Hello dear all.
While reading some articles regarding several satellite sensors, I realized that the terms "imager" and "sounder" are distinctively used.
Unfortunately, I couldn't find a descriptive document where the differences are mentioned [ I only got the following: http://satellite.imd.gov.in/dynamic/satfaq.pdf] and reading several papers to discover the differences will take me more time than that I can invest in.
My second question is: which are the fundamental concepts in the processing of the latter ones to derive the vertical columns?
I really appreciate your input.
Best regards,
Melina.
Hello Melina,
here's a brief answer:
1) an imager can be compared to a photocamera pointed downwards. Instead of RGB matrix one can use more wavelengths to distinguish different objects, but the principle will remain the same.
2) The big family of sounders can be separated to active and passive ones: active sounders emit the pulse and collect the radiance reflected from the atmospheric layers while the passive ones just collect the radiance emitted/reflected by the surface, by clouds, aerosols, and atmospheric gases.
3) passive sounders can collect broad- and narrow-band radiance and the viewing geometry is either nadir- or limb-staring or occultation mode (in this case the absorption of light emitted by the star is measured)
4) atmospheric parameter retrieval approach depends on aforementioned modes, on the object of study, and on the wavelength. Normally one has to solve the radiative transfer equation for a given wavelength to get the parameter. In general, if the outgoing radiance is sensitive to changes of the parameter then the parameter can be retrieved. If the selected spectral channel is too optically thick then the photons carrying the information about the parameter at the given height are trapped, and the retrieval is not possible.
5) technically, an imager can be considered as an array of passive nadir broadband sounders.
Hope this helps :)
An imager produces a 2-dimensional image of an area (usually a surface). An example is landsat which images land surface reflectivity in the Visible/ Near-Infrared part of the spectrum. A sounder is designed to produce a vertical profile, e.g. of water vapor density in the case of an atmospheric sounder.
The satellite imagers include visible, infrared, and microwave based on the wavelength. They usually measures surface or atmospheric parameters. Visible imager sensors measure radiation in the visible band (0.4-0.7 um). Infrared imager sensors usually measure radiation in the 10-12.5 um, in which the atmosphere is relatively transparent to radiation upwelling from the Earth's surface, or water vapor channel center around 6.7 um.
The satellite sounders normally have multiple channels to get information from different atmosphere height, are used to retrieving vertical profiles of temperature and trace-gas concentrations, especially those of water vapor and ozone. To retrieve vertical profile, one have to solver radiative transfer equation. One important concept is weighting function which is related to the atmospheric absorption at specific layer. Using that concept, one can get information from that layer.
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