B. The Purpose of each polarization

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].

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.