  We propose an alternative method to detect the secular aberration drift
induced by the solar system acceleration due to the attraction to the Galaxy
centre. This method is free of the individual radio source proper motion caused
by intrinsic structure variation. We developed a procedure to estimate the
scale factor directly from very long baseline interferometry (VLBI) data
analysis in a source-wise mode within a global solution. The scale factor is
estimated for each reference radio source individually as a function of
astrometric coordinates (right ascension and declination). This approach splits
the systematic dipole effect and uncorrelated motions on the level of
observational parameters. We processed VLBI observations from 1979.7 to 2016.5
to obtain the scale factor estimates for more than 4,000 reference radio
sources. We show that the estimates highlight a dipole systematics aligned with
the direction to the centre of the Galaxy. With this method we obtained a
Galactocentric acceleration vector with an amplitude of 5.2 $\pm$ 0.2 \mu as/yr
and direction $\alpha_G = 281\deg \pm 3\deg$ and $\delta_G = -35\deg \pm
3\deg$.
