Download Chapter 7: Active Microwave and LIDAR

Active Microwave and
LIDAR
Three models for remote sensing
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1. Passive-Reflective: Sensors that rely on EM
energy emitted by the sun to illuminate the
target. The sensor measures the amount of
energy that is reflected from the target. Used
for wavelengths of about 0.4-3.0 μm. Most of
our work this quarter will focus on the use
of this model. We will spend some time
discussing two other models…
Three models for remote sensing
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2. Passive-Emitted: The sun still serves as the
source of illumination (hence, still a passive
system). The sensors measure the amount of
EM energy emitted from the target in the
thermal or emissive portion of the spectrum.
These systems are intended to determine
something about the thermal properties of the
target. Used for wavelengths of about 3.0-30
μm
Three models for remote sensing
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3. Active: These systems do not rely on solar
radiation to illuminate the target. The system
emits EM energy and then records the amount
of this energy that is reflected from the target.
These systems operate in parts of the spectrum
where emissions from the sun are minimal
(longer wavelengths; microwave, radar).
Usually for wavelengths of about 1cm-1m;
some applications (LIDAR) at about 1 μm
Active Systems
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Active Microwave: wavelengths of about 1mm to
1m; RADAR (Radio Detection and Ranging
• Capable of obtaining imagery day and night
and can penetrate clouds
• Certain wavelengths can penetrate some types
of vegetation
• Long wavelengths can penetrate dry soil (or
sand) several meters to reveal subsurface
features
• Not as widely used as optical systems
LIDAR: (Light Detection and Ranging)
wavelengths of about 1 µm; sometimes called
LADAR (Laser Detection and Ranging)
• Gaining widespread use in recent years
LIDAR
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Pulses of energy (usually at about 1 µm)
emitted from a laser, energy is reflected
from the ground back to the sensor, travel
time for the signal is used to measure
distance from sensor to the ground
feature
Inertial measurement unit (IMU) and GPS
are critical for accurately locating ground
footprint of laser pulses
Your text mentions continuous wave lasers
and pulsed laser. My discussion will
focus on use of the pulsed laser.
LIDAR: Laser Altimeter
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First applications of LIDAR were to provide height
above the ground for aircraft
Extension of this idea and improved aircraft
navigation tools (IMU and GPS) made it possible
to generate topographic maps
Use of laser footprints of about 10m over forests
revealed multiple returns – several “heights.”
Why?
Laser energy reflecting off different layers in the
canopy; potential to obtain data about the
vertical distribution of biomass in a forest
canopy!
Airborne LIDAR
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Ground footprint: 0.5m (or less) to
10m
Ground spacing of pulses: variable,
generally 3-5m, sometimes <1m
Horizontal and vertical accuracy in
the range of 0.5m or less
Waveform Recording vs. Discrete-return LIDAR
Lefsky et al., Fig 1
Canopy height
Lefsky et al., Fig 2
Lefsky et al., Fig 3
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Synthetic
Waveform
created from
discretereturn data
Lefsky et al., Fig 4
Beach Erosion Application
Beach profile before and after a
major storm
Lefsky et al., Fig 5