# BDS-3 Triple-Frequency Timing Group Delay/Differential Code Bias and Its Effect on Positioning
## 1 Introduction  
The BeiDou Global Navigation Satellite System (BDS-3) of China, consisting of 3 geostationary earth orbit (GEO), 3 inclined geostationary orbit (IGSO) and 24 medium earth orbit (MEO) satellites, started its service in 2020 [1, 2, 3]. BDS-3 transmits a total of five frequency signals, including new signals, BIC (1575.420 MHz), B2a (1176.450 MHz) and B2b (1207.140 MHz), based on compatibility with the old signals, B11 (1561.098 MHz) and B3I (1268.520 MHz) [4, 5]. The new signals, BIC and B2a, are divided into pilot and data components [6, 7]. Some of the receivers in the International GNSS (global navigation satellite system) Service (IGS) network exclusively use the pilot component to generate pseudorange and carrier phase observations, and others use the pilot and data components jointly. Almost no receivers use the data component directly [8]. Undoubtedly, the new frequencies together with the old frequencies enable users to possess more choices in dataprocessing (e.g., cycle slip detection) and linear signal combinations, with more distinctive features (e.g., lower observation noise) as well [9]. Slight time delays in the satellite signals during the transmission caused by the satellite or the receiver hardware, which are often referred to as hardware delay bias, exist in both code and carrier phase measurements and have different effects on different types of codes and carriers [10]. To address the pseudorange hardware delay problem, a correction is usually performed on the user side using the timing group delay (TGD) parameter from either the real-time broadcast ephemeris or a post-processed high-precision differential code bias (DCB) product [11]. The DCB is the difference between the hardware delay deviations of two code observations, which can be divided into inter-frequency and intra-frequency biases, depending on the frequency of the two code observations [12]. Currently, the only two IGS analysis centers (IAC) that provide multi-GNSS DCB products are the Chinese Academy of Sciences (CAS) and the German Aerospace Center (DLR) [13]. The estimation strategy of the DCB product of CAS and DLR are different; the former generates DCB products by single-site ionospheric total electron content (TEC) modeling through the IGGDCB method proposed by [PERSON] et al. [14; 15], while the latter estimates DCB products after calculating TEC as ionospheric compensation through global ionospheric maps (GIM) [16]. The rapid CAS product lags by approximately two to three days while DLR products are updated once every three months [8]. The estimation and correcting methods of the DCB are hot research topics in geodesy since the DCB is one of the critical errors in GNSS precise positioning and ionospheric modeling [17]. [PERSON] et al. [18] demonstrated the equivalence between SDS-2 TGD and DCB, derived a TGD/DCB correction model and performed standard point positioning (SPP) and precise point positioning (PPP) with and without DCB correction experiments using navigation messages and observations provided by the Multi-GNSS Experiment (MGEX). [PERSON] et al. [19] summarized the TGD correction model for each GNSS and performed SPP and PPP experiments. Before the construction of the SDS-3 system, the experimental satellite constellation (BDS-3e), with two IGSO and three MEO satellites, was established at the end of 2016 [20]. [PERSON] et al. [21] estimated the BDS-3e DCB and analyzed its performance by using the data provided by the MGEX and International GNSS Monitoring and Assessment System (iGMAS). After the fundamental constellation of BDS-3 started its service in 2018, [PERSON] et al. [22] first summarized the TGD correction of the new BDS-3 frequencies and evaluated the performance of the single- and dual-frequency SPP of BDS-2, BDS-3 and BDS-2/BDS-3, respectively. Subsequently, the DCB correction of the new frequencies of BDS-3 was analyzed and the effect of DCB correction on BDS-3 single- and dual-frequency positioning was tested using iGMAS data [23].