# BDS-3 Time Group Delay and Its Effect on Standard Point Positioning
## 1 Introduction  
The BeiDou constellation began providing positioning, navigation and timing (PNT) services at least in the Asia-Pacific region in December 2012 [1]. The development of the BeiDou constellation has three phases: The BeiDou satellite navigation experimental system (BDS-1), the construction of the regional BeiDou system (BDS-2) and the construction of a global BeiDou system (BDS-3) [2]. By the end of 2012, BDS-2 consisted of five geostationary earth orbits (GEOs), five inclined geosynchronous orbits (IGSOs) and four medium altitude earth orbit (MEO) satellites. The BDS-3 constellation started in 2015 and is expected to provide global service with 5 GEO, 3 IGSO and 27 MEO satellites by 2020 [3]. Withthe rapid development of the BDS-2 constellation, BDS-2 has become a hot topic in the GNSS application community [4; 5; 6]. Many studies on BDS-2 have been carried out. [PERSON] et al. [7] presented the initial results of the BDS-2 precise orbit determination (POD). With the precise orbit and clock released by the IGS multi-GNSS experiment (MGEX), it provides the conditions for the study of the precise application of BDS-2 [8; 9]. In addition, [PERSON] et al. [10] investigated the effectiveness of the broadcast time group delay (TGD) in the navigation message and differential code biases (DCBs) provided by the MGEX; both the standard point positioning (SPP) and precise point positioning (PPP) were carried out for BDS signals with and without code bias correction. In addition, the triple-frequency ambiguity resolution performance using real BDS-2 data was investigated by [PERSON] et al. [11]. They illustrated that a minor improvement could be achieved by using triple-frequency observations compared with using dual-frequency observations. Furthermore, [PERSON] et al. [12] concluded that the GPS-BDS PPP combination presented a slightly better performance in comparable conditions and smaller intervals with the static model. Moreover, a special problem for BDS-2 is the satellite-induced code bias, which was investigated by [PERSON] and [PERSON] [13]. Before the launch of the BDS-3 satellites, the experimental BDS-3 (BDS-3e) system consisting of 3 MEO and 2 IGSO satellites was launched [14; 15; 16]. Since 2015, BDS-3e has implemented and validated new modes of navigation signals and inter-satellite links. In the last few years, many studies on BDS-3e have been carried out [14; 15; 16]. [PERSON] et al. [3] analysed the POD for the BDS-3e satellites with the international GNSS continuous monitoring and assessment system (iGMAS) and MGEX tracking networks. In addition, [PERSON] et al. [16] presented the performances of the POD and PPP of the onboard BDS-3e satellites. Precise orbit and clock determination for BDS-3e satellites with yaw attitude using the extend center for orbits determination in Europe (CODE) orbit model (ECOM) has been illustrated by [PERSON] et al. [17]. Interestingly, there is no satellite-induced code bias in the BDS-3e satellites. From multipath-free conditions, the satellite-induced code variations that still exist in the bands of B1C, B2a and B2b of BDS-3e satellites were investigated by [PERSON] et al. [18]. Furthermore, [PERSON] et al. [2] studied the performances of the BDS-3 demonstration system, including the signal-to-noise ratios, code errors and the multipath errors of the civilian signals. Moreover, the DCBs and the performance of both satellites and the receiver DCBs for BDS-3e using iGMAS and MEGX network observation data were evaluated by [PERSON] et al. [19]. The results showed that the DCBs estimated from the BDS-3e satellite were more stable than those from the BDS-2 satellite. However, the BDS-3e satellite is not part of the BDS-3 constellation. From November 2017 to May 2019, 18 MEO, 1 GEO and 1 IGSO BDS-3 satellites (excluding experimental satellites) have been launched successfully. As a global system, BDS-3 has provided basic services to the countries along the Belt and Road and the neighboring regions since the end of 2018.