  Jets of compact radio sources are highly relativistic and Doppler boosted,
making studies of their intrinsic properties difficult. Observed brightness
temperatures can be used to study the intrinsic physical properties of the
relativistic jets, and constrain models of jet formation in the inner jet
region. We aim to observationally test such inner jet models. The very long
baseline interferometry (VLBI) cores of compact radio sources are optically
thick at a given frequency. The distance of the core from the central engine is
inversely proportional to the frequency. Under the equipartition condition
between the magnetic field energy and particle energy densities, the absolute
distance of the VLBI core can be predicted. We compiled the brightness
temperatures of VLBI cores at various radio frequencies of 2, 8, 15, and
86~GHz. We derive the brightness temperature on sub-parsec scales in the rest
frame of the compact radio sources. We find that the brightness temperature
increases with increasing distance from the central engine, indicating that the
intrinsic jet speed (the Lorentz-factor) increases along the jet. This implies
that the jets are accelerated in the (sub-)parsec regions from the central
engine.
