# Observing APOD with the AuScope VLBI Array
## 2 Experiments
### VLBI Station Network and Tracking  
The Australian AuScope geodetic VLBI array [26] consists of three radio telescopes: HOBART12 (Hb), KATH12M (Ke), and YARRA12M (Yg). Their geographical locations and the lengths of the resulting baselines are indicated in Figure 2. The antennas are similar in design with small 12 m reflectors and fast slew rates of up to 5\({}^{\circ}\)/s in azimuth and up to 1.5\({}^{\circ}\)/s in elevation.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment Name** & **Stations** & **Exper. Start [UTC]** & **Duration (Duration [s])** & **APOD Tracks** \\ \hline
316a & Ke, Yg & 11 November 2016, 22:15:00 & 33 min 11 s & 1 (211) \\
317a & Ke, Hb & 12 November 2016, 09:34:00 & 41 min 33 s & 1 (27) \\
317b & Ke, Hb, Yg & 12 November 2016, 21:35:00 & 34 min 49 s & 2 (161, 137) \\
318b & Ke, Hb, Yg & 13 November 2016, 10:48:30 & 25 min 57 s & 2 (41, 241) \\
318c & Ke, Hb & 13 November 2016, 21:01:00 & 25 min 59 s & 1 (44) \\
318d & Ke, Yg & 13 November 2016, 22:35:00 & 22 min 55 s & 1 (153) \\
319a & Ke, Hb, Yg & 14 November 2016, 10:00:00 & 40 min 5 s & 2 (74, 201) \\ a332 & Ke, Hb, Yg & 27 November 2016, 16:00:00 & 24 h & 4 (86, 256, 29, 189) \\ \hline \hline \end{tabular}
\end{table}
Table 1: A list of APOD experiments by AuScope in November 2016. Experiments 316a to 319a lasted for about 40 min at most observing one or two consecutive single-baseline scans to APOD shortly after another. Experiment a332 lasted for 24 h mostly observing quasars in a geodetic schedule plus four APOD tracks in between. The last column (APOD tracks) indicates the number of APOD tracks observed in this session and the duration of each scan in seconds (in brackets).  
Figure 1: Simplified experiment work flow diagram illustrating the external input data (rhomboid boxes, left), the used software and infrastructure (rectangular boxes, right), and the essential interchange data (italic text).  
In addition to the standard tracking mode for natural sources such as quasars, Antenna Control Units (ACUs) provide a dedicated AZEL tracking mode which is applicable for satellite tracking. In this mode, an ACU loads lists of time-tagged azimuth and elevation positions and interpolates the actual pointing angles smoothly between these tracking points. In initial tests, other tracking schemes have been tested: the TLE tracking mode implemented in the NASA Field System (FS, [[https://lupus.gsfc.nasa.gov/fsdoc/fshome.html](https://lupus.gsfc.nasa.gov/fsdoc/fshome.html)]([https://lupus.gsfc.nasa.gov/fsdoc/fshome.html](https://lupus.gsfc.nasa.gov/fsdoc/fshome.html))) and a so-called step-wise tracking approach. The latter enables satellite tracking by defining discrete source positions in terms of topocentric right ascension and declination angles in short intervals of, e.g., 1 s in the VEX file. The high position update rate in combination with the antenna's inertia enable tracking of the satellite more or less continuously. Both schemes failed to track APOD, because they are implemented in the FS, which is simply overcharged by the high rate of positioning commands required to track such a fast satellite. In contrast, the AZEL tracking mode of the ACU directly controls the antenna without any detour via the FS, with the shortcoming that the standard control procedures for experiment automation are not accessible. Hence, manual interaction is required to switch between tracking modes and to load the AZEL tracking files individually at each antenna which is error-prone and time-consuming.  
All AuScope antennas are equipped with modern Digital Base Band Converters (DBBC-2) for digitization and formatting and Mark5B+ recorder systems. The antenna feeds are designed for the S- and X-band domains and receive circular polarized signals. For more technical details, we refer to [26].