remote sensing microwave image. 1. penetration of radar signal ► ► radar signals are able to...
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Remote SensingRemote Sensing
Microwave ImageMicrowave Image
1. Penetration of Radar Signal
► Radar signals are able to penetrate some solid features, e.g. soil surface and vegetative covers
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
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
http://www.crisp.nus.edu.sg/~research/tutorial/freqpol.htm
2. Polarization
► Most radar produce HH images, some are designed to produce both. There are also VH and VV images
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
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
http://www.crisp.nus.edu.sg/~research/tutorial/mw.htm#freq
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
3. Synthetic Aperture Systems (SAR)
► Doppler effect - the frequency shift allows the system to assign reflections to features at their correct positions
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
4. Reflectance
►Specular reflection - when surface is smooth relative to the wavelength
- incident angle = reflection angle
4. Reflectance
► Diffuse reflection - when surface is rough relative to the wavelength
- the signal will be scattered equally all directions
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
5. Radar Image Brightness
►Geometric characteristics - slope facing with respect to sensor determines signal returns
- surface roughness determines the type and amount of returns
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
6. Natural Features Response
► Soil responses - top soil moisture gives high returns and
- limits penetration of radar waves
6. Natural Features Response
► Vegetation responses- when = plant size or plant is dense, the return is high
- high moisture content returns more energy
- HH polarized sensing penetrates vegetation more than HV
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
ReadingsReadings
► Chapter 8Chapter 8
1. Penetration of Radar Signal
<- Long wavelength
Short wavelength ->
http://www.crisp.nus.edu.sg/~research/tutorial/freqpol.htm
Synthetic Aperture Radar
http://www.crisp.nus.edu.sg/~research/tutorial/mw.htm#freq
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