C.  The Purpose of  the Scan Geometry

        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].

Frequencies for SSMI

Figure 10


        The only remaining issue then is to pick a scanning angle that optimizes the 19, 37 , and 85

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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