A new concept for a geostationary microwave sounder has recently been developed at the Jet Propulsion Laboratory and funded by NASA, called the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR), which is a viable approach to obtaining the observations the NRC calls for. The basic measurements essentially duplicate those that are currently provided by low-earth-orbiting systems such as the Advanced Microwave Sounding Unit (AMSU). However, GeoSTAR could provide these measurements from a GEO orbit, which could consist of a continuous series of 2-D radiometric snapshots of a large portion of the visible Earth disc at various sounding frequencies. From those observations we could derive, using mature retrieval algorithms, 3-dimensional maps of temperature, water vapor and liquid water between the surface and the upper troposphere every 30 minutes. Additional information about GeoSTAR is available here.