Measurements from space offer the potential for global-scale observations necessary for understanding weather, climate and the global environment. However, microwave measurements from space have been limited by the large aperture antennas required to obtain reasonable spatial resolution. For example, the 10 km resolution at L-band (1.4 GHz) from an orbit of 800 km desired by hydrologists for the measurement of soil moisture would require an antenna in space of about 20 m x 20 m. For practical applications with periodic global coverage it would also be necessary to scan about 145 degrees with this aperture.
Aperture synthesis is a new technology that helps to overcome some of the limitations of size, weight, and scanning associated with real aperture antennas. Microwave imaging with fine spatial resolution is possible from space using aperture synthesis without the need to scan a large aperture.
Aperture synthesis was first applied in radio astronomy as a means to achieve high resolving power with an antenna array using a limited number of (relatively) small, individual elements. More recently synthetic aperture radiometers have been developed for remote sensing of the earth. The first such instrument, the L-band Electronically Scanned Thinned Array Radiometer (ESTAR), was developed by NASA's Goddard Space Flight Center and the University of Massachusetts. The objective of this research was to demonstrate the utility of aperture synthesis for remote sensing of the earth with specific application to the remote sensing of soil moisture and ocean salinity (two important observations made at L-band).
dT = (A/na)*Tsys/Bt**0.5
where Tsys is the sum of the system and scene noise temperatures and Bt is the time bandwidth product, A is the effective area of a real aperture with the same spatial resolution as obtained with the synthesized antenna, a is the area of the individual real aperture antennas employed in the array, and n is the number of antennas in the array. The term, Tsys/Bt**0.5, is just the conventional formula for sensitivity for a real aperture (total power) radiometer. Since in a practical application na is less than A , the sensitivity of the synthesized beam will be poorer than a real aperture antenna; however, the synthetic aperture radiometer receives energy from all pixels in the field-of-view and as a result, its integration time can be larger than that of a comparable real aperture scanning radiometer.
There are many possible ways to implement aperture synthesis and each configuration must be evaluated for its performance (i.e. sensitivity, number of receivers, coverage in the Fourier space, etc). For example, ESTAR is a hybrid which employs real antennas (long stick arrays) to obtain resolution in the direction of motion (along-track) and uses aperture synthesis to obtain resolution cross-track. One could obtain equivalent resolution using aperture synthesis in two-dimensions, for example with an array of antenna elements along the arms of a cross (+), a tee (T) or (Y). It is also possible in some applications to have the antennas arranged around the circumference of a circle (e.g. a hoola hoop). Such arrays have been studied with application to profiling of atmospheric temperature at 50-60 GHz.
The experience with ESTAR has demonstrated the potential of aperture synthesis for remote sensing of soil moisture from space. However, the measurement of soil moisture requires moderate sensitivity. Applications which require greater sensitivity (e.g. ocean salinity measurement requires (DT0.02 K) or synthesis in two dimensions (necessary from geostationary orbit) will require research to improve the image reconstruction algorithm and calibration methods. Although, accomplished successfully with ESTAR at the level needed to measure soil moisture from an aircraft platform, calibration for applications which require greater sensitivity and autonomous calibration in space require additional work. The transfer of data between the individual antennas and central processor (interconnect problem) is another area requiring work. This includes both the transfer of data back from the individual antennas and the delivery of reference signal (e.g. the LO) to the individual antennas. Finally, advances in correlator technology are needed to reduce the power requirements of the processing unit.
The potential for practical, high resolution microwave measurements from space has raised interest at other frequencies also. For example, studies are underway of a high resolution (1 km) instrument at 18 and 37 GHz to monitor thin ice and open water (leads) in the Arctic to support shipping along the Northern Sea Route. An instrument of the ESTAR-type (hybrid real-and-synthetic aperture) is being built at 37 GHz for the Department of Defense (Navy and Air Force) by Quadrant Engineering in Massachusetts. Research on aperture synthesis in two dimensions has also received much attention recently. In particular, ESA/ESTEC is studying the potential of aperture synthesis in two dimensions for remote sensing of soil moisture. An aircraft prototype is nearly complete (at L-band and using a Y-configuration) and studies are underway to define an instrument for monitoring soil moisture from space. Several laboratory instruments have also been developed for research on aperture synthesis in two dimensions. Instruments have been assembled in Denmark (TUD, N. Skou at 10 GHz), in Germany (DLR, Peichl and Seuss at 37 GHz), and in the U.S.A. at the Goddard Space Flight Center (12 GHz) and TRW (44 GHz, Pearlman and Davidhowser). Plans exist in France at CNES to build an instrument at C-band (6 GHz). A two dimensional instrument with a somewhat different configuration has been built in Japan (ETL, K. Komiyama at 10 GHz) and a related concept originally developed at Hughes (C. Wiley and Edelsohn, RADSAR) is continuing to receive attention.
"The Sensitivity of Synthetic Aperture Radiometers for Remote Sensing of Applications from Space," D.M. Le Vine, Radio Science, Volume 25, Number 4, pp 441-453, July 1990.
Interferometery and Synthesis in Radio Astronomy, A. Thompson, J. Moran and G. Swenson, J. Wiley and Sons, New York, 1986.
Proceedings IGARSS-94, Synthetic Aperture Radiometry for Earth Remote Sensing, Vol III, pp 1311-1331 (Lib Congress #: 93-80348; IEEE Cat #: 94CH3378-7).
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