Remote Sensing ' Lecture 10

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Lecture 10 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
• LIDARs 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 aircrafts
  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