Datensatz

Zusammenfassung

Effect of source structure is the (only) dominant factor that limits the accuracy of the celestial reference frame. The AGNs in absolute astrometric and geodetic observations are still assumed to be an ideally point on the sky although their structure and its evolution over both time and frequency are well understood in radio astronomy. I will report on my study about investigating these effects based on the observations from the new generation geodetic VLBI system, VLBI Global Observing System (VGOS). VGOS makes regular observations since 2019 with a network of 7 to 12 antennas simultaneously at the four bands in the range of 3 to 11 GHz. This network is expanding globally. I have imaged these VGOS observations on a routine basis via closure images because of the known issue of missing antenna calibrations for geodetic antennas. These VGOS images show that nearly all VGOS sources have “invisible” structure at the angular scales of 0.1 milli-arcsecond or higher. Another challenge of modeling source structure in VGOS observations is the image alignment over the four frequencies, where core shift will also play a role. According to my simulations, any misalignment among the highest three frequency bands is magnified in the source position estimates from VGOS. I will report on how to valid the image alignment by using phase delays. It is to be noted that not only for geodesy but also for astrometry modeling source structure is so important and is the necessary step forward.