Atmospheric and Meteorological Lidars Basic principles

Lidar is an active optronic measurement method that uses lasers to probe the depths of sparsely populated environments such as the atmosphere and aquatic zones, and measure the distance to scattering volumes or solid targets (soil, buildings, etc.). The lidar instrument combines optical, electronic and digital components. The word lidar is an abbreviation of "light detection and ranging". The method is based on the propagation of a short laser pulse, followed by detection of the light reflected by the medium. The round-trip time between laser emission and detection determines the distance to the scattering zone or reflective target. The optical properties of the probed media are obtained by spectral and polarimetric analysis of the scattered light, while the dynamic properties (velocity fields, turbulence) are obtained by Doppler analysis. Lidar is used for targets ranging from a few dozen meters to several hundred kilometers (space lidar), or even farther away (Earth-Moon distance measurement).

The principle of the atmospheric lidar method was tested in the 1930s-1940s using very intense light projectors. From 1962-1963 onwards, lidars used exclusively lasers, preferably pulsed. In practical terms, lidar applications took on great importance in the late 1990s, following major technical advances in lasers and detectors, as well as in portable computers for data management, real-time display and analysis.

Since the 2000s, the atmospheric lidar community has diversified, opening up to service applications alongside ongoing innovative research. New applications are due to :

• technical advances leading to the miniaturization of reliable instruments;

• the use of new vectors (aircraft, drones, satellites);

• to coordinated network activities, offering a continuous range of temporal and geographical coverage, from local to global.

These innovations have led to a considerable increase in data volumes, which, combined with the multitude of information available from other sources, has created a big data emulation within the atmospheric and meteorological lidar community. This community has been enriched by the arrival of non-lidar specialists interested in data processing and analysis for more general work on geophysics or climate, for example. In this way, lidar has become commonplace and should be studied as an integral part of remote sensing methods.

This article deals with fundamental lidar principles as a prerequisite for atmospheric and meteorological applications