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A LIDAR is not only used for lIght detection and ranging, but nowadays it is one ofthe most powerful instruments for analysing the atmosphere. A lidarconsists of a transmitter and a receiver. The transmitter is sending a fairly collimated pulse or modulated beam of monochromatic laser light of a certain frequency into the atmosphere where this light is absorbed or scattered by the particles (molecules, water droplets, ice crystals, dust) of the atmosphere. The highly sensitive receiver records light which is reflected by the atmosphere within acertainfield of view(FOV) and with or without a certain state of polarisation. The transmitterand the receiver may be mounted atthe same place (then we calithe lidar monostatic) or they may be located in different places (then we call the lidar bistatic). Furthermore, the FOV may be directed along the same or different direction asthe (FOV of the) transmitted beam. Very often the FOVs of the transmitter and the receiver are coaxial cones; sometimes the receiver has several FOVs (e.g. a narrow central FOV and a ring shaped outer FOV) thus enabling recording of light of several states of polarisation (e.g. parallel and perpendicular to the direction of polarisation of the transmitted beam) simultaneously. Furthermore, some lidars are capable of transmitting and receiving light of severalfrequencies almost simultaneously. In addition to that some lidars provide a possibility of scanning the atmosphere. lidars may be ground-based, airborne or space-based. The primary goal of Iidar sensing is to acquire meteorologically important parameters of the atmosphere from the lidar signal. To be able to obtain such parameters, it is necessary to describe the lidar signal physically sufficiently precisely by a mathematical formula and to solve this formulaforthe interesting parameters. In this report the application for visibility detection is the object.
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