LIght Detection And Ranging

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LIght Detection And Ranging

• LIDAR gathers data through laser light striking
  the surfaces of the earth and measuring the time
  of pulse return
• A LIDAR system is often capable of recording up
  to five (5) returns per pulse
• LIDAR can distinguish not only the canopy and
  bare ground but also surfaces in between (such as
  a forest structure and under story

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A point cloud of multiple LIDAR returnsa topic
for on-going research estimating forest stand
volume for example
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The most common use for LIDAR is the creation of
very high resolution DEMs
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An interesting paper http//www.isprs.org/commiss
ion3/proceedings/papers/paper114.pdf
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Why LIDAR?

• Potential exists for highly accurate Elevation
  data sets.
• Forestry, construction, even modeling radio
  reception for cell towers depend on very accurate
  elevation data
• Many areas are too large (or difficult) to
  manually survey, river flood plains, coastal
  mudflats or sediment deposits

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Conceptually quite simple, a single laser beam
sweeps back and forth beneath an aerial (or
space) platform. As the velocity of light is a
constant, the time from pulse to recording
provides a very accurate measure of height
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Recording the back scatter

• typically use extremely sensitive detectors
  called photomultiplier tubes to detect the
  backscattered light.
• Photomultiplier tubes convert the individual
  quanta of light, photons, first into electric
  currents and then into digital numbers
• The amount of light required for a
  photomultiplier tube to record data is very small
  on the order of picoamps (1 pA 10-12 A a 60 W
  light bulb draws a current of 0.5 A!).

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The geometry of the platform is critical.and
variable (airplanes bounce!)
Accurate LIDAR data is completely dependent on
being able to know, and correct for changes in
platform orientation.
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From the Spencer B. Gross web sitehttp//www.sbg
maps.com/lidar.htm

• The core of the system is the Inertial
  Measurement Unit (IMU). The IMU measures the
  translation and rotational dynamics of the
  sensor. It is entirely solid state for high
  reliability, containing a triad of high quality
  silicon accelerometers, and three low noise Dry
  Tuned Gyros.
• It is non-trivial to actually create a worthwhile
  data set.

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Using LIDAR to understand the atmosphere

• Differential Absorption LIDAR (DIAL) uses 2 laser
  pulses fired simultaneously one wavelength of
  laser light is normal and transits the
  atmosphere with no significant reflection or
  absorption
• The second wavelength of laser light is absorbed
  by a specific gas (ozone or water vapor) in the
  atmosphere

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The UV DIAL system uses five laser (or lidar)
wavelengths IR and visible wavelengths both
measure aerosols and clouds. Two UV wavelengths
determine the profile of ozone by analyzing the
absorption differences due to ozone between the
two lidar returns. http//oea.larc.nasa.gov/PAIS
/DIAL.html