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