This Standard describes the determination of height-resolved profiles of atmospheric backscattering by means of active optical sounding. The measurements allow the following properties of the atmosphere up to several kilometres above ground to be derived: • Cloud bases • Upper boundaries of optically thin clouds • Upper and lower boundaries and internal structures of particle layers: – The height of thermal structures, e.g. inversions – Boundary layer height, mixing layer¬ height (under suitable conditions) • Backscatter coefficients of the particles • Extinction coefficient (requires further assumptions). The Standard goes on to discuss the depolarisation lidar and the use of multi-wavelength systems. This allows further parameters to be determined: • Particle size classification (Ångström expo¬nent, colour ratio) • Shape classification (linear depolarisation degree) The following fields of application are particularly important: • Air quality monitoring (vertical structure of the boundary layer) • Aviation safety (cloud base) • Particle content and transport (e.g. volcanic dust) • Weather forecasting and climate modelling (e.g. atmospheric boundary layer, cloud ¬base) • Satellite remote sensing (validation) Examples that illustrate these applications are discussed in Section 11. This Standard does not describe extended lidar techniques that capture the following effects quantitatively: inelastic scattering effects (Raman scattering), Doppler shift, multiple scattering, modulation techniques and spectral separation of molecular and particle backscattering (HSRL = high spectral resolution lidar). Some of these extended techniques have already been or will be described in other parts of the ISO 28902 series. This Standard does not deal with the special features of aircraft-borne or satellite-aided systems nor those of scanning systems.