1. Sensor Channels and Polarizations
The SSMI has four frequencies: 19.35 GigaHertz (GHz), 22.235 GHz, 37.0
GHz, and 85.5 GHz. The sensor is a passive suite: it only measures the
radiation coming up from the earth. It does not send out a signal , such
as a RADAR. The 19, 37, and 85 GHz frequencies are split into two channels
each : one channel detects vertically polarized microwave energy, while
the other receives horizontally polarized energy. The 22 GHz frequency
only has a vertically polarized channel. Thus, the SSMI has seven total
channels, summarized below in Table 1.
The sensitivity of these channels ranges from +/- 0.37 degrees K to 0.73
K. The SSMI channels measure a quantity known as brightness temperature
. Brightness temperature, or Tb, is the radiometric temperature
of the earth. It is directly related to the actual temperature, via equation
Tb = e T (1)
where T is the Temperature, and -e- is the emissivity of the earth's surface.
Emissivity is a relative, dimensionless measure than varies from 0 to 1.
Objects with a low emissivity (such as water ) will appear colder to the
SSMI than they actually are. Conversely, objects with a high emissivity
will appear closer to the actual temperature. In the ideal case, an object
with a emissivity of 1 (referred to as a Planck blackbody) would emit a
brightness temperature which exactly equalled the surface temperature.
This is a perfect emitter.
With this in mind , one can say identify two primary functions of the SSMI
: it conducts radiometry (the measure of radiant energy) and polarimetry
(the measure of polarization properties of a scene) of the earth's surface.
Go to the next section.
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