  We present a method to measure cosmic magnetic fields with ultra high energy
cosmic rays (UHECRs). We apply an advanced autocorrelation method to simulated
UHECRs which includes their directional as well as energy information. Without
explicit knowledge of the UHECR sources, such measurements are sensitive to the
number of sources and to the magnetic field strength subjected to the UHECRs.
Using a UHECR Monte Carlo model including sources, random walk propagation and
a coherent deflection, we explain the procedure of reconstructing the allowed
phase space of the model parameters from a simulated autocorrelation
measurement.
