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LARGE AREA MAPPING OF SOIL MOISTURE USING THE ESTAR
PASSIVE MICROWAVE RADIOMETER
T. J. Jackson
USDA Hydrology Lab, Beltsville, MD 20705
D. M. Le Vine
NASA GSFC Microwave Sensors Branch, Greenbelt, MD 20771
C. T. Swift
University of Massachusetts, Dept, of Electrical Engineering, Amherst, MA 10004
T. J. Schmugge
USDA Hydrology Lab, Beltsville, MD 20705
ABSTRACT
Recent investigations designed to study land surface hydrologic-atmospheric interactions have shown the
potential of L band passive microwave radiometry for measuring surface soil moisture over large areas.
Satisfying the data needs of these investigations requires the ability to map large areas rapidly. With aircraft
systems this means a need for more beam positions over a wider swath on each flightline. For satellite systems
the essential problem is resolution. Both of these needs are currently being addressed through the development
and verification of electronically scanned thinned array radiometer (ESTAR) technology. The ESTAR L band
radiometer has been evaluated for soil moisture mapping applications in two studies. The first was conducted
over the semiarid rangeland Walnut Gulch watershed located in southeastern Arizona. The second verification
study was performed in the subhumid Little Washita watershed in southwest Oklahoma. Both tests showed that
the ESTAR is capable of providing soil moisture with the same level of accuracy as existing systems.
KEY WORDS: Soil moisture, microwaves, radiometry
1 - INTRODUCTION
There have been significant advances in the passive microwave remote sensing of soil moisture over the past
decade. Many of these are reviewed in Jackson and Schmugge (1991) and Schmugge et al. (1992). There is
a consensus that the optimal sensor system for monitoring and mapping soil moisture is an L band passive
microwave radiometer. Using this wavelength, it is possible to reliably estimate surface moisture within a few
percent even in the presence of moderate vegetation cover, although there are still a number of data
interpretation and algorithm problems that need to be addressed.
Providing surface soil moisture on an operational global basis from a satellite platform involves solving
one of the critical problems in long wavelength passive microwave radiometry. This is the problem of meeting
ground resolution requirements within the constraints of the wavelength-antenna relationship. No practical
solution has been proven using conventional antenna technologies. For this reason, we have been evaluating
the use of synthetic aperture radiometers. This approach has the potential of providing data with adequate
resolution and being compatible with satellite platform limitations. Le Vine et al. (1990) have designed and
constructed an aircraft prototype of one configuration of such an instrument called the electronically scanned
thinned array radiometer (ESTAR).
This paper describes a program designed to evaluate the ESTAR for the remote sensing of soil moisture
through a series of experiments designed to verify its calibration and performance as well as contributing to the
scientific application of the data. These experiments, which were conducted in two diverse environments in the
United States, Arizona and Oklahoma, demonstrated the sensor's capabilities and have contributed new
information on the large scale interpretation and application of the data. 2
2 - ESTAR L BAND INSTRUMENT
The electronically scanned thinned array radiometer (ESTAR) is a synthetic aperture, passive microwave
radiometer. The nominal parameters of the instrument are:
