How AIRS Works
The Atmospheric Infrared Sounder (AIRS) built by BAE SYSTEMS for NASA/JPL, is a cross-track scanning instrument. 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 km on either side of the ground track. Within the AIRS instrument the infrared energy is separated into wavelengths. This information is sent from AIRS to the Aqua spacecraft, which relays it to the ground.
The term "sounder" in the instrument's name refers to the fact that temperature and water vapor are measured as functions of height. AIRS also measures clouds, abundances of trace components in the atmosphere including ozone, carbon monoxide, carbon dioxide, methane, and sulfur dioxide, and detects suspended dust particles.
AIRS measures the infrared brightness coming up from Earth's surface and from the atmosphere. The AIRS optical system splits infrared radiation into its constituent "colors". The effect (but not the technique) is similar to rain drops splitting sunlight into a rainbow. 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 onboard 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.
Animation, from data collection to temperature profile
Animation, light traveling through AIRS optics