RadioAstron gravitational redshift experiment: status update
  A test of a cornerstone of general relativity, the gravitational redshift effect, is currently being conducted with the RadioAstron spacecraft, which is on a highly eccentric orbit around Earth. Using ground radio telescopes to record the spacecraft signal, synchronized to its ultra-stable on-board H-maser, we can probe the varying flow of time on board with unprecedented accuracy. The observations performed so far, currently being analyzed, have already allowed us to measure the effect with a relative accuracy of $4\times10^{-4}$. We expect to reach $2.5\times10^{-5}$ with additional observations in 2016, an improvement of almost a magnitude over the 40-year old result of the GP-A mission. 
D. A. Litvinov, U. Bach, N. Bartel, K. G. Belousov, M. Bietenholz, A.   V. Biriukov, G. Cimo, D. A. Duev, L. I. Gurvits, A. ...
astro-ph.IM gr-qc