Ulrich Schreiber is a Professor at the Technical University of Munich. He has more than thirty years of research experience in the technology of space geodesy, in particular Satellite and Lunar Laser Ranging, ring laser development and optical time transfer. He received the Huygens medal for Instrumentation from the European Geosciences Union in 2016. Jon-Paul Wells is a Professor of Physics at the University of Canterbury in Christchurch, New Zealand. His research interests include large ring laser gyroscopes, optical interferometry and laser spectroscopy of inorganic solids.