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Estimation of the primary variables describing atmospheric state, namely, temperature, moisture and wind, using space-based observations continues to lead to improved delineation of atmospheric conditions. Errors in the estimation of temperature moisture and wind from these observations, for global, regional and mesoscale applications, have been reduced as the spatial temporal and spectral resolution of these observations has increased. Reduction in the errors has also been greatly assisted by improvements in numerical weather prediction (NWP) models, data assimilation methods and continuing increases in computer power. This paper describes the progress, showing results from recent operational sounding and image data. It presents examples of both the improvements in the estimation of the primary analysis variables and the resultant benefits to NWP from these improved data. The importance of satellite direct readout for providing timely data for regional applications where short data cut-off times are present is also noted. The paper also records the important role of in international collaboration in these activities, and describes the key role played by the International TOVS Working Group (ITWG) and more recently by the International Winds Workshop. The ITWWG first met in 1983 and since then the International TOVS Study Conferences (ITSCs) have become a forum for the exchange of information related to atmospheric sounding. Twelve ITSCs have been held, the next one will be in Beijing in 2003. The ITWG has been responsible for the generation and sharing of community software for ingest, preprocessing and application of operational and research sounding data. Without this software much of the recent research and applications related to atmospheric sounding, would not have occurred. Another important role of the ITWG is the provision of guidance and recommendations to agencies involved in atmospheric sounding related areas, such as data collection, frequency protection, data preprocessing, data application and distribution. Key areas considered by the ITWG include NWP, climate monitoring and global change and radiative transfer. In short the activities of the ITWG have been pivotal in the effective collection, processing and application of sounding data will continue to be so for the foreseeable future. Importantly the coming years will bring several new sounders (both infrared and passive microwave) on polar and geostationary satellites, and this will further consolidate the requirement for worldwide cooperative work.
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