# Observing APOD with the AuScope VLBI Array
## 5 Data Analysis  
The multi-band delay observables derived by the processing scheme outlined in Section 4 were analyzed with VieVS 3.0., which is able to handle quasar observations, as well as observations of satellites. All target parameters can be estimated in a least-squares adjustment (Gauss-Markov model, e.g., [39]) based on reduced observations, which are calculated by subtracting the computed (theoretical) delays from the observed delays (observed minus computed, o-c).Theoretical delays for APOD observations were calculated with the near-field delay model outlined in Section 4.1 using the final orbit solution by BACC (see Section 2.2). The analysis is based on the so-called total delays, which were computed as the sum of multi-band delay residuals calculated in fourfit and the a priori delays of the correlator input model. The observation reference epochs are the times of the signal reception at Station 1 (of the baseline), calculated at integer seconds, as common in geodetic processing. Basically, we have three sets of delay results for each experiment: (1) S-band delays, (2) X-band delays, and (3) the ionosphere free linear combination. To minimize the effect of the ionosphere, we decided to use Option 3, although the quality of the S-band observations--in terms of amplitude stability and SNR (see Section 4)--seems to be higher. Multi-band delays were also obtained from the quasars observed in Experiment a332 by standard VLBI data processing with DiFX and fourfit. These quasar delays were written to an Mk3 database, processed in \(\
u\)Solve [40] and exported as a Level 4 NGS file. They were then analyzed in VieVS by estimating a set of standard parameters: station coordinates, source coordinates, EOP, ZWD, and station clock parameters. The WRMS of the post-fit residuals was 38 ps. The obtained ZWD estimates shown in Figure 11 were then applied a priori on the APOD observations to correct for effects caused by the wet fraction of the atmosphere. The Vienna Mapping Function (VMF1 [41]) was used to determine the slant delay corrections. Investigating the o-c delay residuals provide a first impression of how well the observed delays match the theoretical delays modeled in VieVS. The top panel in Figure 12 depicts o-c values for two consecutive APOD scans, 168 and 169, in Experiment a332. The observed delays were corrected for the station clock offsets and rates applied in the correlation step to keep the absolute o-c values small. Figure 11: Zenith Wet Delay (ZWD) estimated from quasar observations in Experiment a332. The time at which the APOD Scans 168 and 169 were observed are highlighted. The bottom panel in Figure 12 depicts the major delay corrections applied on the purely geometric near-field delays (see Section 4.1), which target the effects of the hydrostatic (trop_h) and the wet (trop_w) fraction of the troposphere. The hydrostatic part was modeled according to [42] using in situ pressure measurements and the VMFI\({}_{h}\) mapping function. The wet fraction was modeled by mapping the ZWD estimates derived by the quasar observations in Experiment a332 down to slant directions using the VMF1\({}_{w}\) mapping function. The total troposphere corrections are in the range between \(\sim\)\(-61\) ns and \(\sim\)\(+51\) ns. Other corrections not being illustrated here due to their comparably small magnitudes are thermal antenna deformation (max. 1.8 ps [43]), gravitational effects on the near-field delays according to [37] (max. \(\pm 24\) ps) and the effect of antenna axes offsets (max. 44 ps). In Scan 168, the residual delays show little variation of \(\sim\)1.5 ns over the scan duration. In contrast, the delay residuals in Scan 169 are noisier and they show a systematically curved variation with a magnitude of \(\sim\)12 ns over the scan. Looking at the elevation angles at which APOD was observed during these scans (Figure 12, third panel), we suppose that the more pronounced scatter in the second scan can be explained by tracking issues that are more likely to occur at higher elevations.