Lidars (LIght Detection And Ranging instruments) measure the radiation that is returned either from molecules and particles in the atmosphere or from the Earth’s surface when illuminated by a laser source. Compared with radar, the shorter wavelengths used in a lidar allow greater detail to be observed. On the other hand, the light cannot penetrate optically thick layers such as clouds.
There are different types of lidar instrument:
— the backscatter lidar, in which the laser beam is backscattered, reflected or re-radiated by the target, gives information on the scattering and extinction coefficients of the various atmospheric layers being probed;
— the differential absorption lidar analyses the returns from a tuneable laser at different wavelengths to determine densities of specific atmospheric constituents, as well as water vapour and temperature profiles;
— Doppler lidar measures the Doppler shift of the light backscattered from particles or molecules moving with the wind, thereby allowing the determination of wind velocity;
— the ranging and altimeter lidar provides accurate measurements of the distance from a reference height to precise locations on the Earth’s surface.
The first satellite-borne ranging and altimeter lidar, GLAS, was flown on the NASA ICESat mission which was launched in January 2003 (and completed September 2010) to study the variations of ice topography, as well as cloud and atmospheric properties. In April 2006, the CALIOP backscatter lidar, flying on the NASA CALIPSO platform, was launched to measure cloud and aerosol properties. ESA is currently implementing two laser missions, ADM-Aeolus and EarthCARE. The ALADIN high spectral resolution Doppler wind lidar on board ADM-Aeolus will measure profiles of line-of-sight winds globally. The ATLID high spectral resolution lidar on board EarthCARE will measure cloud and aerosol optical properties.