# The Effect of BDS-3 Time Group Delay and Differential Code Bias Corrections on Positioning
## 1 Introduction  
The current BDS-3 offers all-weather, all-time, high-accuracy positioning and navigation and timing (PNT) services to users worldwide. A hybrid constellation consisting of satellites in three types of orbits was adopted by BDS-3, multi-frequency navigation signals were provided by BDS-3, so that the accuracy of services can be improved by the combination of multi-frequency signals [1, 2, 3]. There are five frequencies capable of transmitting in BDS-3 satellites, which include B1C, B2a, B2b, B1I and B3I [4].  
It is well known that GNSS signals will be affected by hardware equipment during the propagation of satellites and receivers, resulting in time delays, which we call hardware delays [5; 6]. In addition, different types of observation signals for different frequencies or the same frequency can cause different hardware delays due to differences in propagation links. Therefore, time group delay (TGD) or differential code bias (DCB) is often used to represent hardware delay differences for different signals [7]. The effect of TGD and DCB on pseudorange can reach the meter level, which was an important factor affecting the accuracy of navigation, positioning and timing [8]. As the user's requirements for navigation and positioning increase, TGD or DCB must be corrected in navigation and positioning solutions [9]. Thence, TGD and DCB must be taken seriously. The BDS-3 satellite clock is referenced to the B3I and B1I/B3I ionosphere-free combination in broadcast ephemerides and precise products, respectively [10; 11; 12]. The TGD and DCB need to be corrected when using another single-frequency or dual-frequency ionosphere-free combination [8; 13].  
Originally, the control segment provided TGD parameter based on the values measured by the spacecraft contractor. Nevertheless, calibration was performed before the satellite launch, other factors such as the environment in space can cause certain variation [14; 15]. Therefore, [PERSON] et al. [16] presented the expression of TGD estimation error for phase center separation, and the effect on the user ranging measurement accuracy was also analyzed. With continuous monitoring of the BDS-2 system, the BDS-2 TGD parameters were updated on 21 July 2017; the reason was there is a systematic bias between the pseudoranges obtained by the BDS receivers of different correlated algorithms [17]. Thence, [PERSON] et al. [18] presented calibration of BDS TGD, and investigated the impact on the signal in space range error (SISRE) and the accuracy for single point positioning (SPP). The results showed that each satellite should correct the TGD. However, with the continuous development of precise products, the Chinese Academy of Sciences (CAS) have been providing DCB products of GNSS since 2014. Therefore, [PERSON] et al. [19] assessed the performance of BDS-3e DCB based on iGMAS and MEGX network observations. Furthermore, [PERSON] et al. [20] compared the accuracy of broadcast TGD and DCB products for GPS, Galileo and BDS. The research showed that when comparing BDS-3 DCB and \(TGD_{1}\) there was 3.8 ns deviation. For users, it is more important to correct TGD and DCB. Thence, [PERSON] et al. [21] studied the correction models of TGD and DCB for BDS-2. In addition, [PERSON] et al. [22] were given models of multi-GNSS, and analyzed the effect of code biases on SPP and PPP. Nevertheless, the TGD and DCB correction models of BDS-2 are given in previous researches, but such correction models of BDS-3 have not been researched and summarized. In addition, because of the lack of comprehensive research in the available literature, it is unclear the effectiveness of TGD and DCB and their impact on positioning.  
In this contribution, we first analyzed the current BDS-3 DCB and TGD parameters, and demonstrated the relation between TGD and DCB parameters with practice with theory. Moreover, correction models of TGD and DCB parameters in SPP and PPP were derived in detail. Finally, we compared and analyzed the effects of TGD and DCB on BDS-3 SPP and PPP accuracy.