Remote Sensing Microwave Image

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Remote SensingMicrowave Image
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1. Penetration of Radar Signal

• Radar signals are able to penetrate some solid
  features, e.g. soil surface and vegetative covers

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1. Penetration of Radar Signal

• Skin depth
• - the depth to which the strength of a signal is
  reduced to 1/e (approx. 37) of its surface
  magnitude
• - skin depth increases with wavelength, absence
  of moisture, surface roughness, and depression
  angle

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2. Polarization

• HH image
• - radar transmits horizontally polarized signals
  and receives the horizontally polarized return
  signal
• HV image
• - radar transmits horizontally polarized signals
  but receives vertically polarized signals

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http//www.crisp.nus.edu.sg/research/tutorial/fre
qpol.htm
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2. Polarization

• Most radar produce HH images, some are designed
  to produce both. There are also VH and VV images

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2. Polarization

• Depolarization
• - the horizontally polarized microwave energy is
  changed into vertically polarized energy
• - depolarizers on the ground appear brighter on
  HV image
• - rough surface and inhomogeneous subsurface are
  depolarizers

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3. Synthetic Aperture Radar Systems (SAR)

• Real aperture SLAR system
• - the oldest, simplest, and least expensive of
  radar systems
• Synthetic Aperture Systems (SAR)
• - use an array of real antennas to synthesize
  the effect of a very long antenna

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http//www.crisp.nus.edu.sg/research/tutorial/mw.
htmfreq
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3. Synthetic Aperture Systems (SAR)

• Doppler effect
• - objects experience different frequency shifts
  in relation to their distances from the aircraft
  track
• - objects at the leading edge of a beam reflect
  pulses with higher frequency than those at the
  trailing edge

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3. Synthetic Aperture Systems (SAR)

• Doppler effect
• - the frequency shift allows the system to
  assign reflections to features at their correct
  positions

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4. Reflectance

• Incidence angle
• - the angle between the axis of the incident
  radar signal and a perpendicular to the surface
  that the signal strikes
• Specular reflection
• Diffuse reflection
• Corner reflection

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4. Reflectance

• Specular reflection
• - when surface is smooth relative to the
  wavelength
• - incident angle reflection angle

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4. Reflectance

• Diffuse reflection
• - when surface is rough relative to the
  wavelength
• - the signal will be scattered equally all
  directions

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4. Reflectance

• Corner reflection
• - a double reflection caused by adjacent smooth
  surface
• - high reflectance appears as sparkles on the
  image
•   - tends to be proportionately larger than its
  real size  

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5. Radar Image Brightness

• Geometric characteristics
• - slope facing with respect to sensor determines
  signal returns
• - surface roughness determines the type and
  amount of returns

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5. Radar Image Brightness

•   Electrical characteristics
• - Complex Dielectric Constant water 80, most
  material 3-8 when dry
• - moisture content is more significant than
  material themselves in increasing reflectance
• - metal objects have high returns, e.g. metal
  bridges, silos

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6. Natural Features Response

• Soil responses
• - top soil moisture gives high returns and
• - limits penetration of radar waves  

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6. Natural Features Response

• Vegetation responses
• - when l  plant size or plant is dense, the
  return is high
•   - high moisture content returns more energy
•   - HH polarized sensing penetrates vegetation
  more than HV

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6. Natural Features Response

• Water and ice responses
• - smooth water yields specular reflectance with
  no return
• - rough water surface returns with various
  strength
• - ice age, surface roughness, snow cover etc.
  affect returns

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Readings

• Chapter 8

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1. Penetration of Radar Signal
lt- Long wavelength
Short wavelength -gt
http//www.crisp.nus.edu.sg/research/tutorial/fre
qpol.htm
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Synthetic Aperture Radar
http//www.crisp.nus.edu.sg/research/tutorial/mw.
htmfreq