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AMSR-E Validation Data

Validation Campaigns for the AMSR-E Sensor on the NASA Aqua Satellite


SMEX02 Electronically Steered Thinned Aperture Radiometer (ESTAR) Aircraft Data

The Electronically Steered Thinned Aperture Radiometer (ESTAR) was deployed during the Soil Moisture Experiment 2002 (SMEX02) for estimating surface soil moisture with an L-band radiometer. These data are not available for distribution. Distribution is restricted because problems with condensation within the instrument affected performance and quality control. Although some of the data may be recoverable, several days of data were lost. Processing is no longer being completed. This study was conducted during June and July 2002 in the area of the Ames, Iowa, USA.

These data were collected as part of a validation study for the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E). AMSR-E is a mission instrument launched aboard NASA's Aqua Satellite on 04 May 2002. AMSR-E validation studies linked to SMEX are designed to evaluate the accuracy of AMSR-E soil moisture data. Specific validation objectives include assessing and refining soil moisture algorithm performance; verifying soil moisture estimation accuracy; investigating the effects of vegetation, surface temperature, topography, and soil texture on soil moisture accuracy; and determining the regions that are useful for AMSR-E soil moisture measurements.

Detailed Instrument Description

The Electronically Steered Thinned Aperture Radiometer (ESTAR) is a synthetic aperture, microwave radiometer operating at a center frequency of 1.413 GHz (L-band) and a bandwidth of 20 MHz. The instrument flown during the SMEX02 mission is horizontally polarized.

Aperture synthesis is an interferometric technique in which the product (complex correlation) of the output voltage from pairs of antennas is measured at many different baselines. 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. ESTAR is a hybrid real-and-synthetic aperture radiometer that uses real antennas (stick antennas) to obtain resolution along-track and aperture synthesis (interferometric processing between pairs of sticks) to obtain resolution across-track (Le Vine et al., 1994). This hybrid configuration could be implemented on a spaceborne platform.

The effective swath created in the ESTAR image reconstruction (essentially an inverse Fourier transformation) is about 45° wide at the half power points. The field of view is restricted to 45° to avoid distortion of the beam but could be extended to wider angles if necessary. The image reconstruction algorithm in effect scans this beam across the field of view in 2° steps. The beam width of each step varies depending on look angle from 8° to 10°, therefore, the individual original data are not independent, since each data point overlaps its neighbors. That is, the image is over sampled in the across track dimension. Roughly, there are approximately nine independent beam positions. For this experiment the swath will be restricted to approximately 35. Another approach to using the data, especially in a mapping mode, is to interpret each of the original nonindependent observations as a sample point and then use a grid overlay to average the data. The final product of the ESTAR is a time referenced series of data consisting of the set of beam position brightness temperatures at 0.25 second intervals.

Calibration of the ESTAR is achieved by viewing two scenes of known brightness temperature. By plotting the measured response against the theoretical response, a linear regression is developed that corrects for gain and bias. Scenes used for calibration include black body, sky, and water. During aircraft missions, a black body is measured before and after the flight and a water target during the flight. The match in level and pattern is quite good and in general the ESTAR calibration has been accurate and reliable. For interpretation purposes it should be noted that the sensitivity of soil moisture to brightness temperature is 1 percent for 3° K.

Spatial Coverage

Southernmost Latitude: 41.7 N
Northernmost Latitude: 42.7 N
Westernmost Longitude: 93.8 W
Easternmost Longitude: 93.2 W

Temporal Coverage

06 June 2002 to 17 July 2002

References and Related Publications

Le Vine, D. M., A.J. Griffiths, C. T. Swift, and T. J. Jackson, 1994. ESTAR: A Synthetic Aperture Microwave Radiometer for Remote Sensing Applications. IEEE Proc. 82:1787-1801.

Please see the United States Department of Agriculture (USDA) SMEX02 Web site for more information, and the NSIDC SMEX02 Data Sets Web site to access data.


Many graduate students and volunteers worked to collect the field data. We would like to thank the Soil Moisture Experiment 2002 Science Team, the National Soil Tilth Laboratory for their assistance. We would also like to thank the National Aeronautics and Space Administration for their generous contributions to the study. This work was supported by the NASA Aqua AMSR, Terrestrial Hydrology and Global Water Cycle Programs.

Technical Contact

David LeVine
NASA Goddard Space Flight Center
Greenbelt, MD 20771