What is LIDAR?
LIght Detection And Ranging uses the same
principle as RADAR. The lidar instrument transmits light out to a target.
The transmitted light interacts with and is changed by the target. Some
of this light is reflected / scattered back to the instrument where it is
analysed. The change in the properties of the light enables some property
of the target to be determined. The time
for the light to travel out to the target and back to the lidar is used
to determine the range to the target.
There are three basic generic types of lidar:
- Range finders
- DIAL
- Doppler lidars
Range finder lidars are the simplest lidars. They are used to measure
the distance from the lidar instrument to a solid or hard target.
DIfferential Absorption Lidar (DIAL) is used to
measure chemical concentrations (such as ozone, water vapor, pollutants)
in the atmosphere. A DIAL lidar uses two different laser wavelengths which
are selected so that one of the wavelengths is absorbed by the molecule
of interest whilst the other wavelength is not. The difference in intensity
of the two return signals can be used to deduce the concentration of the
molecule being investigated.
Doppler lidar is used to measure the velocity of a target. When the light
transmitted from the lidar hits a target moving towards or away from the
lidar, the wavelength of the light reflected/scattered off the target will
be changed slightly. This is known as a Doppler shift - hence Doppler Lidar.
If the target is moving away from the lidar, the return light will have
a longer wavelength (sometimes referred to as a red shift), if moving towards
the lidar the return light will be at a shorter wavelength (blue shifted).
The target can be either a hard target or an atmospheric target - the atmosphere
contains many microscopic dust and aerosol particles which are carried by
the wind. These are the targets of interest to us as they are small and
light enough to move at the true wind velocity and thus enable a remote
measurement of the wind velocity to be made.
More information on LIDAR.
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