Microwave Remote Sensing

1. Passive microwave sensors
        - microwave emission is related to temperature and emissivity
        - microwave radiometers are sensitive to l= 0.1mm - 30cm
        - the interpretation requires knowledge of the system, the
                atmosphere, and the thermal property of the objects

2. Active microwave sensors - radars
        RADAR: RAdio Detection And Ranging

        - transmitter: transmits repetitive pulse of microwave energy

        - receiver: receives the reflected signal through antenna and
                filters and amplifies the signal

        - antenna array: transmits a narrow beam of microwave energy

        - recorder: records and displays the signal as an image
 

3. Side-looking airborne radar (SLAR)
        Ranging:
        - distance from the antenna to the features can be calculated
                by measuring the time delay between the signal is transmitted to the
                time its echo is received

        Detecting frequency and polarization shifts:
        - by comparing the transmitted signal of known properties to
                the received signal

        The "all weather" capability:
        - the l used by SLAR is long enough to penetrate clouds and
                light rain, e.g. applications in tropical area
        - SLAR systems are independent from solar illumination, which
                makes night missions possible

        Spatial resolution
        - spatial scale at 1:100,000, between Landsat and air photo
        - different spectral information from other sensor systems
 

4. Geometry of the radar image
        - Depression angle

        - Far, mid, and near-range portion of the radar image

        - Radar shadow, more severe in the long range
 

        Slant range distance (F6.7, Campbell, 1987)
        - direct distance from the antenna to an object on the ground
                measured by time delay

        Ground range distance
        - distance of correct scaling as we would measure on a map

 
        Geometric errors
        - because radars collect information in slant range distance

        Radar layover
        - the top of a tall object appears closer to the antenna than
                its base
        - the antenna receives the echo of the top before the base
        - it is more severe in the near range

        Radar foreshortening
        - with modest or high relief in the mid or far range portion
        - features maintain relative position but incorrect distance
            causing near range slope appear steeper and far range slope
            gentler
 

5. Resolutions
   two determinant parameters: pulse length and antenna beam width
        - the pulse length dictates the spatial resolution in the
                direction of energy propagation

        - the width of the antenna beam determines the resolution
                cell size in the flight direction

   Range resolution (along Track)
        - slant-range resolution is consistent

        - equal to half the transmitted pulse length: PL/2

        - ground range resolution changes with distance from the
                aircraft

        - inversely related to the cosine of depression angle
          R_r = slant range resolution /cosq_d, q_d-depression angle

   Azimuth resolution (cross track)
        - is determined by the angular beam width b and slant range R_r
          while beam width b is inversely related to antenna length AL
          R_a = R_r·b,      b = l/AL, l-pulse wavelength 

        - near range portion has finer resolution than the far range
 

6. Reading: Chpt 8
 
 
 
 
 
 

...Back to Ling Bian top page.