Soil Moisture

Soil Properties

Soil Temperature

Vegetation and Land Cover

Aircraft Remote Sensing

Satellite Remote Sensing

Overview The Science The Data and Files Data Access and Contacts FTP Site Contacts References

Overview

Microwave radiometry at long wavelengths can be used to measure and monitor surface soil moisture. A key issue in implementing this approach has been the inherent spatial resolution problem of long wavelength microwave radiometry at spacecraft altitudes. Synthetic aperture techniques can solve this problem. As part of SGP99, the Electronically Scanned Thinned Array Radiometer (ESTAR) was flown to study soil moisture. Data were collected using this L-band passive microwave mapping instrument over a 10,000 km² region for about two weeks (July 8-20). The major objective of this investigation was to support the evaluation of C-band instruments and related retrieval algorithm development for remote sensing of soil moisture from space. A secondary objective was to extend the unique SGP97 data set to support collaborative efforts to understand surface and boundary layer interactions and the physical processes controlling the spatial and temporal variability at these scales. The ESTAR instrument performed well and provided a high-quality data set for analysis, although a hardware problem caused the loss of one day's data (July 11) and problems with an aging data system (since replaced) caused some loss of data. Meteorological conditions were good. A significant and spatially variable rainfall event was followed by a dry-down sequence. Calibration of ESTAR was verified using ground observations and results of previous campaigns in this region. An established soil moisture algorithm was implemented using ancillary data bases. This algorithm was validated using ground observations at several scales. Error levels were nominally 3%, which was similar to previous investigations (Jackson et al., 1999). Results clearly demonstrated the performance of both the ESTAR instrument and the soil moisture algorithm. Additional information on the ESTAR SGP99 campaign results can be found in Le Vine et al. (2001)

The Science

The electronically scanned thinned array radiometer (ESTAR) is a synthetic aperture, microwave radiometer operating at a center frequency of 1.413 GHz (21 cm) with a bandwidth of 20 MHz. It provides horizontally polarized data. This instrument is the most efficient microwave mapping device currently available.

Aperture synthesis is an interferometric technique in which the product (complex correlation) of the output voltage from pairs of antennas is measured at several different antenna spacings (baselines). Each baseline produces a sample point in the Fourier transform of the scene, and an image of the scene is obtained by applying the inverse transform to the measurements. ESTAR is a hybrid real and synthetic aperture radiometer which obtains real aperture resolution along track and synthetic aperture resolution across track using a linear array of stick antennas (Le Vine et al., 1994). Each set of correlation measurements produces a one-dimensional image strip extending perpendicular to the direction of motion. A two-dimensional image is formed by accumulating strips as the aircraft moves forward. This hybrid configuration could be implemented on a spaceborne platform.

The effective swath created in the ESTAR image reconstruction (essentially the inverse Fourier transformation) is limited only by the changes in the effective beam with incidence angle that can be tolerated. The field of view in processing this data has been restricted to ± 45^o to avoid any distortion of the synthesized beam with incidence angle but could be extended to wider angles if necessary. The image reconstruction algorithm in effect scans the beam across the field of view in 2^o steps. The beam width at each step is about 8-10^o, increasing with look angle. The interval between scans (0.25 seconds) is determined by the integration time fixed by the instrument hardware, corresponding to a distance of about 40 m. The along-track beam width is about 16^o, corresponding to a footprint size of about 2 km. Therefore, the data in each scan are not independent. For the final data product, a grid overlay was used to average the data.

Calibration of ESTAR is achieved by viewing two scenes with known brightness temperature. Gain and bias are determined by a linear least-squares fit of the measured response to the theoretical response. Scenes used for calibration include blackbody, sky, and water. During aircraft missions, a black body is measured before and after the flight and a water target during the flight. Water temperature is measured in situ where possible, and is measure using a thermal infrared sensor when available. For SGP99, calibration was obtained using blackbody measurements made in an anechoic chamber prior to departure and water measurements (open ocean over a research ship) made following the mission.

The ESTAR instrument was flown on a P-3B aircraft operated by the NASA Wallops Flight Facility. ESTAR was installed in the bomb bay portion of the aircraft during this mission. Flights were conducted at an altitude of 7.5 km. It should be noted that radiometer calibration is sensitive to its operating temperature. At a particular aircraft altitude this is stable and all SGP99 flights were conducted at a single altitude to aid in getting consistent performance.

As in the case of SGP97 (Jackson et al. 1999) the planned four parallel lines were modified to compensate for strong RFI in the vicinity of Oklahoma City. This was a critical problem because the area affected included the El Reno study area. The flight plan was modified to include two east-west lines in this area (these are flown as a deviation in the last of the four long parallel lines). See Le Vine et al. (2001) for more details on the flightlines. This reconfiguration eliminated the strong RFI.

An attempt was made to conduct the flights exactly the same way on a daily basis. For the most part this was accomplished; however, instrument, weather, and logistical constraints resulted in some deviations. Also, as mentioned above, some data were lost due to problems with the data system.

During the SGP99 field campaign, a preliminary calibration was used for ESTAR. Data were processed into an image product within twelve hours of collection. This product provided valuable information for mission planning and quality control. The first step in quality control was the review of spatial and temporal features in these images.

Post processing of the ESTAR data consisted of refining the calibration, RFI removal, georegistration, and a correction for incidence angle (all data is given as brightness temperature at nadir). It was decided not to use the daily water and blackbody calibrations because of inconsistencies (likely due to contamination by the shore and changes in instrument operating temperature associated with the change in altitude needed to fill the beam as much as possible with water). Calibration scenes chosen were a pre-mission laboratory blackbody measurement (GSFC anechoic chamber) and a post-mission open-ocean water measurement flown with salinity and sea-surface temperature ground truth supplied by shipboard measurements.

An ESTAR data record, corresponding to one complete cross-track scan, is obtained every 0.25 seconds, except for gaps of a few seconds at five-minute intervals set aside for internal calibration. A calibrated ESTAR data record consists of the time and TB values for each beam position at that time. Global Positioning System (GPS) data and aircraft pitch, roll and yaw data collected during flight are used to georegister each beam position.

Criteria were established for identifying data records that were contaminated with RFI. These records were dropped from the data set. The strong RFI encountered around Oklahoma City resulted in a section being removed from the north-south flightlines in this region. Coverage of this area was provided by east-west lines added to the flight profile in the vicinity of the El Reno site. RFI outside of this region caused less significant loss of data.

All data were normalized to nadir using the method described in Jackson et al. (1995), and Le Vine et al (1994). These data were resampled to a Lat/Lon grid at a pixel resolution of approximately 500 m. Small areas that had no observations were filled by interpolation.

The Data and Files

The tabulated brightness temperature data provided here are from a mapping of the calibrated and incidence angle normalized ESTAR brightness temperature onto a georeferenced grid. Each pixel corresponds to a box 0.005 degrees Latitude by 0.005 degrees Longitude (approximately 555 m X 450 m). The pixel value is the unweighted average of all brightness temperatures falling within the box. The tabulated Lat/Lon are the coordinates of the center of each box. The image size is 301 pixels by 801 lines. The data type is ASCII text.

(The limits given below refer to pixel centers. Therefore, the image edges extend (pixel size)/2 = 0.0025 degrees beyond these values; e.g. upper left corner pixel covers -98.5025 to -98.4975 Longitude, 37.9975 to 38.0025 Latitude)

Data for the following dates were judged to be of acceptable quality; July 8, 9, 14, 15, 19 and 20, 1999. There is one file per day. These are named as sgpmdd99.txt where m is the month and dd is the day. Each file consists of all pixels (that had a brightness temperature value assigned) as individual text records containing the latitude (degrees), longitude, and brightness temperature (degrees Kelvin).

Data Access and Contacts

References

Jackson, T. J., Le Vine, D. M., Swift, C. T., Schmugge, T. J., and Schiebe, F. R. Large area mapping of soil moisture using the ESTAR passive microwave radiometer in Washita’92. Remote Sensing of Environment. 53:27-37. 1995.

Jackson, T. J., Le Vine, D. M., Hsu, A. Y., Oldak, A., Starks, P. J., Swift, C. T., Isham, J. D., and Haken, M. Soil moisture mapping at regional scales using microwave radiometry: the Southern Great Plains hydrology experiment. IEEE Trans. on Geoscience and Remote Sensing, 37(5): 2136-2151. 1999.

Le Vine, D. M., Griffis, A., Swift, C. T., and Jackson, T. J. ESTAR: a synthetic microwave radiometer for remote sensing applications. Proceedings of the IEEE. 82:1787-1801. 1994.

Le Vine, D.M., Jackson, T. J., Swift, C. T., Haken, M., and Bidwell, S. ESTAR measurements during the Southern Great Plains experiments. IEEE Trans. on Geoscience and Remote Sensing, 39:1680-1685. 2001.