21 December 1994 Radiometer aperture synthesis for soil moisture mapping
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A major application for a 21 cm radiometer is the remote sensing of soil moisture which is possible because of the large contrast between the dielectric constant of dry soil (approximately equals 3.5) and that of liquid water ( approximately equals 80). One of the major problems with the utilization of long wavelength radiometers from satellite platforms has been the large antenna size required with its substantial mass. For example, at satellite altitudes an antenna size of at least 10 m is required to obtain resolutions in the 10-20 km range. The size requirement is fundamental but the mass can be reduced by using unfilled arrays or as will be described here a thinned array antenna. Such a system operating at L-Band ((lambda) equals 21 cm or 1.42 GHz) has been developed and tested from an aircraft platform. It is called ESTAR (Electronically Scanned Thinned Array Radiometer) and it uses linear (stick) antennas in the along-track direction and aperture synthesis between pairs of sticks separated by odd multiples on half wavelengths in the cross track direction. The approximate dimensions of the antenna are 1 meter by 1 meter. Results from an evaluation series of flights over a study watershed in Oklahoma indicate that such a system can provide useful soil moisture information.
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Thomas J. Schmugge, Thomas J. Jackson, David M. Le Vine, Calvin T. Swift, "Radiometer aperture synthesis for soil moisture mapping", Proc. SPIE 2313, Microwave Instrumentation and Satellite Photogrammetry for Remote Sensing of the Earth, (21 December 1994); doi: 10.1117/12.197342; https://doi.org/10.1117/12.197342

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