# Study of Fast and Reliable Time Transfer Methods Using Low Earth Orbit Enhancement
## 4 Example Analysis
### Analysis of the Influence of LEO Enhancement Degree on Time Transfer Performance  
To further investigate the influence of the enhancement level of LEO satellites on time transfer performance, experiments were designed with different numbers of LEO satellites. Four scenarios were considered for the LEO enhancement level, corresponding to 1, 2, 3, and 4 LEO satellites. Regarding the LEO satellite number control method involved in this paper, the selection strategy of three LEO satellites is taken as an example to illustrate the following: When the number of LEO satellites is more than 3, the first three satellites are retained according to the elevation angle. If a retained LEO satellite has an elevation angle lower than the cut-off elevation angle, the satellite with the higher elevation angle is selected for replacement.  
Simultaneously, three time transfer links, namely pie1-ons1, harb-sydn, and ohi2-syog, were selected for conducting experiments on the LEO enhancement level. The diagrams depicting clock offsets for the pie1-ons1 link across the four scenarios are detailed in Figure 11, with the first-order differences shown in Figure 12. Similarly, the clock offset diagrams for the harb-sydn link under the four scenarios are shown in Figure 13, with the corresponding first-order difference results displayed in Figure 14. Lastly, the clock offset diagrams for the ohi2-syog link in the four scenarios are depicted in Figure 15, and the first-order difference results are exhibited in Figure 16.  
Figure 12: First-order difference diagram of pie1-ons1 time transfer link clock offset in the four enhancement scenarios.  
Figure 13: Clock offset diagram of harb-sydn time transfer link in the four enhancement scenarios.  
Figure 14: First-order difference diagram of harb-sydn time transfer link clock offset in the four enhancement scenarios.  
Figure 15: Clock offset diagram of ohi2-syog time transfer link in the four enhancement scenarios.  
As the LEO satellite count increases, the effect of LEO enhancement was further strengthened, resulting in an enhanced convergence speed for the time transfer link. To examine the convergence speed, the convergence times of the three links in the four scenarios were tabulated, as shown in Table 8 and Figure 17. Under the four different LEO enhancement conditions, the trend in convergence of the three time transfer links was consistent, and convergence speed increased with satellite number. Combined with the data in Table 5, the convergence speed of the time transfer link clock offset under different LEO enhancement conditions was calculated and compared with that of the GPS. The results are summarized in Table 9. The largest improvement in convergence speed was observed in the harb-sydn link, which uses cesium as its clock frequency source. A sole LEO satellite increased convergence speed by 13.6%, whereas four satellites improved it by up to 77.3%. The ohi2-syog link with the mixed clock group showed little change in convergence speed according to the number of satellite. With one satellite, the convergence speed was almost the same as that of GPS. With three and four, the convergence speed increased by 33.3%. The aforementioned results further corroborate that LEO-augmented PPP has different degrees of applicability to time transfer links depending on the clock frequency source.  
Simultaneously, the standard deviation of the clock offset sequence of the time transfer link in the convergence interval is analyzed under the condition of different numbers of LEO satellites, and the results are shown in Table 10. The findings demonstrate that the stability of the time transfer link in the convergence interval increased with the number of satellites. Accordingly, with the increase in the number of LEO satellites, the combination of GPS + LEO has a better geometric shape, which is more conducive to improving the performance of PPP time transfer.