Keith Barnham1, Kaspar Knorr2, Massimo Mazzer3. 1. Physics Department, Imperial College London, London SW7 2BW, UK. 2. Fraunhofer Institute for Wind Energy and Energy System Technology, 34119 Kassel, Germany. 3. CNR-IMEM, Parco Area delle Scienze 37/A, 43124 Parma, Italy.