Measuring the Beam Energy with Radiative Return Events
  This paper studies the possibility to measure the centre of mass energy using e+e- -> Z,gamma -> mu+mu-,gamma events at the ILC. With L = 100 fb^-1 at sqrt(s) = 350 GeV a relative error of around 10^{-4} is possible. The potentially largest systematic uncertainty comes from the knowledge of the aspect ratio of the detector. 
A.Hinze, K.Moenig