The AIRS Instrument Suite
The Atmospheric Infrared Sounder (AIRS) instrument suite is designed to measure the Earth’s atmospheric water vapor and temperature profiles on a global scale. It is comprised of a space-based hyperspectral infrared instrument (AIRS) and two multichannel microwave instruments, the Advanced Microwave Sounding Unit (AMSU-A) and the Humidity Sounder for Brazil (HSB). The AIRS instrument suite along with several other instruments was launch aboard NASA's Earth Observing System Aqua satellite on May 4, 2002.
The AIRS instrument, 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's constituent wavelengths, or “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.
High spectral resolution of the AIRS instrument improves vertical resolution, however the resolution degrades in the presence of clouds. Microwave energy sensed by the AMSU and HSB instruments is insensitive to clouds and provides valuable soundings in all weather conditions. 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.