## 1 Introduction
The main challenges of such future clock types are still the high costs and the complex manufacturing to be able to be carried by a satellite. For real-time processing and, thus, for clock prediction, it is important to have stable clocks. However, with an epoch-wise clock processing, a more stable clock does not give an improvement for clock estimation. To achieve improvements in the estimation, a possibility is to optimize the algorithm for clock modeling. Optical two-way Inter-Satellite Links (OISL) is one measurement technique to perform high-precision dual one-way satellite-to-satellite tracking with a laser [8; 15; 16; 17]. The impact of different ISL connectivity schemes was simulated in [15; 18]. An alternative to OISL is optical two-way /dual one-way ground-to-satellite links. European Laser Timing (ELT; [19]), as part of the ACES mission, and T2L2 (Time Transfer by Laser Link; [20]) are two of the ground-to-satellite ranging and time transfer techniques. BeiDou already involved an optical measurement technique for two-way ranging, named Laser Time Transfer (LTT; [21]), and a two-way microwave technique, named Two-way Satellite Time and Frequency Transfer (TWSTFT; [22]), to achieve accurate time transfer. In [16; 23], we introduced the Optical Two-Way Link (OTWL) measurement concept for a future GNSS MEO satellite constellation. This concept builds on dual one-way measurements and is the ground-to-space counterpart of OISL. In [16], we compared the OISL and OTWL measurement techniques and showed the benefits of synergistically using both techniques with Galileo L-band measurements for the POD of MEO satellites. In [17], we expanded the MEO satellite constellation by using inclined geosynchronous orbit (IGSO) and GEO satellites and analyzed the orbit errors. The resulting orbit errors from [17] showed that combinations of these measurement techniques also increase the orbit accuracy of IGSO and GEO satellites to the millimeter level. While we only analyzed the orbit errors in [16; 17], in this study we focus on the analysis of the satellite clocks from the simulation scenarios, which are identical to [17]. Our target is to estimate the satellite clocks, with respect to the reference clock, with an accuracy at the picosecond level. We assumed that all satellites in the constellation each carry a PHM. The clocks were estimated epoch-wise using a least-squares adjustment. We analyzed the results of the clock estimation by the mean error and standard deviation of the estimated (apparent) clocks as well as the Allan deviation [24]. For comparison, we performed a further simulation using the ACES clock as an example of a clock with high stability for the future. Last, we performed the prediction of the estimated clocks for all scenarios up to one day. We analyzed the influence of the OTUVL and/or OISL measurements being used in addition to the GNSS L-band observations as well as the expansion of the MEO constellation with either the IGSO or GEO satellites for the clocks' estimation and prediction. We want to indicate that this work is a simulation study and is still far from a realistic project study. We take many systematic errors for the observations' simulation and the orbit modeling into account, but the complexity of physical reality cannot be fully transported into a simulation environment. The work is structured as follows. In Section 2, we described the simulation setup and initialize the simulation and estimation parameters. Section 3 presents the results of the clock analysis. Our conclusions are given in Section 4.