Observing Gravitational Radiation with QSO Proper Motions and the SKA
  We discuss the ability of the SKA to observe QSO proper motions induced by long-wavelength gravitational radiation. We find that the SKA, configured for VLBI with multiple beams at high frequency (8 GHz), is sensitive to a dimensionless characteristic strain of roughly $10^{-13}$, comparable to (and with very different errors than) other methods in the 1/yr frequency band such as pulsar timing. 
Andrew H. Jaffe