Systematic effects in apparent proper motion of radio sources
  The galactocentric rotation of the Solar system generates the systematic effect in proper motions known as 'secular aberration drift'. This tiny effect (about five microseconds per year) in the quasar proper motion can be measured by VLBI. However, the motions of relativistic jets from the active extragalactic nuclei can reach several hundred microseconds per year and mimic the proper motion of the observed radio sources. These apparent motions exceed the secular aberration drift by factor of 10-100. In this paper we search the ways to overcome the difficulties and discuss our estimates of the secular aberration drift using OCCAM software. 
O. Titov