In its recent “Earth Science Decadal Survey” the National Research Council (NRC) recommended that NASA undertake a mission to place a microwave sounder in geostationary orbit. This has been a long sought goal for NOAA as well. The NRC identified a “MW array spectrometer” as the preferred instrument payload. A prototype for such an instrument, the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR), has been developed and demonstrated at NASA's Jet Propulsion Laboratory, and a second phase of risk reduction and development of elements required for a space mission is now under way, sponsored by the NASA Earth Science Instrument Incubator Program and co-funded by NOAA's National Environmental Satellite Data and Information Service (NESDIS). GeoSTAR is a microwave sounder with nearly the same performance as the Advanced Microwave Sounding Unit (AMSU) system now operating on NASA and NOAA low-earth-orbiting (LEO) satellites. Providing such a capability in geostationary orbit (GEO) has long been a goal for space agencies, since GEO offers key advantages over LEO and the value of microwave sounders has been firmly established with AMSU. Due to the very large antenna aperture needed in GEO, it has not previously been possible to develop such an instrument, but GeoSTAR overcomes the large-aperture problem by synthesizing a large aperture and thus makes a GEO microwave sounder possible. With NASA's stated commitment to implement the NRC “Decadal Survey” missions, a confluence of common interests with NOAA and other agencies, and the growing maturity of the necessary technology, it is likely that GeoSTAR will be implemented in the near future. Plans for a GeoSTAR mission are now being developed, and a realistic mission study was recently completed by JPL for NASA. A similar study has been done for NOAA as well, and both conclude that GeoSTAR can meet all relevant measurement requirements within reasonable mass, power and cost constraints. The emergence of a viable approach to implement a “GEO AMSU” is of particular interest to NOAA, which is currently developing the next generation of GOES satellites. This satellite system was intended to carry an advanced hyperspectral infrared sounder (the Hyperspectral Environmental Suite – HES), but HES was recently cancelled for the first two satellites (GOES-R and GOES-S). A strong need for an atmospheric sounder remains, and the possibility of flying GeoSTAR in conjunction with the first GOES-R satellites – possibly as a “Mission Of Opportunity” – is now under investigation. A joint effort by NASA and NOAA, where NASA would provide the instrument and NOAA would provide the platform and launch, would be a compelling demonstration of a new “Research to operations” paradigm and would fill the “advanced sounder” gap as well. GeoSTAR will provide a number of measurements that are key in monitoring and predicting hurricanes and severe storms – including hemispheric 3-dimensional temperature, humidity and cloud liquid water fields, rain rates and rain totals, tropospheric wind vectors, sea surface temperature, and parameters associated with deep convection and atmospheric instability – everywhere and all the time, even in the presence of clouds – and will also provide key measurements related to climate research.