Satellite sounding instruments measure radiation at infrared and microwave wavelengths that have primarily been emitted by the atmosphere itself. The different wavelengths provide information on the temperature and composition (e.g. humidity, ozone amount) of the atmosphere over a range of altitudes. Infrared sounding instruments provide very high quality information in cloud-free areas, but can provide no information below cloud, which is where microwave instruments are essential.
The capability of these instruments is still advancing and work continues to improve the exploitation of the data they provide. Recently, information from a new generation of advanced infrared sounding instruments (spectrometers and interferometers) became available.
Infrared sounding radiometers
The High-resolution InfraRed Sounder (HIRS), a 20-channel radiometer, provides information on the temperature, water vapour and ozone structure of the atmosphere. Together with microwave instruments it forms part of the ATOVS sounding system which, following on from the previous TOVS sounding system, was the main source of satellite sounding data for more than 25 years, and remains valuable even today.
High spectral resolution sounders
A new generation of infrared sounding instruments have become available since 2003. These instruments, such as the Atmospheric Infrared Sounder (AIRS) on EOS-Aqua and the Infrared Atmospheric Sounding Interferometer (IASI) on Metop, boast very high spectral resolution and thousands of individual channels. These allow much finer scale structure in the vertical profiles of temperature and humidity to be successfully analysed (about one km, compared to three km for the previous generation of sounders).
Infrared sounding — technical details
- The infrared region of the spectrum used for atmospheric remote sensing ranges from 16.6 to 3.3 microns in wavelength
- Carbon dioxide (CO2) spectral bands at 15 and 4.3 microns give us information on the temperature structure of the atmosphere
- Information on water vapour content can be gained from a large number of H2O lines between five and eight microns
- Parts of the infrared spectrum are sensitive to ozone (8.9-10.1 micron band), and other trace gases such as CH4 and N2O
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