Optical Clocks in Space
  The performance of optical clocks has strongly progressed in recent years, and accuracies and instabilities of 1 part in 10^18 are expected in the near future. The operation of optical clocks in space provides new scientific and technological opportunities. In particular, an earth-orbiting satellite containing an ensemble of optical clocks would allow a precision measurement of the gravitational redshift, navigation with improved precision, mapping of the earth's gravitational potential by relativistic geodesy, and comparisons between ground clocks. 
S. Schiller, A. Goerlitz, A. Nevsky, J.C.J. Koelemeij, A. Wicht, P.   Gill, H. A. Klein, H. S. Margolis, G. Mileti, U. Sterr...