The “ core ” of a radio source is believed to mark the frequency-dependent location where the optical depth to synchrotron self–absorption \tau _ { s } \approx 1 .
The frequency dependence can be used for derive physical conditions of the radio emitting region and the ambient environment near the central engine of the radio source .
In order to test and improve the models to derive this information , we made multi-frequency dual-polarization observations of 3C 309.1 in 1998.6 , phase-referenced to the QSOs S5 1448+76 and 4C 72.20 ( S5 1520+72 ) ( 4 ^ { \circ } 26 ^ { \prime } and 1 ^ { \circ } 49 ^ { \prime } away , respectively ) .
We present here preliminary results from these observations : total and polarized intensity maps , spectral information deduced from these images , and the relative position of 3C 309.1 with respect to S5 1448+76 at different frequencies .
Finally , we discuss briefly the observed shift of the core position .