D. Lipka \emph{et al.}, "FLASH Undulator BPM Commissioning and Beam Characterization Results", in Proc. 3rd International Beam Instrumentation Conference, Monterey, CA, USA, paper TUPF07, pp. 315–319, ISBN: 978-3-95450-141-0, \url{http://jacow.org/IBIC2014/papers/tupf07.pdf}, 2014.