# Observing APOD with the AuScope VLBI Array
## 4 Correlation and Post-Processing
### Correlation  
The data were first correlated using the DiFX 2.5 correlation software [28] installed on a small computing cluster at the Mt Pleasant observatory in Hobart. A final correlation iteration was computed on the Vienna Scientific Cluster 3 ([[http://vsc.ac.at/systems/vsc-3/](http://vsc.ac.at/systems/vsc-3/)]([http://vsc.ac.at/systems/vsc-3/](http://vsc.ac.at/systems/vsc-3/))) in Vienna, Austria, using the same software components.  
A first correlation of the quasar signals was made to establish the clock model using a standard approach with the HOPS package. This initial correlation used a 1 s integration time and a spectral resolution of 62.5 kHz. Yarragadee was chosen as the reference station and the a priori clock model of Hobart and Katherine were established from strong detections selected from quasar scans across the duration of the experiment.  
For the APOD scans, the a priori model was inserted via modified IM-files, as described in Section 4.1. To properly handle the narrow DOR tones of the satellite, the zoom-band option in DiFX was used. This enabled the extraction of the narrow bandwidth tones of the APOD signal from the recorded 16 MHz channels (see Figure 3). Ten 32 kHz wide zoom-bands centered on the APOD DOR and carrier tones, as listed in Table 3, were used to extract the APOD signal. These scans were correlated at a fine spectral resolution of 1 kHz and a 0.1 s integration time.  
Detailed investigations give us confidence on the consistency between quasar and APOD data. There are no intrinsic differences between the data. All data follows identical observing configuration with no changes to the analog pathway. Zoom-bands as implemented in DiFX do not cause any systematic differences.  
Figures 4 and 5 show the magnitudes of the auto-spectra of Scans 168 and 169 in Experiment a332 for all zoom-band channels in the S- and X-bands, respectively. The signal magnitudes in the S-band are much smoother and more constant over time compared to the X-band. Presumably, these perturbations are caused by tracking inaccuracies caused by the low quality of the predicted APOD orbits, which were used to calculate the tracking data, and potential deficiencies of the AZEL tracking mode of the ACU (see Section 3). In general, tracking issues have a much larger effect on the X-band, as the width of the directional receiving pattern's main lobe, referred to as the antenna beam-width, is much narrower in the X-band. The beam-width can be approximated by \(BW\approx\lambda/D\), where \(D\) is the antenna diameter and \(\lambda\) denotes the wavelength of the observed signal. Hence, the beam-width in the S-band  
The cross correlation spectra depicted in Figure 6 (S-band) and Figure 7 (X-band) show a similar pattern in the magnitude: again, magnitudes in the S-band are more stable over time compared to the X-band.  
The a priori delay model used for correlation was not accurate enough to stop phase wrapping due to residual delay rate, as evident from the phase plots in Figure 6-8. Due to the smaller wavelength, the phases wrap about four times faster in the X-band, limiting the integration time to 0.1 s in the correlation process.  
Figure 5: Auto-spectrum magnitudes of the X-band zoom-bands of Scans 168 (upper panels) and 169 (lower panels) of Experiment a332. The frequency axes (horizontal axes) depict the five 32 kHz wide DiFX zoom-bands described in Table 3 next to one another. The time axes indicate seconds since the start of Scan 168 (10:41:04 UTC).  
Figure 6: Cross-spectrum magnitudes (left column) and phases (right column) of the S-band zoom-bands of Scans 168 (upper panels) and 169 (lower panels) of Experiment a332. The frequency axes (horizontal axes) depict the five 32 kHz wide DiFX zoom-band described in Table 3 next to one another. The time axes indicate seconds since the start of Scan 168 (10:41:04 UTC).