# Observing APOD with the AuScope VLBI Array
## 6 Summary and Outlook
Presumably, systematic signatures in the delay residuals were mainly caused by errors in the a priori orbits used to model the computed delays. Additionally, the described tracking issues could have caused small systematic effects, as the satellite beacon was observed with slight off-axis angles. In general, the delay observables become noisier with increasing elevation, especially in the X-band, indicating that improved tracking features utilizing more accurate orbit data would be beneficial. Considering the experience gained through the presented experiments, we provide some suggestions for future experiments. In general, the common visibility of APOD was limited to elevations lower than 40\({}^{\circ}\) due to the observation geometry, which was determined by continental-wide baselines (between 2360 and 3432 km long) and the low satellite orbit (about 450 km). Shorter baselines would enable one to observe longer tracks with a larger variation in the elevation angle. Nevertheless, a global distribution of observation sites is a prerequisite for the estimation of satellite orbits. Further (simulation) studies are recommended to investigate suitable observation networks for LEO satellites. The VLBI signal emitted by APOD, consisting of narrow band carriers plus symmetric DOR tones at redundant spacing, is not ideal for obtaining delay observables as common in the geodetic VLBI, as it yields a very narrow ambiguity spacing. For classical VLBI processing, wide band noise would be preferable, although the present signal is suitable for other processing schemes, such as Doppler ranging. Furthermore, our experiments clearly showed that adequate satellite tracking features, which enable the antennas to track continuously and accurately are a basic requirement for observations of LEO satellites. It is important to consider this when designing the future global VLBI network in the view of co-location satellite missions. The next step towards fully automated satellite observations is the integration of the AZEL tracking mode provided by the ACU in the Field System (station-specific code). In combination with the full support of the new VEX2.0 format ([[https://safe.nrao.edu/wiki/bin/view/VLBA/Vex2](https://safe.nrao.edu/wiki/bin/view/VLBA/Vex2)]([https://safe.nrao.edu/wiki/bin/view/VLBA/Vex2](https://safe.nrao.edu/wiki/bin/view/VLBA/Vex2))), which will enable one to integrate satellites in the observation schedules and control files, satellites could then be observed like quasars in geodetic sessions. Our work showed that a sophisticated dynamic orbit model is highly beneficial for the analysis of future experiments. Considering a potentially low number of observed tracks--caused by the restricting observation geometry for LEO satellites--and a non-ideal distribution of stations, a kinematic orbit model is not suitable. To be prepared, we plan to upgrade VieVS with a suitable orbit model. Although the results of this study are not suitable yet to study actual frame ties, this work is valuable due to the gained experience in terms of observation, data processing, and analysis strategies. The developed observation and data processing procedures can now serve as guidelines for other observatories, hopefully better preparing the global VLBI network for the next co-location satellite mission. Conceptualization: [PERSON]; investigation: [PERSON]; methodology: [PERSON], L.M., and J.M. ; project administration: [PERSON]; resources: [PERSON], J.M., J.S., and J.C.; software: [PERSON] and J.M. ; writing--original draft: [PERSON]; writing--review & editing: [PERSON], [PERSON], and [PERSON]. The authors thank the Austrian Science Fund (FWF) for funding projects J 3699-N29 and SORTS I 2204. This work was supported by the Australian Research Council (FS1000100037 and FS110200045). This study made use of AuScope VLBI infrastructure. AuScope Ltd is funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme.