Passive Microwave Remote Sensing

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Passive Microwave Remote Sensing

• Lecture 10
• Nov 06, 2007

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Principals

• While dominate wavelength of Earth is 9.7 um
  (thermal), a continuum of energy is emitted from
  Earth to the atmosphere. In fact, the Earth
  passively emits a steady stream of microwave
  energy as well, though it is relatively weak in
  intensity due to its long wavelength.
• The spatial resolution usually low (kms) since
  the weak signal.
• A suit of radiometers can record it. They measure
  the brightness temperature of the terrain or the
  atmosphere. This is much like the thermal
  infrared radiometer for temperature.
• A matrix of brightness temperature values can
  then be used to construct a passive microwave
  image.
• To measure soil moisture, precipitation, ice
  water content, sea-surface temperature, snow-ice
  temperature, and etc.

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Rayleigh-Jeans approximation of Plancks law
Thermal infrared domain (Plancks law)
Microwave domain (Rayleigh-Jeans approximation)
Recall
Let
We have
We have
Unit is Wm-2Hz
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• For a Lambertian surface, the surface brightness
  radiation B(v,T),
• The really useful simplification involves
  emissivity and brightness temperature

Unit is Wm-2Hzsr
In comparison with thermal infrared (TB)4 e?
(T)4
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Some important passive microwave radiometers

• Special Sensor Mirowave/Imager (SSM/I)
• It was onboard the Defense Meterorological
  Satellite Program (DMSP) since 1987
• It measure the microwave brightness temperatures
  of atmosphere, ocean, and terrain at 19.35,
  22.23, 37, and 85.5 GHz.
• TRMM microwave imager (TMI)
• It is based on SSM/I, and added one more
  frequency of 10.7 GHz.

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AMSR-E

• Advanced Microwave Scanning Radiometer EOS
• It observes atmospheric, land, oceanic, and
  cryospheric parameters, including precipitation,
  sea surface temperatures, ice concentrations,
  snow water equivalent, surface wetness, wind
  speed, atmospheric cloud water, and water vapor.
• At the AMSR-E low-frequency channels, the
  atmosphere is relatively transparent, and the
  polarization and spectral characteristics of the
  received microwave radiation are dominated by
  emission and scattering at the Earth surface.
• Over land, the emission and scattering depend
  primarily on the water content of the soil, the
  surface roughness and topography, the surface
  temperature, and the vegetation cover.
• The surface brightness T (TB ) tend to increase
  with frequency due to the absorptive effects of
  water in soil and vegetation that also increase
  with frequency. However, as the frequency
  increase, scattering effects from the surface and
  vegetation also increase, acting as a factor to
  reduce the TB

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AMSR-E
Najoku et al. 2005
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Example1 Snow depth or snow water equivalent
(SWE)

• The microwave brightness temperature emitted from
  a snow cover is related to the snow mass which
  can be represented by the combined snow density
  and depth, or the SWE (a hydrological quantity
  that is obtained from the product of snow depth
  and density).

?Tb Tb19V-Tb37V
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Kelly et al. 2003
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3. Study Area (1)
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Impact of snow density (4)-mean SD
Snow density 0.4 g/cm3 Multi-snow density
Xianwei, Xie, and Liang 2006
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Results AMSR-E vs ground- SD at individual
stations (snow density 0.4 g/cm3)
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Results AMSR-E vs ground- SD at individual
stations (snow density 0.4 g/cm3)
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Results Annual change of SWE in YWR
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Antarctic sea ice
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Mike and Xie, 2006
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Maximum SD values exceed 50-60 cm in most data
sets, (outside range of retrievable snow depth
for 37GHz) and are likely noise
Mike and Xie, 2006
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Mike and Xie, 2006
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Seasonal Comparison of Locations of Max SD Areas,
2002
Max Areas 2s
7/20/02
10/20/02
11/18/02
8/20/02
9/24/02
12/20/02
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Seasonal Comparison of Locations of Max SD Areas,
2003
4/20/03
1/20/03
7/20/03
10/20/03
5/20/03
2/20/03
8/20/03
11/18/03
Oct 1, 2005
Oct 1, 2004
3/20/03
6/20/03
9/20/03
12/20/03
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Seasonal Comparison of Locations of Max SD Areas,
2004
1/20/04
4/20/04
7/22/04
10/20/04
11/17/04
5/20/04
2/20/04
8/20/04
3/20/04
6/19/04
9/17/04
12/20/04
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Seasonal Comparison of Locations of Max SD Areas,
2005
4/20/05
7/20/05
10/20/05
1/20/05
5/20/05
2/20/05
8/20/05
11/16/05
3/20/05
6/20/05
9/20/05
12/20/05
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Example2 Radio-frequency interference
contaminate the 6.9 and 10.7 GHz channels

• Radio-frequency interference (RFI) the cable
  television relay, auxiliary broadcasting, mobile.
  RFI is several orders of magnitude higher than
  natural thermal emissions and is often
  directional and can be either continuous or
  intermittent.
• Radio-frequency interference (RFI) is an
  increasingly serious problem for passive and
  active microwave sensing of the Earth.
• The 6.9 GHz contamination is mostly in USA,
  Japan, and the Middle East.
• The 10.7 GHz contamination is mostly in England,
  Italy, and Japan
• RFI contamination compromise the science
  objectives of sensors that use 6.9 and 10.7 GHz
  (corresponding to the C-band and X-band in active
  microwave sensing) over land.

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radio-frequency interference (RFI) index (RI)
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Li et al. 2004
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6.9 GHz contamination
Najoku et al. 2005
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