Radiance, reflection and the atmosphere

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Radiance, reflection and the atmosphere

• January 27, 2003

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Definition of Remote Sensing

• Remote sensing is the use of the electromagnetic
  spectrum to acquire information about the planet
  without being in physical contact with it.

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Specifically, we measure radiation produced in
three ways1. Emitted from the surface (thermal
IR)2. Reflected from the surface (solar)3.
Reflected from energy pulses directed at the
surface (RADAR)
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Coordinate system used with satellite sensors
qZ Zenith angle q Look or incidence angle
qS Solar zenith angle
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Three kinds of scanners

• Whisk-broom (cross-track)
• Push-broom (along-track)
• Hybrid cross-track

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Whisk-broom or cross-track scanner Examples
AVHRR, SeaWiFS. Notice how field-of-view (FOV)
changes with look angle.
Advantages single detector scans across the
entire path, can be calibrated on each rotation.
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Because of off-nadir look angle and Earths
curvature, field-of-view increases with zenith
angle.
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Push-broom or along-track scanner examples
include Landsat-7, MERIS
Advantages longer dwell time, better
resolution Disadvantages unwieldy number of
sensors for wide swaths
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Hybrid cross-track MODIS, VIIRS
Advantages multiple sensors in along-track
direction increase dwell time by allowing for a
slower rotation rate. MODIS has 10 such sensors
per channel. But wide swath width leads to a
phenomena called the bowtie effect.
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Im going to describe radiation in four ways 1.
Radiant flux 2. Plane irradiance 3. Radiant
intensity 4. Radiance.
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Irradiance
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Black and graybody emission
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Blackbody emission Plancks law
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Solar blackbody comparison
5900 Kelvin
Solar irradiance above the Atmosphere
Solar irradiance at the surface (solar zenith 60o)
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Comparison of the intensity and radiance emitted
by a black and gray body
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MODIS Sea Surface Temperature
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Kinds of reflection
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Water properties, especially in the visible
Real and Imaginary parts of the index of
refraction for water
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Two kinds of solar reflection in the visible
Direct surface reflection, diffuse sub-surface
backscatter
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Scatter angle as a function of wind speed (Mobley)
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Daily MODIS
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Vertical structure of the Atmosphere
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Atmospheric constituents
Columnar water equivalents Ocean
4 km
Atmosphere
10 m Water vapor
10 cm
Non-raining cloud liquid water
0.25 mm
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Ocean and atmospheric properties
Atmospheric water vapor and liquid water
distribution
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UV
Visible
Near IR
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SeaWifs Individual Bands
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End
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True Color SeaWifs
Processed SeaWifs
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