Related to the issue of polarization is the concern over the 'look
angle' or scanning geometry of the SSMI. The basic problem is that a direct
downward (nadir viewing ) scan method will not observe any significant
difference in polarization. Elachi [1987] stated that '.... at an incidence angle of
37 degrees from vertical, an optical wave polarized perpendicular to the plane
of incidence will reflect about 7.8% of its energy from a smooth water surface,
while an optical wave polarized in the plane of incidence will not reflect any
energy from the same surface. All the energy will penetrate into the water. This
is the Brewster effect." In general, "... lower incidence angles show much less
difference between the horizontal and vertical polarizations of a channel than
higher ones [See figure 10]. At nadir, where the incidence angle is 0 degrees,
the polarization difference disappears entirely" [Grant, 1991].
GHz channels. Rough inspection of figure 9 will show the reader the rationale behind the selection
of 51-53 degrees.
As mentioned earlier, the conical scanning method with constant incidence angle makes the
sensor 'footprint' on the surface a constant shape and resolution. This is a significant advantage
over the DMSP OLS and NOAA AVHRR, since both those sensors sweep across nadir from left
to right.
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