The "Precipitation and All-weather Temperature and Humidity" (PATH) mission is one of the 15 NASA "decadalsurvey"
missions recommended by the U.S. National Research Council in 2007 and will implement the first microwave
sounder in geostationary orbit. This is possible with a new sensor being developed at the Jet Propulsion Laboratory, the
Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR). Adequate spatial resolution is achieved by using
aperture synthesis instead of a large parabolic reflector as is used in conventional systems. A proof-of-concept prototype
was developed at JPL in 2005 under the NASA Instrument Incubator Program and used to demonstrate that this new
concept works well at sounding frequencies. Another IIP effort is now under way to advance key technology required for
a full space system. The maturity of the concept and technology is now such that mission development could be initiated
in 2010-11. The possibility of flying GeoSTAR as an "instrument of opportunity" on NOAA's new series of "GOES-R"
geostationary weather satellites is being actively pursued. Other low-cost options are under study as well.
PATH/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.