Technology
The term "sounder" in AIRS's name refers to the fact that the instrument measures temperature and water vapor as a function of height.
AIRS measures the infrared brightness coming up from Earth's surface and from the atmosphere. Its scan mirror rotates around an axis along the line of flight and directs infrared energy from the Earth into the instrument. As the spacecraft moves along, this mirror sweeps the ground creating a scan swath that extends roughly 800 kilometers on either side of the ground track. Within the instrument, an advanced, high-resolution spectrometer separates the infrared energy into wavelengths.
Each infrared wavelength is sensitive to temperature and water vapor over a range of heights in the atmosphere, from the surface up into the stratosphere. By having multiple infrared detectors, each sensing a particular wavelength, a temperature profile, or sounding of the atmosphere, can be made. While prior space instruments had only 15 detectors, AIRS has 2378. This greatly improves the accuracy, making it comparable to measurements made by weather balloons.
Thick clouds act like a wall to the infrared energy measured by AIRS. However, microwave instruments on board Aqua can see through the clouds with limited accuracy. Using a special computer algorithm, data from AIRS and the microwave instruments are combined to provide highly accurate measurements in all cloud conditions resulting in a daily global snapshot of the state of the atmosphere.
Read more about this topic: Atmospheric Infrared Sounder
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