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Instrument: GFC RADIOMETER : Gas Filter Correlated Radiometer
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The Gas Filter Correlation Radiometer (GFCR) is an
optoelectrical sensor able to detect airborne pollutants such as
carbon monoxide, methane, and nitrogen oxides. The device makes
use of a polarization modulator, in conjunction with a
polarization beam splitter, to enable rapid optical-path
switching without the use of moving parts. As light reflected
from the atmosphere enters the instrument the light radiation
passes through a polarization-sensitive beam splitter, the
splitter reflects the polarized photons and transmits
horizontally polarized photons. The beamis thus alternated
between two optical paths: one comprised of a vacuum cell and
the other a correlation cell that contains a small quantity of
gas, which the sensor is calibrated to detect.

The gas in the correlation cell acts as a defacto filter for
incoming radiation. As radiation from the two paths is
recombined and analyzed, the difference in signal strengths
indicates the amount of target gas present in the outside air.