Microwave Image

1. Penetration of the radar signal
        - radar signals are able to penetrate some solid features,
            e.g. soil surface and vegetative covers

        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

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

        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 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

        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

        - 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
        - when surface is smooth relative to the wavelength

        - incident angle = reflection angle

        Diffuse reflection
        - when surface is rough relative to the wavelength

        - the signal will be scattered equally all directions

        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
 

Microwave Image Interpretation

1. Radar image brightness
   Geometric characteristics
        - slope facing relative to sensor determines signal returns

        - surface roughness determines type and amount of returns

   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
 

2. Natural features response
   soil response
        - top soil moisture gives high returns and

        - limits penetration of radar waves

   vegetation response
        - 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
 

    water and ice response
        - smooth water yields specular reflectance with no return

        - rough water surface returns with various strength

        - ice age, surface roughness, snow cover etc. affect returns

Radar Remote Sensing
use synthetic aperture principle

SEASAT SAR 1978
Shuttle Image Radar-A (SIR-A) 1981
        - a synthetic aperture radar carried by the Shuttle
            Transportation System
        - designed to observe land information
        - 260km altitude, 40m range and azimuth resolutions,
          l=23.5cm, HH, fixed large look angle (47-53)

Shuttle Image Radar-B (SIR-B) 1984
        - l=23.5cm, HH, 25m azimuth resolution, 15-45m range resolution
        - varied look angle 15-60o to acquire stereo images
        - obtain roughness and moisture info at different look angles

Shuttle Image Radar-C (SIR-C) 1994
        - multiple radar bands l=23.5, 5.8, 3.1cm, color composite
        - varied look angle 15-60^o, HH, HV, VV, and VH
          25m azimuth resolution, 15-45m range resolution

        - ocean: waves, wind motion, current motion, and sea ice,
            ecosystems: land use, vegetation, fire effects, flood,
                                and clear cutting
            hydrology: water, wetland, soil moisture, snow and glacier
            geology: geologic structures, soil erosion, transportation,
                                and deposition, active volcanoes
            rain and clouds

ALMAZ-1 1991
        Soviet Union satellite radar system
        - 300km orbit, l=10cm, HH, varied look angle 20-70^o,
          10-30m range and azimuth resolutions

ERS-1, ERS-2,
        European Space Agency, 1991, 1995
        - sun-synchronous orbit, 785km
        - 16 ~ 18 days temporal resolution
        - three sensor systems including a C-band active microwave
            instrumentation with 30m resolution, VV, 23^o look angle

ENVISAT-1
        European Space Agency, 2002
        - Advanced Synthetic Aperture Radar (ASAR)
        - 30m resolution, 14-45^o look angle

JERS-1
        National Development Agency of Japan, 1992
        - sun-synchronous orbit, 568km
        - a four-band optical sensor
        - 23cm L band SAR, HH, 18m resolution, 35^o look angle

ALOS
        National Development Agency of Japan
        Advanced land Observing Satellite (ALOS)
        - a Phased Array L-band Synthetic Aperture Radar system
        - varied look angle and multi-polarization

RADARSAT
        Canadian Space Agency, 1995
        - designed to observe sea ice, coastal line, land cover,
            agriculture, and forest
        - 798km orbit, 1-3days, l=5.6cm, HH
        - various spatial resolutions and look angles
 

Interferometric Radar
        - based on the phase difference of radar signals received by
            antennas located at different positions in space
        - with a known interferometric baseline, the phase difference
            is used to calculate elevation
        - single-pass interferometry: two antennas on a single
            aircraft
        - repeat-pass interferometry: single antenna with multiple
            passes

        Shuttle Radar Topography Mission (SRTM)
        - single-pass interferometry
        - covers 60^oN-56^oS, 30m resolution DEM, C and X bands
 

LIDAR
        LIght Detection And Ranging
        uses pulses of laser light directed toward the ground and measured
        return time to measure distances
        - rapid pulsing
        - can record up to five returns per pulse, thus
            discriminating multiple surfaces per pulse
        - equipped with GPS, recorded data are georeferenced
        - large quantity of data
        - used for generating DEMs, contours, and feature extraction

        Ice, Cloud, and Land Elevation Satellite (ICESat) 2003

        Vegetation Canopy Lidar (VCL), the first mission of NASAs Earth
                System Science Pathfinder (ESSP) program
        - measure ground surface and canopy-top elevation, vertical
            distribution of canopy elements
        - no longer actively developed

Reading; chpt 8
 
 
 
 
 
 
 

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