Among the 15 Earth missions the National Research Council recently recommended that NASA undertake in the next decade is the “Precipitation and All-weather Temperature and Humidity” (PATH) mission, which will place a microwave sounder in geostationary orbit. The NRC identified a “MW array spectrometer” as the preferred instrument payload for PATH. A prototype for such an instrument, the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR), has been developed 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. GeoSTAR is a microwave sounder with the same capabilities as those of the Advanced Microwave Sounding Unit (AMSU) system now operating on low-earth-orbiting (LEO) satellites. Providing such a capability in geostationary orbit (GEO) has long been a goal for NOAA, NASA and other space agencies, since GEO offers key advantages over LEO. Due to the very large antenna aperture needed for a microwave sounder to provide the required spatial resolution, it has not previously been possible to develop such instruments for GEO. GeoSTAR overcomes the large-aperture problem by synthesizing a large aperture, and the development of the GeoSTAR concept therefore 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 PATH and GeoSTAR will be implemented in the near future. Plans for a PATH/GeoSTAR mission are now under development, 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 (named GOES-R after the first in the new series). 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 on one of the first GOES-R satellites 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. 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.