What is a Sounding Instrument?
  
   _Answer posted by Wayne Boncyk (boncyk@edcsgw4.cr.usgs.gov) to
   IMAGRS-L_
   
   Satellite-borne remote sensing instruments may be used for more than
   imaging; it is possible to derive information about the constituents
   of the local atmosphere above a ground target, for example. One common
   area of study is to observe atmospheric emissions in the spectral
   neighborhood of the 183GHz water absorption line (millimeter-wave;
   in-between microwave and thermal IR). These channels can be monitored
   by an appropriate collection of narrow passband radiometers, and the
   data that are returned can be analyzed to deduce the amount of water
   vapor present at different levels (altitude layers) in the atmosphere.
   The reference to "sounding" is an application of an old nautical term,
   the investigation of the state of a medium at different depths
   (original application: the ocean - specifically determination of the
   depth of the ocean floor).

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