# Calibration and Impact of BeiDou Satellite-Dependent Timing Group Delay Bias
## 5 Conclusions and Discussions  
Unlike relative positioning, single point positioning needs to take all involved errors into account, among which the timing group delay or differential code bias cannot be ignored. For newly launched satellites such as the BDS, the precision and bias in the timing group delay and differential code bias need to be comprehensively investigated. In this contribution, we found the satellite-dependent timing group delay bias in the BDS broadcast ephemeris by comparing the BDS timing group delay with the differential code bias provided by the DLR. It was shown that the timing group delay bias is not only dispersed by BDS-2 and BDS-3, but also by each individual satellite. Meanwhile, our analysis results show that besides the TGD\({}_{1}\) bias, there also exists a bias at the B3I frequency. The bias may come from the receivers adopted at the BDS monitoring stations, which are different from the commercial receivers at multi-GNSS experiment stations. Moreover, the different timing group delay update epoch of BDS-2 and BDS-3 also implies that the timing group delays of BDS-2 and BDS-3 satellites are determined independently in the BDS control segment, which may also result in the systematic timing group delay bias between BDS-2 and BDS-3.  
To further calibrate the timing group delay bias, we proposed a network solution strategy based on raw observations from 87 multi-GNSS experiment stations. The results show that the two sets of the timing group delay bias exhibit good agreement with correlation coefficients of 0.821 and 0.985. The calibrated timing group delay bias also indicates that the timing group delay bias of BDS-3 is more dispersed than that of BDS-2.  
For the validation of the calibrated timing group delay bias, we corrected the calibrated biases in the timing group delay for signal-in-space range error assessment and SPP. It was shown that the BDS signal-in-space range error improved from 0.71, 0.81, and 1.40 m to 0.64, 0.66, and 0.64 m at B1I, B3I, and B1I/B3I frequencies, and the corresponding SPP performance improved by 5.6%, 8.4%, and 21.6%, respectively. This indicates that after the satellite-dependent timing group delay bias correction, the accuracy of the BDS is comparable with that of the timing group delay. Meanwhile, although a significant systematic bias is observed between BDS-2 and BDS-3, it is recommended to correct the timing group delay bias by individual satellite.  
It should be pointed out that as the timing group delay bias results from the different receivers and the strategy employed at the BDS control segment, the timing group delay bias needs to be monitored continuously. Besides, considering the fact that a code bias always exists among different receivers, the timing group delay or differential code bias should be calibrated after receiver classification in the future.  
[PERSON] conceived the research framework; [PERSON] analyzed the data wrote the manuscript, [PERSON] and [PERSON] reviewed and revised the manuscript; [PERSON] acquires the funding; [PERSON] and [PERSON] collected the data and analyzed part of the data. All authors have read and agreed to the published version of the manuscript.  
This work is mainly funded by the National Natural Science Foundation of China (No. 41874042, No. 11673050 and No. 61571309). We thank the GNSS observation data and products provided by MGEX.  
The authors declare no conflict of interest.