# Observing APOD with the AuScope VLBI Array
## 3 Scheduling and Observations  
The starting point of each VLBI experiment is the observation planning, referred to as scheduling. In general, VLBI schedules define which antennas observe which source at what time. The scheduling task itself is complicated due to the large number of different observation criteria that have to be considered, such as common visibility of the target from remote sites, slew times between consecutive scans, and the determination of required on-source time to reach the target SNR. For planning the discussed experiments, the VieVS satellite scheduling module [32] was used. It allows for the scheduling of VLBI observations of near-field targets along with observations of natural extragalactic radio sources, which are routinely observed in geodetic VLBI campaigns. The natural sources are selected automatically by optimizing the sky coverage at all stations as common for geodetic VLBI sessions [33]. The observation geometry that was determined by continental-wide baselines and the very low orbit of APOD was a major limiting factor for scheduling. On average, the APOD satellite was visible by individual AuScope telescopes four times a day for a couple of minutes only. The projected fields of view are displayed in terms of shaded red circles in Figure 2. Only in the intersecting areas was APOD simultaneously visible by two stations. As indicated in Figure 2, common visibility from all three AuScope telescopes was restricted to very low elevation angles and scan durations as short as a couple of minutes at most. In general, APOD was simultaneously visible by only two of the three antennas, limiting the experiment design to single-baseline scans. Due to these restrictions, it was not possible to observe more than two single-baseline scans shortly after another during a flyover, as exemplarily indicated in Figure 2 for two consecutive scans (168 and 169) in Experiment a332. In this example, APOD was observed first on the baseline Hb-Yg for 86 s followed by a second scan on Ke-Yg lasting for 256 s. The next occasion for an APOD scan only existed about 10.5 h later that day. Despite these circumstances, we took every chance from November 11 to 14, 2016, and observed APOD whenever common visibility from two AuScope antennas was given. All observed experiments are listed in Table 1, which also provides details on the observation durations. During observations, APOD was tracked continuously by the antennas as long as common visibility was given and the signal was recorded continuously. (Each of these recorded APOD tracks is referred to as _scan_ in this paper, which must not be mixed up with the term _observation_. Due to the high SNR, delays could be derived in a 1 s interval, i.e., a single scan is chopped up into multiple observations.) Due to the observation geometry APOD scans were restricted to low elevation angles in general with most experiments being observed at elevations between 5 and 20\({}^{\circ}\), at some rare occasions at higher elevations up to 37\({}^{\circ}\) at most. In general, the data analysis becomes more challenging due to the consistently low elevations, as the effects of the neutral atmosphere and the ionosphere on the observed signals increase with decreasing elevation. Furthermore, estimates of station heights, zenith wet delays, and station clocks do not decorrelate properly without observations at varying elevations (e.g., [34]). On the other side, the change rates of the topocentric antenna pointing directions (azimuth and elevation rates) decrease as the distance between antenna and satellite increases with lower elevation angles. Hence, the demands on the tracking and on the pointing accuracy of the antenna decrease at lower elevation angles. Therefore, the APOD satellite was easier to keep within the field of view via the observing antennas at low elevations than at high elevations. We were able to assess the received signal live during APOD tracks by a spectrum analyzer connected to the intermediate frequency (IF) channels at the recorder racks.