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While analyzing decades of very long baseline interferometry (VLBI) data, wedetected the secular aberration drift of the extragalatic radio source propermotions caused by the rotation of the Solar System barycenter around theGalactic center. Our results agree with the predicted estimate to be 4-6 microarcseconds per year ({\mu}as/yr) towards {\alpha} = 266\circ and {\delta} =-29\circ. In addition, we tried to detect the quadrupole systematics of thevelocity field. The analysis method consisted of three steps. First, weanalyzed geodetic and astrometric VLBI data to produce radio source coordinatetime series. Second, we fitted proper motions of 555 sources with longobservational histories over the period 1990-2010 to their respectivecoordinate time series. Finally, we fitted vector spherical harmonic componentsof degrees 1 and 2 to the proper motion field. Within the error bars, themagnitude and the direction of the dipole component agree with predictions. Thedipole vector has an amplitude of 6.4 \pm 1.5 {\mu}as/yr and is directedtowards equatorial coordinates {\alpha} = 263\circ and {\delta} = -20\circ. Thequadrupole component has not been detected. The primordial gravitational wavedensity, integrated over a range of frequencies less than 10-9 Hz, has a limitof 0.0042 h-2 where h is the normalized Hubble constant is H0/(100 km s-1).

VLBI measurement of the secular aberration drift