This document specifies 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:
• height of structures, e.g. inversions, boundary layer height, mixing layer height (under suitable conditions);
• attenuated backscatter of the particles;
• particle backscatter and extinction coefficients (requires further assumptions).

The document also addresses the depolarisation lidar and the use of multi-wavelength systems. This allows further parameters to be determined:

• particle size classification (Ångström exponent, 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 and visual range) (see ISO 28902-1[8]);
• particle content and transport (e.g. volcanic dust);
• weather forecasting and climate modelling (e.g. atmospheric boundary layer, cloud base, cloud microphysics);
• satellite remote sensing (validation).

Examples that illustrate these applications are discussed in Annex A.

The benefits of scanning systems for parameters mentioned above are also discussed in Clause A.7.

In addition, particle backscatter lidars that measure at least two carefully selected wavelengths can be used to determine atmospheric gas concentrations. This is known as the differential absorption lidar “DIAL” technique. This technique is not part of this document and has been described in VDI 4210-1[12].

This document does not specify extended lidar techniques that monitor the following parameters quantitatively: inelastic scattering effects such as, Raman scattering, Doppler broadening, Doppler shift, multiple scattering, modulation techniques, and spectral separation of molecular and particle backscattering [high spectral resolution lidar (HSRL)]. Some of these extended techniques are or will be described in other parts of the ISO 28902 series.

This document does not address special features of airborne or satellite-borne systems.