The Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR) was conceived to fill a gap in our Earth remote sensing capabilities - the lack of a microwave atmospheric sounder in geostationary orbit. This capability is highly desired because of the advantageous observing point GEO offers, with continuous views of the entire visible Earth disc - crucial for observation of hurricanes and other rapidly evolving atmospheric phenomena. The GeoSTAR concept is a radical departure from the conventional way of constructing a microwave sounding radiometer, which typically uses a scanning parabolic reflector that focuses a narrow receiving beam into a single receiver. Instead, the STAR approach is to use a 2-D array of a large number of receivers to synthesize a large aperture and operate the system as a spatial interferometer that images the scene in two dimensions without requiring scanning. Such systems have been investigated before, and the European Space Agency is developing a soil moisture satellite mission based on this concept, but none had been adequately demonstrated either on the ground or in space. This is a perfect case for a prototyping effort. An effort has been under way at the Jet Propulsion Laboratory for the last 3-4 years to build a GeoSTAR prototype and demonstrate that the 2-D STAR approach works. The prototype, which is a moderate-aperture implementation of a fully functional temperature sounder, was completed in early 2005 and has since undergone a number of tests. This prototyping effort, which involved both technology and system development as well as development of new testing methodologies, has been very successful. The GeoSTAR concept, even though it has not been tested in a space environment and therefore is not formally at TRL 6 (i.e. per NASA's definition), is now ready to be considered for space mission opportunities. A companion effort to assess the expected impact on weather forecast accuracy through a so-called Observational System Simulation Experiment (OSSE) is also under way. Together, the prototyping and the OSSE constitute crucial steps on the road to a space mission.
GeoSTAR could potentially be the payload of the PATH mission. If PATH goes forward, the GeoSTAR payload with be the first microwave sensor in geostationary orbit. It would help to improve weather and climate observations and models, while improving our understanding and forecasting of hurricane intensity.
The GeoSTAR team is also working with several partners for instrument development, such as OmniSys Instruments of Sweden who recently released test results for a correlator ASIC.
Additionally, there is also an opportunity to fly a GeoSTAR on a GOES satellite. Currently there is no HES payload on GOES-R and GOES-S (and maybe -T) - GeoSTAR may be a good fit to fill this slot, as a GEO microwave sounder could have a high value to NOAA.