  Relativistic jets in active galactic nuclei represent one of the most
powerful phenomena in the Universe. They form in the surroundings of the
supermassive black holes as a by-product of accretion onto the central black
hole in active galaxies. The flow in the jets propagates at velocities close to
the speed of light. The distance between the first part of the jet that is
visible in radio images (core) and the black hole is still a matter of debate.
Only very-long-baseline interferometry observations resolve the innermost
compact regions of the radio jet. Those can access the jet base, and combining
data at different wavelenghts, address the physical parameters of the outflow
from its emission. We have performed an accurate analysis of the
frequency-dependent shift of the VLBI core location for a multi-wavelength set
of images of the blazar CTA 102 including data from 6 cm down to 3 mm. The
measure of the position of the central black hole, with mass $\sim
10^{8.93}\,M_\odot$, in the blazar CTA 102 reveals a distance of $\sim
8\times10^4$ gravitational radii to the 86 GHz core, in agreement with similar
measures obtained for other blazars and distant radio galaxies, and in contrast
with recent results for the case of nearby radio galaxies, which show distances
between the black hole and the radio core that can be two orders of magnitude
smaller.
