# Observing APOD with the AuScope VLBI Array
## 3 Scheduling and Observations
Our experience was that the signal amplitude became increasingly unstable at higher elevation angles, especially in the X-band due to the narrower field of view. While the S-band signals were rather stable throughout all experiments, we even experienced signal loss in the X-band occasionally at elevations above \(\sim\)30\({}^{\circ}\). We think that these tracking issues were mainly caused by the low accuracy of the predicted APOD orbits used for tracking, which show offsets to the final orbits of up to 1000 m (see Section 2.2). At an elevation of 30\({}^{\circ}\) the max. pointing error caused by an offset of 1000 m in the APOD orbit was about 4.2\({}^{\prime}\). With a beam-width of about 10.2\({}^{\prime}\) in the X-band, pointing errors of a few arc-minutes can already be critical. Furthermore, the internal interpolation of the tracking data in the ACU might not be accurate enough to precisely follow such a fast satellite. If there is already a pointing offset caused by the tracking data, additional inaccuracies due to a non-optimal interpolation of the data in the ACU may be enough to cause severe pointing problems in the X-band--even at moderate elevation angles. On average five quasars were observed before and after the APOD track(s) in Experiments 316a to 319a. (For the sake of brevity, we only mention quasars when we talk about natural radio sources, as the most common type of active galactic nucleus (AGN) observed in the geodetic VLBI. To be precise, we do not exclude the possibility that other types of AGN were observed as well in the described experiments.) The main reason for the inclusion of quasars in the observation plan was to use them as calibrator sources in order to establish an initial clock model in the correlation process as outlined in Section 4.2). Experiment a332 was designed differently: Basically, it is a geodetic 24 h VLBI session consisting of 761 three-station scans to strong quasars with a minimum flux density of 0.65 Jy. For the scheduling we optimized the sky coverage at each site in order to decorrelate estimates of station clocks, station heights, and troposphere delays. Whenever visible, APOD was observed in between the quasars, resulting in four single-baseline scans to APOD in 24 h. The first two scans (168 and 169), which are illustrated in Figure 2, are used as a generic example in this paper. This particular session design with satellite scans being embedded in quasar scans allows for additional analysis options, such as using Zenith Wet Delays (ZWDs) estimated in a quasar-only solution, to correct the APOD observations a priori (for more details, we refer to Section 5). The observing mode used for all described experiments was designed pursuing two purposes: (1) to record the full APOD S- and X-band signals and (2) to derive reasonable geodetic results from standard observations of quasars at the same time. Changing the observation mode when switching between quasars and APOD was not an option as it might have introduced unknown systematic biases. The observation mode has been slightly modified from the mode used for the AUSTRAL sessions described in [35]. Data was recorded in 16 channels with 16 MHz bandwidth each, 10 in the X-band and 6 in the S-band, applying 2-bit sampling, which results in a recording rate of 64 Mbps per channel. The frequency allocation is illustrated in Figure 3. Due to strong Radio Frequency Interference (RFI) in the lower S-band at Hobart, the S-band channels were allocated contiguously, yielding continuous frequency coverage. Only 4 of 16 channels were dedicated to record the APOD tones. The purpose of the others was to increase the frequency coverage for quasar observations to enable the calculation of reasonable multi-band delays. Publicly available two-line element (TLE) data sets (e.g., at [[https://celestrak.com/](https://celestrak.com/)]([https://celestrak.com/](https://celestrak.com/))) were used for the initial orbit determination by VieVS and enable one to plan the experiments up to several weeks in advance.