New source flux density and antenna sensitivity monitoring in AUSTRAL VLBI sessions for improved feedback and results

Abstract In very-long-baseline interferometry (VLBI) observations, two or more telescopes observe distant quasars and record the radio emission at specific frequency ranges. The source flux densities and telescope sensitivities are two essential parameters for determining the duration of observation on each source to yield the target signal-to-noise ratio (SNR). The source flux density varies with the baseline length, viewing angle and changes in the structure of the source itself, whereas the antenna sensitivity is often assumed to be constant at the zenith. A geodetic VLBI session is typically 24-hours long and is comprised of as many short observations as possible on various sources. In the data processing stage, the recorded signal of each observation needs to reach an SNR threshold to produce useful geodetic products. An SNR target of at least two times higher than required is usually in place during the planning of the VLBI session. However, we find that the Australian VLBI sessions (AUSTRALs) still consist of observations with insufficient SNR, which suggests either the inaccuracy of the assumed source flux densities, antenna sensitivities, or both. In this work, we analysed the SNR performance for 72 S/X AUSTRAL sessions since 2014 and investigated the possibility to increase the number of observations and session performance. We start by monitoring the sensitivity of the three 12m AuScope VLBI telescopes and the source flux density directly from these sessions based on a source- and observation-specific approach. This research shows an improvement of more than two standard deviations in the SNR prediction compared to the current utilised practice, which means the prediction for the sensitivity of telescopes and source flux density is more precise. This research can potentially improve the observing efficiency from the current global average of about 80%. We can also better define good sources in the southern sky for geodesy. This new approach (a scan-by-scan or `scan-wise' procedure) is the first step to realise continuous feedback and observation improvement.