A Multi-Frequency Study of 3C309.1

Here we summarize our results from a detailed multi-frequency study of the QSO 3C309.1 based on the Very Long Baseline Array (VLBA) observations made in mid 1998. From our images, we find a curved jet extending up to 100 milliarcseconds (mas) to the east at low frequencies with two main components, A and B. A preliminary astrometric analysis (Ros and Lobanov 2001) provides a determination of the core position at different frequencies by phase-referencing to a nearby radio source, QSO S5 1448+76. The changes of the core position with frequency suggest high opacity close to the core caused by synchrotron self-absorption. Due to the large astrometric uncertainties we cannot draw any conclusions about the values of the opacity gradients at high frequencies. We believe that a detailed analysis of the frequency depedence of the core position will reveal the profile of the matter distribution in the broad line region, as was initially suggested by Lobanov (1998)