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. Satellites involved in GPS limb sounding have been: METSAT, OERSTED (Danish), and several others. See also Error analysis for the Global Positioning System Links and references GPS meteorology Meteorological instrumentation and equipment Global Positioning System