Opacity in parsec-scale jets of active galactic nuclei

In extragalactic jets, the apparent position of the bright/narrow end (the core) depends on the observing frequency, owing to synchrotron self-absorption and external absorption.The effect must be taken into account in order to achieve unbiased results from multi-frequency VLBI data on AGN jets.Multi-frequency core shift measurements supplemented by other data enable estimating the absolute geometry and a number of fundamental physical properties of the jets and their environment.We have previously measured the shift between 13 and 3.6 cm in a sample of 29 AGNs to range between 0 and 1.4 mas.In these proceedings, we present and discuss first results of our follow-up study using VLBA between 1.4 and 15.4 GHz.