Interactions with the Atmosphere and Surfaces

1. Interactions with the Atmosphere
  Scattering
        The redirection of EM energy by particles suspended in the
        atmosphere or large molecules of atmospheric gases

        - Rayleigh scattering:
        It occurs when atmos. particles' diameters are much smaller than the wavelength of the radiation
        It is common high in the atmosphere
        Radiation with shorter wavelength is easier to be scattered
        Black vs. blue vs. red skies

        - Mie scattering:
        Particles' diameters are equivalent to the wavelength
        It is common in lower atmosphere
        It is wavelength dependent

        - Nonselective scattering:
        Particles are much larger than the wavelength
        All wavelength are scattered equally

        - Effects of scattering:
        It causes haze in remotely sensed images
        It decreases the spatial detail on the images
        It also decreases the contrast of the images

   Refraction
        The bending of light rays at the contact between two media that
transmit light but with different density; when light enters the denser
medium, it is refracted toward surface normal.

   Absorption
        The atmosphere prevents, or strongly attenuates, transmission of
radiation through the atmosphere
        - three gases:
                Ozone (O3): absorbs ultraviolet radiation high in atmos.
                Carbon-dioxide (CO2): absorbs mid and far infrared
                                                 (13-17.5microm) in lower atmosphere
                Water vapor (H2O): absorbs mid-far infrared
                                                 (5.5-7.0, >27microm) in lower atmosphere

   Atmospheric windows
        Those wavelengths that are relatively easily transmitted through
the atmosphere.
- the windows:
                UV & visible:   0.30-0.75microm
                Near infrared:  0.77-0.91microm
                Mid infrared:   1.55-1.75microm,  2.05-2.4microm
                Far infrared:   3.50-4.10microm,  8.00-9.20microm,  10.2-12.4microm
                Microwave:      7.50-11.5mm,  20.0+mm

        - The atmospheric windows are important for RS design

2. Interactions with surfaces
        All EM energy reaches earth's surface must be reflected, absorbed,
or transmitted
        The proportion of each depends on:
type of features, wavelength, angle of illumination

   Reflection
        Light ray is redirected as it strikes a nontransparent surface

        - Specular reflection:
        When surface is smooth relative to the wavelength,
        incident radiation is reflected in a single direction
        incidence angle = reflection angle

        - Diffuse (isotropic) reflection:
        When surface is rough relative to the wavelength,
        energy is scattered equally in all directions
        - Lambertian surface

        spectral reflectance r_l =E_R(l)/E_I(l)

        Energy of wavelength l reflected from the object
       -------------------------------------------------- x 100%
        Energy of wavelength l incident upon the object

   Transmission
        Radiation passes through a substance without significant
attenuation
        - transmittance(t):
                  transmitted radiation
           t =  ---------------------
                    incident radiation

   Absorption
 

3. Spectral reflectance curve
   Vegetation
        - chlorophyll absorbs blue and red, reflects green
        - high reflection and transmission at NIR
        - reflection and absorption at MIR, water absorption bands

         Biophysical sensitivity of spectrums
        - the palisade cells absorb blue and red light
          and reflect green light at a peak of 0.54mm

        - the spongy mesophyll cells reflect near infrared light that is
            related to vegetation biomass because
            the intercellular air space of spongy mesophyll layer is
            where photosynthesis and respiration occur

        - vegetation moisture content absorbs mid infrared energy

        Reference: Jensen, J. R. "Biophysical Remote Sensing." Annals,
        73:(1),111-132.

   Soil
        -soil moisture/texture decreases reflectance
        -coarse soil (dry) has a relatively high reflectance
        -surface roughness, organic matter, iron oxide
   Water
        -transmission at visible and strong absorption at NIR
        -water surface, suspended material, and bottom of water body

4. Reading: Chpt 1
 

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