II. SENSOR DESIGN PRINCIPLES
The previous section gave useful information regarding the functionality
of the SSMI. However, no information was given as to the purpose for the
sensor to be designed this way. The design of the sensor will be discussed
in the following section. The SSMI design is deeply rooted in radiative
physics. Thus, some time will be spent discussing the physics behind the
SSMI.
In general, there are numerous processes we are concerned with in atmospheric
radiation such as reflection, scattering, absorption, and emission. With
respect to each of these, there can be ( and often is ) great dependence
on frequency (and, thus, wavelength), viewing angle, emissivity, temperature,
size and type of material, etc. The resultant path traversed by the radiation
can be complex indeed. These different paths are representative of different
interactions that the microwave energy will have with mass (such as water
vapor, ice, oxygen, etc.). Figure 7 shows the significant microwave interactions
associated with the four SSMI frequencies.
Figure 7
Microwave Interactions: taken from Weather Satellites: Systems, Data, and
Environmental Applications, By P. K. Rao et al, American Meteorological
Society, 1990
Again, in general, we can characterize the radiation using the concept
of a Planck blackbody mentioned earlier. Ideally, a blackbody can be represented
by equation (2) below, known as the Planck blackbody function:
Equation (3) is the Rayleigh-Jeans Approximation to the Planck blackbody
formula. It is the functional equation for applications with the SSMI.
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