D.M. Le Vine

Soil moisture mapping at regional scales using microwave radiometry: the Southern Great Plains Hydrology Experiment

Surface soil moisture retrieval algorithms based on passive microwave observations, developed and verified at high spatial resolution, were evaluated in a regional scale experiment. Using previous investigations as a base, the Southern Great Plains Hydrology Experiment (SGP97) was designed and conducted to extend the algorithm to coarser resolutions, larger regions with more diverse conditions, and longer time periods. The L-band electronically scanned thinned array radiometer (ESTAR) was used for daily mapping of surface soil moisture over an area greater than 10000 km/sup 2/ for a one month period. Results show that the soil moisture retrieval algorithm performed the same as in previous investigations, demonstrating consistency of both the retrieval and the instrument. Error levels were on the order of 3% for area Integrated averages of sites used for validation. This result showed that for the coarser resolution used that the theory and techniques employed in the algorithm apply at this scale. Spatial patterns observed in the Little Washita Watershed in previous investigations were also observed. These results showed that soil texture dominated the spatial pattern at this scale. However, the regional soil moisture patterns were a reflection of the spatially variable rainfall and soil texture patterns were not as obvious.

Interferometric synthetic aperture microwave radiometry for the remote sensing of the Earth

Interferometric aperture synthesis is presented as an alternative to real aperture measurements of the Earths brightness temperature from low Earth orbit. The signal-to-noise performance of a single interferometric measurement is considered, and the noise characteristics of the brightness temperature image produced from the interferometer measurements are discussed. The sampling requirements of the measurements and the resulting effects of the noise in the measurements on the image are described. The specific case of the electronically steered thinned array radiometer (ESTAR) currently under construction is examined. The ESTAR prototype is described in detail sufficient to permit a performance evaluation of its spatial and temperature resolution. Critical aspects of an extension of the ESTAR sensor to a larger spaceborne system are considered. Of particular important are the number and placement of antenna elements in the imaging array. >

Aquarius: An Instrument to Monitor Sea Surface Salinity From Space

Aquarius is a combined passive/active L-band microwave instrument that is being developed to map the salinity field at the surface of the ocean from space. The data will support studies of the coupling between ocean circulation, global water cycle, and climate. Aquarius is part of the Aquarius/Satelite de Aplicaciones Cientiflcas-D mission, which is a partnership between the U.S. (National Aeronautics and Space Administration) and Argentina (Comision Nacional de Actividades Espaciales). The primary science objective of this mission is to monitor the seasonal and interannual variation of the large-scale features of the surface salinity field in the open ocean with a spatial resolution of 150 km and a retrieval accuracy of 0.2 psu globally on a monthly basis.

ESTAR: a synthetic aperture microwave radiometer for remote sensing applications

ESTAR represents a new technology being developed for passive microwave remote sensing of the environment from space. The instrument employs an interferometric technique called aperture synthesis in which the coherent product from pairs of antennas is measured as a function of pair spacing. Substantial reductions in the antenna aperture needed for a given spatial resolution can be achieved with this technique. As a result, aperture synthesis could lead to practical passive microwave remote sensing instruments in space to measure parameters such as soil moisture and ocean salinity which require observations at long wavelengths and, therefore, large antennas. ESTAR is an L-band, aircraft built as part of research to develop this technique ESTAR is a hybrid real-and-synthetic aperture radiometer which employs stick antennas to achieve resolution along track and uses aperture synthesis to achieve resolution across track. Experiments to validate the instruments ability to measure soil moisture have recently been conducted at the USDA watersheds at Walnut Gulch in Arizona and the Little Washita River in Oklahoma. The results of both experiments indicate that a valid image reconstruction and calibration have been obtained for this remote sensing technique. >

The determination of surface salinity with the European SMOS space mission

The European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission aims at obtaining global maps of soil moisture and sea surface salinity from space for large-scale and climatic studies. It uses an L-band (1400-1427 MHz) Microwave Interferometric Radiometer by Aperture Synthesis to measure brightness temperature of the earths surface at horizontal and vertical polarizations (T/sub h/ and T/sub v/). These two parameters will be used together to retrieve the geophysical parameters. The retrieval of salinity is a complex process that requires the knowledge of other environmental information and an accurate processing of the radiometer measurements. Here, we present recent results obtained from several studies and field experiments that were part of the SMOS mission, and highlight the issues still to be solved.

Soil moisture and rainfall estimation over a semiarid environment with the ESTAR microwave radiometer

The application of an airborne electronically steered thinned array L-band radiometer (ESTAR) for soil moisture mapping was investigated over the semiarid rangeland Walnut Gulch Watershed in southeastern Arizona. During the experiment, antecedent rainfall and evaporation were very different and resulted in a wide range of soil moisture conditions. The high spatial variability of rainfall events within this region resulted in moisture conditions with distinct spatial patterns. Analysis showed a correlation between the decrease in brightness temperature after a rainfall and the amount of rain. The sensors performance was verified using two approaches. First, the microwave data were used to predict soil moisture, and the predictions were compared to ground observations of soil moisture. A second verification used an extensive data set collected the previous year at the same site with a conventional L-band push broom microwave radiometer (PBMR). Both tests showed that the ESTAR is capable of providing soil moisture with the same level of accuracy as existing systems. >

Initial Results In The Development Of A Synthetic Aperture Microwave Radiometer

A radiometer that measures the complex correlation of the voltage from pairs of antennas at many different baselines is being developed. Each baseline produces a sample point in the Fourier transform of the scene, and a map of the scene is obtained after all measurements have been made by inverting the transform. A substantial reduction in the antenna collecting area required compared to a conventional imaging radiometer can be obtained in this manner. An aircraft prototype being developed is a hybrid which uses real aperture antennas to obtain resolution along-track (stick antennas) and uses aperture synthesis to obtain resolution across-track. The prototype was flight-tested aboard the NASA P-3 in June 1988. During this flight a map was made of the Delmarva Peninsula south of NASAs Wallops Flight Facility. This initial map shows the major land/water features and compares very favorably with a Landsat image of the area, suggesting a bright outlook for the development of this technique in the future.

The effect of the ionosphere on remote sensing of sea surface salinity from space: absorption and emission at L band

The purpose of this work is to examine the effects of Faraday rotation and attenuation/emission in the ionosphere in the context of a future remote sensing system in space to measure salinity. Sea surface salinity is important for understanding ocean circulation and for modeling energy exchange with the atmosphere. A passive microwave sensor in space operating near 1.4 GHz (L-band) could provide global coverage and complement in situ arrays being planned to provide subsurface profiles. However, the salinity signal is relatively small and changes along the propagation path can be important sources of error. It is shown that errors due to the ionosphere can be as large as several psu. The dominant source of error is Faraday rotation but emission can be important.

Discrete scatter model for microwave radar and radiometer response to corn: comparison of theory and data

As part of the Multisensor Aircraft Campaign, MACHYDRO, two microwave sensors, NASAs Airborne Synthetic Aperture Radar (AIRSAR) and Pushbroom Microwave Radiometer (PBMR) collected data over the same corn fields during the summer of 1990. During these flights, measurements were made on the ground of soil moisture and plant parameters. In this paper the measured canopy and soil parameters are used in a discrete scatter model to predict the response of both sensors (radar and radiometer). A distorted Born approximation is used to compute the scattering coefficient for the corn canopy. The backscatter coefficient gives the radar response and the radiometer response is obtained by integrating the bistatic coefficient over all scattering angles above ground. The objective of this analysis is to test the model and, in particular, to determine how well a single set of plant parameters and single model can yield agreement with both the radar and radiometer measurements. The model values are in reasonably good agreement with the measurements at horizontal polarization and reflect observed changes in soil moisture. >

A multifrequency microwave radiometer of the future

A description is given of a passive microwave remote sensing system designed to meet the observational needs of Earth scientists in the 1990s. This system, called the high-resolution multifrequency microwave radiometer (HMMR), is to be part of a complement of instruments in polar orbit that will form the Earth Observing System (Eos). As presently envisioned, the HMMR will consist of three separate instruments. These are the advanced microwave sounding unit (AMSU) presently under development for NOAA (US National Oceanic Atmospheric Administration), the advanced mechanically scanned radiometer (AMSR), and the electronically scanned thinned array radiometer (ESTAR). The AMSU will provide atmospheric soundings of temperature and water vapor using channels in the oxygen resonance band (50-60 GHz) and water vapor resonance line at 183 GHz. The AMSR is to be a microwave imager including channels at approximately 6, 10, 18, 21, 37, and 90 GHz, each with dual (linear) polarization. The ESTAR is an imaging radiometer operating near 1.4 GHz designed to obtain global maps of surface soil moisture with spatial resolution on the order of 10 km. >