Monitoring sensitivity parameters for improved AUSTRAL VLBI

Abstract

Within this work a detailed investigation of the sensitivity by means of the system temperature (Tsys) and the system equivalent flux density (SEFD) is carried out. 130 AUSTRAL VLBI sessions of the AuScope VLBI array are analyzed and a significant elevation dependency of the sensitivity is found. To account for this systematics, mapping function coefficients are estimated for each station which can be implemented in the state-of-the-art VLBI scheduling packages. An increase in both the VLBI measurement precision and in schedule performance is expected. A method is presented to examine these expectations based on the re-estimation of the SEFD. The study of the past AUSTRAL sessions shows a strong scatter of the re-estimated SEFDs. This indicates the difficulties in the prediction of VLBI scheduling parameters. Comparing the Tsys measurements and the SEFD reveals that the absolute magnitudes of the Tsys measurements must be under critical examination, due to systematics in the calibration signal. The relative variations in the Tsys measurements show reliable values and can be applied for real-time sensitivity determination of the AuScope VLBI antennas. Only Yarragadee shows unexpected offsets in the time series of Tsys measurements and must be reviewed critically. Besides the elevation systematics, a daily and yearly signal of the sensitivity are found. This correlates with the atmospheric temperature signal at the site. An impact on the position accuracy level due to unconsidered variations of the VLBI antennas sensitivity cannot be found. This work provides a better understanding of the AuScope VLBI scheduling characteristics based on the elaboration of the AuScope VLBI network sensitivity parameters. Additionally, a software is developed to monitor these parameters.