Status
During the 1980s, in the framework of its Earth observation program, NASA organized several workshops at which scientists demonstrated the roles of soil moisture and ocean salinity in the global environmental system. Passive microwave radiometry could be used to measure these two geophysical parameters, but the most suitable frequency bands were those below 5 GHz and it was difficult to achieve the required spatial resolution with an antenna of reasonable size. NASA's Goddard Spaceflight Center, in collaboration with the University of Massachusetts at Amherst and the US Department of Agriculture, proposed the use of aperture synthesis as a solution to this problem for the first time and started to build an aircraft-borne prototype to test the concept. This NASA ESTAR (Electronically Scanned Thinned Array Radiometer) sensor was designed to be an L-band hybrid real- and synthetic-aperture radiometer and the instrument's validity was demonstrated in several USDA campaigns.