Calvin T. Swift

Professor of Electrical and Computer Engineering
University of Massachusetts

ESTAR Research

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Electronically Steered Thinned Array Radiometer

In the area of earth observations, there is a demand for the combination of fine spatial resolution coupled with large area coverage. These requirements place great stress on passive microwave observations of the earth. Unlike Synthetic Aperture Radar, where time-Doppler processing can be used to achieve the fine resolution comparable to that achieved with optical systems, microwave radiometers require large antennas as the only means to improve spatial resolution. The need for antennas as large as 100 m or more becomes even more evident if L-Band frequencies are to be used in low earth orbit for soil moisture studies. Furthermore, mechanical scanning of large, filled aperture antennas is problematic.

Because of the potential expense associated with placing large, filled apertures into space, the concept of aperture synthesis was explored in the 1980's as a means to address this problem. This concept greatly reduces the number of elements in a phased array antenna, and performs a large number of correlations between elements in order to synthesize an image corresponding the full aperture. In this way, antenna simplicity is achieved, at the expense of system complexity, a reasonable trade-off in view of the low cost electronics that are now available. As a demonstration of the thinned array concept, the University of Massachusetts and Goddard Space Flight Center developed a prototype L-Band system named ESTAR. This instrument demonstrated the success of aperture synthesis and provided scientific results for soil moisture.