## 4 Results
### PPP Time Transfer in Static Model with HAS Service  
Figure 2 illustrates the receiver clock offsets at the PTBB station, calculated using GPS PPP time transfer in a static model, and Figure 3 provides a comparative analysis of the receiver clock offsets for six additional stations under static conditions. Synthesizing the information from both figures, we can discern several key takeaways. Notably, a striking consistency in the trends of receiver clock offsets across various stations is apparent when utilizing GPS PPP in conjunction with the Galileo HAS service. This uniformity suggests that the receiver clock offsets, as determined by PPP, largely reflect the discrepancy between the local time at each station and the reference time indicated by the HAS service. Given that the stations under study are primarily from timekeeping laboratories, where the local time is carefully regulated, the observed trends in receiver clock offsets are predominantly shaped by the reference standards inherent in the HAS service products. Furthermore, it was observed that although the receiver clock offsets at various stations exhibit a pronounced similarity in their trends, there are noticeable systematic variations among them. These variations are largely attributed to the inconsistencies in hardware delays across different receivers. As the calibration of hardware delays is a complex process and does not constitute the main focus of this paper, and considering that hardware delays tend to be stable over short durations, their impact on the research presented here is deemed negligible [30]. Thirdly, a close examination of Figure 2, especially the enlarged detail, reveals that the GPS PPP calculations relying on the Galileo HAS service exhibit minor yet discernible fluctuations in the receiver clock offsets. While these fluctuations are not significant, they can impact the precision of one-way timing derived from GPS PPP. Consequently, it can be deduced that GPS PPP reliant on the HAS service may not be adequate for applications requiring high-precision one-way timing. The primary reason behind this is the absence of a standard time reference for the GPS clock offsets in the Galileo HAS service [27], a characteristic it shares with the BDS-3 PPP-B2b [7]. Figure 2: The receiver clock offset from GPS PPP solutions in the static model for PTBB station. Figure 4 illustrates the time transfer outcomes for the six time chains designed with the PTBB station as the central node. Several conclusions can be inferred from this figure. Firstly, the time transfer results have resolved the apparent discontinuities present in Figure 2, thereby corroborating that the erratic fluctuations in the receiver clock offsets observed earlier were attributable to the GPS reference discrepancies within the HAS service data. Secondly, there is a noticeable inconsistency in the time transfer results across different chains, suggesting that even though all the stations selected are synchronized with UTC(k), each station exhibits unique characteristics. Thirdly, occasional jumps in the time transfer results, such as those seen in the NIST-PTBB link, are primarily due to a paucity of available satellites at certain times, necessitating a reinitialization of the PPP calculations. Lastly, it is evident that the time transfer results for the USN7-PTBB and USN8-PTBB chains are nearly identical, which, beyond differing hardware delays, can be mainly attributed to the fact that the USN7 and USN8 stations are both connected to the same atomic clock source. Figure 3: The receiver clock offset from GPS PPP solutions in the static model for six stations. In order to delve deeper into the performance of single-constellation Galileo and combined Galileo/GPS PPP time transfer in static mode, Figure 5 depicts the receiver clock offsets at the PTBB station derived from single-constellation Galileo and Galileo/GPS combined PPP solutions in static mode.