estimated based on the international GNSS Monitoring and Assessment System (iGMAS) and the Multi-GNSS Experiment (MGEX) stations. Due to restrictions in the tracking networks, the results suggested that the radial and along-directions of one-day overlapping arc errors were between 0.10 m and 0.25 m. In addition, the RMS of the SLR residuals was at the 0.1-0.3 m level. However, the impacts of orbit models, such as solar radiation pressure and yaw model, were not analyzed in their studies. In [15], three-month BDS-3 orbits were acquired based on the same methods as BDS-2 orbit determination, which was analyzed in terms of yaw attitude and solar radiation pressure models. Moreover, to further improve BDS-3 orbit accuracy, inter-satellite link (ISL) and autonomous orbit determination were taken into account [16; 17; 18]. However, compared with the BDS-2 orbit, only preliminary studies on BDS-3 orbit determination, which should be focused on, were conducted. To improve the accuracy of BDS-3 orbits, it should be noted that the data availability and the quality of tracking networks used are the two main factors that restrain the orbit accuracy in the BDS-3 orbit determination, which directly affect the accuracy of orbit determination [19]. Therefore, in [20; 21; 22], the BDS-3 signals and its quality were evaluated, and the results, compared to BDS-2, showed that the data quality of the BDS-3 improves significantly. In addition, the satellite-induced multipath effects along with the elevation in BDS-2 disappeared in the BDS-3 observations. However, because the new signals of BDS-3 satellites remain in the internal test stage, the BDS-3 signals are mainly tracked by the iGMAS network [13], the data quality of which was not involved in the related analysis.