Indirect Methods
The more challenging case involves sensors, primarily satellite-mounted, such as radiometers, optical sensors, RADAR, LIDAR and similar light-based instruments as well as sonar since these cannot measure the quantity of interest, such as temperature, pressure, humidity etc., directly. By understanding emission and absorption processes, we can figure what the instrument is looking at between the layers of atmosphere. While this type of instrument can also be operated from ground stations or vehicles—optical methods can also be used inside in situ instruments—satellite instruments are particularly important because of their extensive, regular coverage. The AMSU instruments on three NOAA satellites, for instance, can sample the entire globe at better than one degree resolution in less than a day.
We can distinquish between two broad classes of sensor: active, such as RADAR, that have their own source, and passive that only detect what is already there. There can be a variety of sources for a passive instrument including scattered radiation, light emitted directly from the sun, moon or stars—both more appropriate in the visual or ultra-violet range, as well light emitted from warm objects which is more appropriate in the microwave and infrared. These three sources will also roughly distinquish the viewing geometry of passive satellite instruments: an occultation instrument tracks the sun, moon or a star through the limb of the atmosphere, a limb-sounder also looks at the limb but measures scattered radiation while a nadir-looking instrument looks down through the atmosphere at the surface. The SCIAMACHY instrument operates in all three modes (it slices, it dices...)
Read more about this topic: Atmospheric Sounding
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