GPS Limb Sounding From Orbit
One can receive on a low flying satellite the signals from the much higher orbiting (20 000 km) GPS satellite constellation. As the low flying satellite orbits the Earth in 1.5 hours, many of the GPS satellites will "rise" and "set" during the time of the orbit. When they do, their signal will traverse the atmosphere.
A signal delay is produced which grows or decays exponentially with time, just as the atmospheric density is an exponential function of height above the Earth's surface. In fact, this so-called limb sounding technique allows us to determine the scale height, the constant describing the steepness of this atmospheric density decay. This makes the technique extremely valuable for climatological studies, as the scale height is directly related to the temperature in the upper atmosphere, where the limb sounding signals do their sensing. The technique works best in the lower stratosphere and upper troposphere; it breaks down close to the Earth surface especially in the tropics, due to water vapour extinction.
Getting a precise measure for the mean atmospheric temperature by an absolute method which is truly global and not affected by the effects of either the non-random (Euramerican-centred, land biased) global distribution of weather stations, or the effects of urban s, is the holy grail of climate change studies. Unfortunately the techniques has only been in use for a small number of years now, offering only a short time base.
Satellites involved in GPS limb sounding have been: METSAT, OERSTED (Danish), and several others.
Read more about this topic: GPS Meteorology
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