## 4 Tests and Results
### \"End-Loss\" Scenario
compared with the BLS clocks using the CODE final clocks, as shown in Figure 8. It can be clearly seen that when only the GNSS orbits are lost (seen in the green lines for Scenario D), the LEO satellite clocks experience the least precision loss. This is partly due to the relatively smaller prediction loss of orbits compared to clocks. Comparing scenarios A and E, it can be seen that when \"end-loss\" occurs in the GNSS clocks, whether it is predicted or not, it significantly impacts the determination of the LEO satellite clocks. Table 4 displays the error budget for LEO satellite clocks resulting from various scenarios of the \"end-loss\" situations of the GNSS real-time products due to latency. It can be observed that when both the GNSS orbital and clock products are lost for durations between 30 and 120 s and no prediction is performed for the GNSS satellite clocks, the precision of the LEO satellite clocks degrades from 0.2 ns without latency to 0.61-1.15 ns. Comparing Scenario A with C and D, it can be seen that the GNSS clock latency is the major cause of this degradation. In Scenario B, the missing GNSS clocks are extrapolated, which reduces the degradations by about 10-20%. Still, with a latency of 30, 60, 90, and 120 s, one needs to expect a degraded precision of 0.53, 0.72, 0.85, and 0.93 ns, respectively. In scenario E, only the GNSS satellite clocks are considered delayed and then predicted. As shown in Table 5, the precision of the predicted GNSS clocks varies from 0.1 to 0.16 ns for latencies from 30 to 120 s. Correspondingly, the precision of the LEO satellite clocks is degraded to 0.28-0.57 ns for a latency of the GNSS clocks from 30 to 120 s. Comparing Scenarios C and E, it can be seen that linearly extrapolating the GNSS satellite clocks significantly reduces the degradation in the LEO satellite clock prediction. Figure 8: Sentinel-3B clocks in the five scenarios (with a latency of 120 s, see Table 3) and the clocks without latencies in the CNES products, compared with the BLS clocks using the CODE final products. Figure 7: Sentinel-3B clocks in the five scenarios (see Table 3) with a latency of 120 s compared with the clocks estimated using the CNES real-time products without latencies. Predicting the LEO satellite clocks instead of the GNSS orbits and clocks is another way to solve the problem of delays of the products. LEO satellite clocks can be predicted with periodic terms and polynomials, as expressed in [25]. The error budget of the LEO clock prediction errors is summarized in Table 6. From Tables 4-6, it can be seen that the precision of the LEO satellite clocks is very sensitive to the precision of the GNSS satellite clocks. Comparing Tables 4 and 6, it can be concluded that for latency of the real-time GNSS products within 2 min, better results can be achieved when waiting for the GNSS real-time products, processing the near-real-time LEO satellite clocks, and performing a LEO satellite clock prediction over this ultra-short period. The short-term predicted clocks for certain GPS satellites show larger noise than the predicted Sentinel-3B clocks connected with an Ultra-Stable Oscillator (USO) that exhibits rather good short-term noise [33]. This cannot be completely reflected in the STDs in Table 5 but can be reflected in the Modified Allan Deviations (MDEVs) [34], as shown in Figure 9. In contrast, the good short-term stability of the USO in Sentinel-3B leads to much smaller degradations for short-term prediction, as shown in Table 6.