# Observing APOD with the AuScope VLBI Array
## 6 Summary and Outlook  
The Chinese APOD-A nano satellite can be considered as the first prototype of a co-location mission enabling VLBI with SLR and GNSS on an LEO satellite.Unfortunately, it has hardly been observed by VLBI, so major studies towards the actual frame ties could not be performed. The main reason for the lack of observations was that VLBI observations of satellites are non-standard, and suitable observing strategies were not in place for this mission. This work now presents the first serious attempt to observe the satellite with a VLBI network over multiple passes. We discuss a series of APOD observations with the AuScope geodetic VLBI array carried out in November 2016. We describe all steps--from the initial experiment design, to the observation and data acquisition procedures, to the correlation and post-processing scheme, and to the analysis of the derived delay observables in VieVS. By the example of Experiment a332, we discussed all applied processing steps, yielding o-c residuals on the level of a few nanoseconds. One major limitation was the bad quality of the APOD orbit solutions due to the absence of GNSS raw data caused by malfunction of the on-board receiver. The predicted orbit data used for tracking show offsets of up to 1000 m compared to the final orbit solutions, which were used to calculate theoretical near-field delays for correlation and analysis. We assume that the low quality of the orbit data, in combination with potential deficiencies of the AZEL tracking mode provided by the antenna controllers, lead to pointing inaccuracies on the level of a few arc-minutes. The mis-pointing caused wide amplitude variations in the received X-band signals, resulting in a lower and more variable SNR of the X-band data compared to the S-band (where the antenna field of view is about four times wider). On the data processing level, the low quality of the a priori orbit limited the accuracy of the computed theoretical delays. Hence, the correlator input model was not accurate enough to fully stop phase wrapping. However, the delay model was sufficient to decrease the phase rate to such an extent that a practicable integration time could be used without introducing decorrelation. Facing the problems caused by the inaccurate orbit solutions, it would be beneficial to use SLR observations (in addition to the available GNSS measurements) to improve the orbit determination. SLR measurements could not only be used to calibrate orbit errors in radial direction but also along-track errors due to the low elevation angles at which most of the LEO tracking data are collected. According to [PERSON] et al. [46], SLR can contribute along-track corrections on the millimeter level and thus substantially improve the overall orbit accuracy. Unfortunately, SLR measurements were sparsely available at the times VLBI observations were performed, precluding this option. It is highly advisable that, for future experiments, VLBI observations are coordinated with SLR tracking campaigns. For the sake of brevity, only one experiment is discussed in detail. Experiment a332 was chosen as a representative example. However, delay observables could be obtained from all experiments using the presented process chain. (Results in terms of total multi-band delays for all sessions listed in Table 1 are publicly available in the IVS working group 7 Wiki at [[http://auscope.phys.utas.edu.au/opswiki/doku.php?id=wg?:apod:apod_obs_auscope](http://auscope.phys.utas.edu.au/opswiki/doku.php?id=wg?:apod:apod_obs_auscope)]([http://auscope.phys.utas.edu.au/opswiki/doku.php?id=wg?:apod:apod_obs_auscope](http://auscope.phys.utas.edu.au/opswiki/doku.php?id=wg?:apod:apod_obs_auscope)). In addition to the results, all session control files, e.g., VEX schedules, are provided along with operator notes.) All residual multi-band delays show similar characteristics in terms of magnitude, scatter (S-band is always smoother than X-band), and SNR (higher and more stable in the S-band).