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. 

SSMI Channels

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:
SSMI Channels

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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