## 3 Results
### Clock Prediction
For long prediction times of around 12 h to one day, the clock error does not increase as much as for the MEO satellites, resulting in around a 22-27% lower clock error. For the ACES clocks in the simulation, the fitting of a constant bias gives the best prediction results up to one day overall, as shown in Figure 10. The clock error barely decreases for the different combinations of observation techniques. Hence, all scenarios can  
Figure 9: The 95 th percentile at certain prediction times using PHM clocks. Results are for MEO satellites in the MEO-only constellation and for the different combinations of observation techniques. The black bars refer to the true PHM clocks. benefit from the high stability of the ACES clock. For the 20 min prediction, the clock errors from the different scenarios improve by about 1 cm compared to the scenarios with the PHM clocks. This is expected as, for short prediction times, the results are still dominated by the mean errors of the estimated clocks, which are similar to the ones from the scenarios with the PHM clocks in the simulation (see Section 3.2.1). For comparisons, the clock prediction errors at 20 min are at the low mm level for the true ACES clocks. After one day, the L-band-only scenario improves by a factor of around 2.4, and the L-band+OISL scenario improves by a factor of around 4-5, compared to the scenarios using the PHM clock. While the clock errors decrease by only a few millimeters for the different combinations of observation techniques, the true clock prediction error at one day is at around 1.7 cm. The L-band+OTWL scenario achieves a clock prediction error of 3.5 cm at one day. This is an improvement by a factor of about 28 compared to the scenario with the PHM clock. The L-band+OTWL+OISL scenario, which gives the best results, is at around 2.1 cm. This indicates that for this prediction interval, the L-band+OTWL+OISL scenario is approaching the stability of the ACES clock. Analogous to the PHM analysis, for the ACES clocks we only analyze the L-band+OISL and L-band+OTWL+OISL scenarios for the GSO satellites. Here, the IGSO and GEO satellites achieve almost the same clock prediction errors as the MEO satellites throughout the analyzed prediction interval. For today's procedures, performing clock synchronization from ground to space, the prediction results can be reduced by the mean, which is known from the last epoch of the estimated clocks. The mean clock error is still present when comparing the MEO system time with external time systems, but it cancels within the MEO system time and, hence, is irrelevant for the user. This leads to 35% lower clock prediction errors for the L-band-only scenario for short prediction times. The L-band+OTWL scenario only improves by about 13%. This indicates that the mean clock errors are already well-determined from the collocated use of the L-band and OTWL measurements. The L-band+OISL scenario achieves about the same results as the L-band+OTWL+OISL scenario. In the case of the PHM clocks, both scenarios reach the true clock prediction errors. For the ACES clocks, the L-band+OISL scenario's clock prediction errors are at around 10 mm, and the L-band+OTWL+OISL scenario's clock prediction errors are at around 6 mm, for short prediction times. Hence, the additional OTWL measurements help with further improvement, compared to the L-band+OISL scenario, when having the ACES clocks. Figure 10: The 95 th percentile at certain prediction times using ACES clocks. Results are for MEO satellites in MEO-only constellation and for the different combinations of observation techniques. The black bars refer to the true ACES clocks. However, when a time system should be realized in space instead of on the Earth's surface, then the mean does not cancel, so having a small mean error of the estimated clocks is important.