# On the Integration of VLBI Observations to GENESIS into Global VGOS Operations
## 3 Materials and Methods
### Scheduling  
In the VLBI technique, individual stations are only able to make one observation of one radio source on the sky at any given time. To form a radio interferometer, it has to be guar  
Figure 4: Current status of VGOS station network based on [PERSON] et al. (2018). Blue circles () mark the telescopes in the operational 12-station network. Red diamonds () mark the additional 8 telescopes that are expected to be operational soon, building a 20-station network. Black triangles () depict another 9 additional telescopes, representing a future 29-station VGOS network. anteed that two or more stations simultaneously observe the same source. In geodetic VLBI experiments, the participating stations actively need to be coordinated to observe sources in a certain sequence and for a certain duration to achieve an optimal geometric stability and highest possible precision for the estimated geodetic parameters during analysis. At the same time, individual antenna characteristics, such as slew speeds, sensitivities, and source visibility, need to be accounted for. The concept of subnetting, where the network is divided into subnetworks and multiple sources are observed simultaneously, increases the complexity of this optimization problem exponentially [22]. Scheduling plays a crucial role as it significantly impacts the precision of geodetic parameter estimates (e.g., [23; 25]). Therefore, comprehensive scheduling is an integral element of this work. We use the well-established scheduling software VieSched++. The software generates a schedule scan after scan using a brute force approach, calculating all possible next scans and evaluating them. The best next scan is selected based on weight factors. Weight factors can compete against each other, and there is no ideal combination of weight factors that is optimal for all experiments in general. Therefore, they must be determined individually for each experiment. In this work, four weight factors are tested following the recommendations by [PERSON] and [PERSON] [22]. These are the idle time of stations, the improvement in sky-coverage per scan, the number of observations per scan, and the duration of a scan. The idle time criterion gives stations that have not observed a higher weight in a while. The sky-coverage criterion favors an even distribution of scans across the sky of stations, leading to a better estimation of tropospheric delays [27]. The weight factor for the number of observations maximizes the number of observations. The weight factor on the duration of a scan reduces the duration between the end of the previous scan and the end of the tested scans. It includes time for the field system commands, slew time, possible idle time, calibration time, and observation time. Further details on weight factors and their impact on scheduling are available in Schartner and Bohm [22] and Schartner and Bohm [23]. As is typical for VLBI experiments, we create schedules with lengths of 24 h. VieSched++ is capable of including observations of satellites together with observations of geodetic sources in a common schedule. As commonly performed in current VGOS schedules, we choose a fixed scan length of 30 s for observations of geodetic sources [25]. For GENESIS observations, we choose a fixed scan length of 10 s. Based on the values given in [28], we assume that this is a sufficient integration time. For the generation of mixed schedules, scans of GENESIS are always prioritized over scans of geodetic sources if the satellite is visible from at least five stations. At the same time, a minimum repeat time, further referred to as MRT, is introduced for the satellite. Once a scan of GENESIS has been scheduled, the next scan of GENESIS can only be scheduled once the MRT has elapsed. That way we include observations of GENESIS in regular intervals while ensuring we include scans of geodetic sources in between. In this work, we evaluate MRTs from 1 to 10 min in 1 min intervals.