## 4 Analysis and Evaluation of the PPP-B2b and Broadcast Ephemeris Performance
### Evaluation of the RT Clock Offset Accuracy  
The RT clock offset performance is assessed using the GFZ precise clock offset product as a reference. Before comparison, it is necessary to eliminate the reference time differences among the broadcast clock offset, PPP-B2b clock offset, and precise clock offset of the BDS-3 system using the time group delay and the DCB. We then select G32 and C40 as the reference satellite clocks for the GPS satellites and BDS-3 satellites, respectively, to make the time datum consistent between the broadcast clock offset and precise clock offset. The same method is applied to the PPP-B2b clock offset. The comparison results are presented in Figure 7.  
Owing to the monitoring stations being distributed in small local areas, only part of the arc of the PPP-B2b clock correction is broadcast. In the case of the GPS PPP-B2b clock offset, there are jumps in the error at regular intervals that compensate for the large slips in the broadcast clock offsets resulting from the updating of the broadcast ephemeris. In addition, the broadcast clock offset errors are greater after correction, mainly because of systematic biases in the PPP-B2b corrections. Furthermore, system biases are included in the corrected clock offset. Although the BDS-3 PPP-B2b clock offsets also contain system biases, such systematic biases do not affect the RT POD accuracy. Figure 7 shows that the PPP-B2b clock offset errors tend to be more continuous and smoother than the errors of the  
Figure 6: RMS errors in GPS satellite orbits (**top**) and BDS-3 satellite orbits (**bottom**).  
broadcast ephemeris, which effectively reduces the errors due to data updating. Owing to the systemic bias, the systematic STD is used in the evaluation of the nine-day clock offset datasets. All the evaluation results are shown in Figure 8. After correction, the STD obtained for the GPS PPP-B2b clock offset is 0.03 m, which is 50% less than that of the GPS broadcast clock offset. Similarly, the STD of the BDS-3 PPP-B2b clock offset is 6.8 cm, which is 70% less than that of the BDS-3 broadcast clock offset. Figure 7 shows that the errors in the GPS PPP-B2b clock offsets contain systematic biases and are much larger than those in the BDS-3 clock offsets. Aiming at reducing the effect of systematic biases in evaluating the PPP-B2b performance, a segmented statistics strategy is applied to the GPS PPP-B2b and broadcast clock offset errors. The GPS-PPP B2b and broadcast clock offset error series are divided into the several identical segments based on the GPS PPP-B2b clock offset error jump caused by the broadcast ephemeris update. The STD of each segment of the clock offset error for each satellite was calculated, then the average of these STDs of each satellite was calculated and the average of these STDs of all satellites was taken to obtain the STD of the entire satellite system at last. The resulting STDs of the GPS PPP-B2b offset errors are smaller than those of the BDS-3 PPP-B2b offset errors.  
Figure 7: Error series in the GPS (**top**) and BDS-3 (**bottom**) broadcast clock offsets and PPP-B2b clock offsets.