The Vaisala CT75K is a lidar ceilometer system that makes elastic scattering measurements of clouds and aerosols. It operates at 905 nm and comprises four transmitter/receiver units that operate simultaneously. By ceilometer standards it has relatively high sensitivity, and so has the ability to detect ice as well as liquid water clouds. It is a low maintenance system capable of virtually unattended operation and is operated continuously at Chilbolton. It is normally operated close to vertically pointing, typically around 4° off zenith, to reduce specular reflections from ice clouds.