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6a_RadiometerSM.pdf
1. Passive Microwave Sensing of Land
u Spatial resolution
The spatial resolution, or field of view
(FOV), of a satellite radiometer is dictated
by the orbital altitude, the incidence angle,
the wavelength, and antenna size. Among
the majority of radiometers flown to date,
the FOV has ranged between about 5 km
for radiometers operating in the 100 GHz
range to about 50 km for radiometers
operating at 1.4 GHz. Land parameters
vary at scales of tens of meters to tens of
kilometers, depending on the specific
variable.
uRelative complexity
Among the three media, land, ocean,
and atmosphere, land is by far the most
complex; surface features—such as
vegetation cover—vary widely in shape,
orientation, and density, as do their
dielectric properties.
3. Emission Components
u Our goal is to measure the reflectivity because it is the
quantity related to the physical parameters of the land surface
contained in the FOV.
u The physical temperature is estimated from either recorded
weather data or from satellite observations made by the 37 GHz
radiometric channel on the TRMM or WindSat satellites.
4. Soil Surface
Emission
For a bare soil surface,
emissivity depends on
soil moisture content
and surface roughness
6. Semi-Empirical Model( 1979-2013)
where Q is a polarization mixing factor, h′ is an equivalent rms height, and n is an
angular exponent (which may be polarization dependent). The rationale for Q is that
when an h-polarized radiometer observes the surface, most of the h-polarized energy
emitted by the surface originates as h-polarized energy incident upon the surface
from below, but a small part of the h-polarized emitted energy originates as
v-polarized energy incident upon the surface from below which gets converted to
h-polarized energy through the process of transmission across the rough surface.
11. Weakly Scattering Medium
This is downward emission at the lower boundary
Next, we compute upward
emission at lower boundary
Z=0
Z=-d
12. Weakly Scattering
Medium
Z=0
Z=-d
2. Upward emission at the lower boundary:
Hence:
3. Upward emission at any height z:
1. Downward emission at the lower boundary:
4. Upward emission at z=0:
18. Optical Thickness
u The vegetation water content is the
single-most important physical
parameter influencing the optical
depth of a vegetation canopy
