5/9/2015 GEM 3366 1 Lecture 12 Content LIDAR. 5/9/2015 GEM 3366 2 –The LIDAR instrument transmits light out to a target –The transmitted light interacts.

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Lecture 12 Content

LIDAR

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– The LIDAR instrument transmits light out to a target

– The transmitted light interacts with and its direction is changed by the target

– Some of this light is reflected / scattered back to the instrument where it is analyzed

– 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

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– LIDAR is an acronym for Light Detection And Ranging

LIDAR sensors measure elevation by bouncing laser light off of a surface and measuring the time the light pulse takes to return.

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What can you do with LIDAR?– Measure distance – Measure speed – Measure rotation – Measure chemical composition and

concentration of a remote target where the target can be a clearly defined object, such as a vehicle, or a diffuse object such as a smoke plume or clouds

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A lidar sensor transmits and receives electromagnetic radiation, but at a higher frequency. Lidars operate in the ultraviolet, visible and infrared region of the electromagnetic spectrum.

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– LIDAR transmits and receives electromagnetic radiation, but at a higher frequency

– LIDAR operate in the ultraviolet, visible and infrared region of the electromagnetic spectrum

– LIDAR’s transmitter is a laser, while its receiver is an optical telescope

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– Different kinds of lasers are used depending on the power and wavelength required

– The lasers may be both cw (continuous wave, on continuous like a light bulb) or pulsed (like a strobe light)

– For some LIDAR applications more than one kind of laser is used

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– The receiving system records the scattered light received by the receiver at fixed time intervals

– LIDARS typically use extremely sensitive detectors called photomultiplier tubes to detect the backscattered light

– photocounts are received and recorded for fixed time intervals during the return pulse

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– The times are then converted to heights called  range bins since the speed of light is well known

– range bins from time: 

where c is the speed of light, 3 x 108 m/s

– if each range bin is 160 ns long the height of each bin is 24 m (note: 1 ns = 1 x 10-9s))

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– LITE stands for LIdar Technology Experiment

– LITE flew on shuttle mission STS-64 in September, 1994

– LITE  was the first successful demonstration of operating a LIDAR from space

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LITE was designed to measure clouds and aerosols, the small particles of "stuff" in the air that includes cloud droplets.

Operating a LIDAR from space allows large regions of the atmosphere to be rapidly sampled.

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– 5 minutes of LITE observations over the Sahara on September 18, 1994.

– The Atlas Mountain range to the  right of the thin black line at 31oN separates air masses thick in aerosols (left) and low in aerosols (clear air, right).

– A complex aerosol structure exists over the desert, with aerosol plumes reaching over 5 km in altitude

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LIDAR systems– There are three basic generic types of

lidar:Range finders

DIAL

Doppler lidars

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– Range finder LIDARS are the simplest LIDARS. They are used to measure the distance from the LIDAR instrument to a solid or hard target.

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– 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.

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– 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.

– The true wind velocity and thus a remote measurement of the wind velocity is computed

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Basic LIDAR airborne components– Consists of:

Airborne vehicles

LIDAR

An inertial navigation system- records aircraft’s pitch, roll, etc.)

GPS receiver – records x,y,z positions of the LIDAR system

Computer – performs the computations to transform positions in the air to unknown positions on the ground

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– The final result from the LIDAR system is the coordinates of a dense pattern of ground points and an image of the ground points

– Current LIDAR accuracies range between 10-15cm

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– LIDAR data examples

LIDAR light intensity map

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– LIDAR intensity data Willapa Bay,Washington

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– Forested area LIDAR image

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… The End …