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LIDAR (= light detection and ranging)

Powerful laser pulses are sent out into the atmosphere. The light scattered back to earth by distant particles or molecules is collected and analyzed as a function of time. The distance between the LIDAR system and the measurement volume can be determined because we know the speed of light.

Instruments for remote sensing of the atmosphere can be sorted into two categories: instruments with active sensors and instruments with passive sensors. Whereas passive instruments depend on natural light sources like the sun or the stars, active instruments themselves provide the radiation they require.

The LIDAR approach uses laser radiation to remotely observe those key parameters of the atmosphere, biosphere or hygrosphere which we have to know precisely in order to understand our earth system. Elastic or inelastic scattering of laser light rays, sometimes also in combination with absorption, is the basis for a large number of different LIDAR applications. There are different variations of the basic LIDAR principle depending on which particular physical interactions of light with the atoms, molecules or aerosol particles in the atmosphere are utilized:

• Aerosol LIDAR to measure dust particles, haze, clouds, etc. for example visibility-range LIDAR to measure the sighting distance at airports or on streets,
• Gas concentration LIDAR using the absorption principle to measure ozone, water vapor, etc.,
• Gas concentration LIDAR using the Raman principle to measure methane, water vapor, etc.,
• Doppler-Wind LIDAR to measure the wind vector and turbulence,
• Molecule LIDAR to measure pressure and temperature.

The LIDAR measuring principle

LIDAR measurement principle

Figure a) shows a typical reception signal in which four regions can be recognized:

1. Up to about 30 meters distance the receiver cannot capture any radiation because the transmission cone has not yet entered the receiver's field of view. The length of this "dead region" depends on the geometry of the transmission and reception optics. In this case the overlap integral is O(x)=0.
2. At a distance of 60 meters there is a maximum intensity of radiation corresponding to the backscatter and the extinction by the atmospheric aerosols. For x>60 meters, the overlap integral is O(x)=1.
3. There is another maximum at 90 meters caused by an inhomogeneity in the atmosphere, for example a fog bank or cloud. Without this inhomogeneity the signal would follow the dotted line.
4. The detected cloud or fog bank causes a reduction in the light coming from behind it, so the signal from the area beyond the inhomogeneity is quickly absorbed by the background noise, i.e. approaches 0.

Figure 2b) shows a typical reception signal for a measurement in homogenous fog. In order to determine the situation close to the visibility-range LIDAR, a compact instrument with a wide-angle transmission and reception cone is used. Because of the differing geometries of the two LIDAR systems, the regions 1 to 3 of figure a) are contained in the first 30 meters of figure b).