The GOP includes 1) a cooperation with European Aerosol Research Lidar Network (EARLINET) and 2) the use of lidar ceilometers which are based on much simpler techniques compared to research lidars but
Up to three cloud base heights of the ceilometer network of the German weather service. For no clouds detected -999. is the _FillValue. For instrument failure -9.e+33f. is taken. http://gop.meteo.uni-koeln.de/gop/doku.php?id=gop_4