A. The Purpose of each channel

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The SSMI has seven channels, but only four distinct frequencies: the 19, 22, 37, and 85 GHz channels. The reason for each frequency is based on the absorption, emission, and scattering properties of the earth's surface and atmosphere. The radiometric aspects of the SSMI were chosen to accomplish specific objectives. This absorption of microwave energy is displayed in figure 8 (courtesy of Aerojet Electro Systems). Looking at this curve, note the 22 GHz region. Here, there is a relative maximum in the absorption. This is caused by water vapor in the atmosphere. Thus , the 22V channel was chosen to provide an estimate of water vapor in theatmosphere.

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Figure 8

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Profiling of Atmosphere is accomplished in water vapor andoxygen absorption bands. (Courtesy of Aerojet Electro Systems).

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The 85 GHz channels were chosen to provide an estimate of the precipitation from a given cloud complex. The following excerpt from Fitzpatrick [1994] provides a discussion of this phenomenon:

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"Large raindrops and ice particles scatter microwave radiation, effectively lowering the background brightness temperature by scattering emitted radiation away from the satellite. This effect is most pronounced at high frequencies at which the particle size becomes comparable to the wavelength. This lowering of the brightness temperature due to scattering by precipitation- sized particles is equally effective over water and land. This relationship enables the 85.5 GHz observations from the SSM/I to play a key role in identifying precipitation."

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Thus , the 85 GHz channels take advantage of the scattering properties of the raindrops and ice particles in the atmosphere, since at a wavelengthof 3.5 mm, the radiation is close to the size of water droplets and ice particles.

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For the 19 GHz channels, the intention was to find a regime that wouldpermit radiation from the surface of the earth to pass directly to the sensor. If the 85 GHz (3.5 mm) channel was close to the size of water droplets, then the 19 GHz (15.5 mm) would be an order of magnitude larger than these species, and as a direct result, the least affected by the water vapor/droplets/ice. Instead, the 19 GHz region is sensitive to the characteristics of the earth's surface, such as ocean surface roughness, land surface moisture,land type, etc. Because of this feature, the 19 GHz is a phenomenal channelfor a variety of applications: Ocean surface wind speed ( to be discussed later), presence of snow / ice on the surface, detection of flooded regions,and many others. The 19 GHz channel is perhaps the most useful, flexible,and exciting channel on the SSMI.

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The 37 GHz was selected for the most part as a direct result of previousexperience from earlier NASA microwave sensors flown aboard satelliteslike the Nimbus series of the 1970's. Primarly, the 37 GHz channels are a good 'midpoint' between the 19 GHz and 85 GHz. These channels are insensitive to some atmospheric species like water vapor and oxygen, yetthey are sensitive to rain, cloud water content, and surface ice cover. Because of these factors, the 37 GHz provides a good detector to helpidentify regions where selected atmospheric phenomena might degradethe determination of numerous environmental products, such as oceansurface wind speeds.