We describe a new tool for studying the structure and physical characteristics of ultracompact AGN jets and their surroundings with \mu as precision .
This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources , where the variability is caused by interstellar scintillation .
We apply this method to PKS 1257–326 to resolve the core-shift as a function of frequency on scales well below \sim 12 \mu as .
We find that the frequency dependence of the position of the scintillating component is r \propto \nu ^ { -0.1 \pm 0.24 } ( 99 \% confidence interval ) and the frequency dependence of the size of the scintillating component is d \propto \nu ^ { -0.64 \pm 0.006 } .
Together , these results imply that the jet opening angle increases with distance along the jet : d \propto r ^ { n _ { d } } with n _ { d } > 1.8 .
We show that the flaring of the jet , and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with p \propto r ^ { - n _ { p } } and n _ { p } \gtrsim 7 .
Such steep pressure gradients have previously been suggested based on VLBI studies of the frequency dependent core shifts in AGN .