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The Campaign
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SGP99 Polarimetric Scanning Radiometer with C-band Scanhead (PSR/C) Brightness Temperature and Derived Soil Moisture Data (Document Version: 09/06/01)
OverviewThe NOAA Polarimetric Scanning Radiometer (PSR) is an airborne microwave imaging radiometer developed and operated by NOAA Environmental Technology Laboratory (Piepmeier and Gasiewski, 2001). The PSR C-band radiometer was flown for the first time as part of SGP99. The PSR/C band radiometer is similar to the low frequency channel of the Advanced Microwave Scanning Radiometers (AMSR, 6.92 GHz) on the NASA Aqua and Japanese ADEOS-II satellites. Soil moisture retrieval algorithms utilizing AMSR data have been proposed but not rigorously evaluated since there are few data sets available for this purpose. The Southern Great Plains 1999 Experiment (SGP99) was designed to provide data sets for both algorithm development and validation purposes. The objective for the PSR/C flights was to provide brightness temperature information for soil moisture retrieval algorithms. More information on the PSR/C portion of SGP99 can be found in Jackson et al. (2001). Instrument Description The PSR/C is a fully polarimetric C-band radiometer housed within a gimbal-mounted scanhead drum. However, only the horizontal and vertical polarization channels are provided here. The scanhead drum is rotatable by the gimbal positioner so that the radiometers can view any angle within ~70o elevation of nadir at any azimuthal angle, as well as external hot and ambient calibration targets. The configuration thus supports conical, cross-track, along-track, fixed-angle stare, and spotlight scan modes. Conical scanning at 55o incidence from nadir was used in SGP99. The PSR/C instrument was installed on the NASA P3-B aircraft. A network of precision clocks within the scanhead and controlling computers provides position/sample synchronization to better than one millisecond. An aircraft Inertial navigation unit (INU) provided roll and pitch information at ~10 msec intervals. These data were later used to correct the observed imagery for minor aircraft attitudinal variations. The PSR/C system provided simultaneous four-Stokes' vector measurements within four adjacent frequency bands at 5.80-6.20, 6.30-6.70, 6.75-7.10, and 7.15-7.50 GHz. Only the H and V polarization data are used in the current study. The 3-dB beamwidths are all 8o, and center frequencies are 6.0, 6.5, 6.925, and 7.325 GHz. The multiband capability of PSR/C allows the study of using frequency agile radiometry for observations over interference prone regions. The primary lens/feedhorn antenna is located adjacent to a co-boresighted video camera and longwave (10 um) IR sensor. Additional details on the PSR/C can be found at the following website http://www1.etl.noaa.gov/radiom/psr/. Flightlines The P3-B flew at a nominal altitude of ~ 8230 m. At this altitude, for the PSR/C the average footprint size was 2.3 km and the swath was 25.5 km. The nominal daily time window for aircraft coverage was 8:30-11:30 am CDT (13:30-16:30 UTC). The flight lines were flown in the order and directions as listed in the following table. This pattern was based upon interference problems first recognized during SGP97 (Jackson et al., 1999). The aircraft was available from July 8 to 20, 1999. Successful mapping flights with the PSR/C were accomplished on July 8, 9, 11, 14, 15, 19 and 20 (PSR/A). SGP99 P-3B Flightlines
PSR/C Data Processing Channel Selection: An initial review of the PSR/C data for the four frequencies available indicated that anthropogenic radio frequency interference (RFI) was present in all channels and both polarizations. RFI is manifested by higher than expected brightness temperatures (TB), sometimes exceeding the nominal geophysical brightness temperatures by hundreds of Kelvins. In most cases the RFI was spatially localized and often present in all channels simultaneously. In general, there was less RFI in the H channels than in the V channels. Based upon a review of the images, the 7.325 GHz frequency was judged to be the least contaminated by RFI and was selected for use in soil moisture analysis. Removal of Localized RFI: For each day and each polarization, the images were examined to identify areas with high TB values that could not be explained by geographic features. An area surrounding each RFI contaminated site was defined and data within this polygon were deleted from the data base. Image analysis also revealed that some individual scans had higher then expected TB values by up to several Kelvins. A scan consists of full 360 degree circle of pixels, including both front and back views. These anomalous circles were easily detected against the background TB of the region by virtue of their shape. Each flight line was reviewed to identify and locate these scans, which were then deleted from the data base. The anomalous scans were the result of strong RFI leakage observed during calibration views. Several RFI mitigation techniques, including a nonlinear filtering algorithm are currently being developed to address these problems. Temporal Normalization: Collecting high-resolution radiometric data over a large region takes considerable aircraft flight time. For SGP99 it took approximately 2.5 hours from the beginning to the end of mapping on a given morning. During this time interval it was suspected that both the surface emissivity and physical temperature increased from their early morning values. Other long-term drifts in instrument characteristics not accounted for in the calibration process could have also caused some brightness variation over the flight period. Since we wish to have the equivalent of an instantaneous snapshot of the region, it was necessary to normalize the observed imagery to a single time. This technique had been employed in previous aircraft missions (Jackson et al., 1995, Jackson et al., 1999, Le Vine et al., 2001). The final PSR/C data set provided and used to compute soil moisture consisted of each observation that was not removed by the RFI procedures, then normalized using the appropriate correction factor. RFI removal was performed separately for the H and V channels. Soil Moisture Retrieval The processed 7.325 GHz H polarization data were used for soil moisture algorithm calculations, as described in Jackson et al. (2001). The algorithm required different data layers, which combined together result in soil moisture images. The Data and FilesThree different data sets are provided; a version of the original PSR/C data, the processed PSR/C brightness temperature data, and soil moisture image files that include all data sets used in the soil moisture retrieval process (gridded brightness temperature, predicted soil moisture and associated parameter files). Original PSR/C Brightness Temperature Data Characteristics: For each day, all PSR/C data collected were compiled into a single file. Each set of observations was assigned a flightline number and geographic coordinates. PSR/C data files are in ASCII text format. Each file includes the horizontal and vertical brightness temperatures for all four C band channels measured. Full polarimetric data are not available at this time. Two types of georeferenced files are provided, the original in latitude and longitude and another containing just the 7.325 GHz channels and UTM georeferencing. There are six days of observations.File Format: The name of the original files with latitude-longitude georeferencing is sgp99orMDD.dat where MDD refers to the month and day of observation and sgp99utmMDD.dat with latitude and longitude converted to UTM coordinate system, Zone 14 S, Ellipsoid: Clarke 1866, datum: NAD27. The following table shows the record format and descriptions of the data. Each file contains approximately 300,000 data points. Structure of the original PSR/C brightness temperature files
Processed PSR/C Data Characteristics: Data for 7.325 GHz were judged to be best for further analyses. Several processing procedures, described in a later section, were performed to remove radio frequency interference (RFI) and to normalize data to a single time window. The processing resulted in different data sets depending upon whether only H or V polarization data were used or both.Files Format: For each day there will be three files described below and in the following table. For all files MDD refers to the month and day of observation. sgp99hpMDD.dat - 7.325 GHz data, horizontal polarization data after processing. sgp99vpMDD.dat - 7.325 GHz data, vertical polarization after data processing. sgp99vhpMDD.dat - 7.325 GHz data, vertical and horizontal polarization from the same sampling points after data processing . All data are ASCII text. Structure of the processed PSR/C brightness temperature files.
Soil Moisture Estimates Characteristics: Brightness temperature observations (7.325 GHz H) were processed using the algorithm described in Jackson et al. (1999) to estimate surface soil moisture. The H polarization data were used to produce a georeferenced gridded product for each day. This grid is exactly the same on each day. It has a pixel resolution of 800 m. The images are all 8 bit binary consisting of 176 pixels wide by 371 lines. The georeferencing information is:
Files Format: All data are 8 bit binary images consisting of 176 pixels wide by 371 lines with no headers. The directory path to the Soil Moisture product data ftp site is /ftphttp://disc.sci.gsfc.nasa.gov/data/sgp99/air_remote_sensing/PSR/sgpprod All files are contained in four subdirectories of the path and are described in the table below. For brightness temperature, soil moisture, and soil temperature MDD refers to the month and day of the observations.
Data Access and Contacts
Technical Inquiries about this Data should be addressed to,
For Information about SGP99 data at the Goddard DAAC, contact Hydrology Data Support Team Jackson, T. J., Le Vine, D. M., Swift, C. T., Schmugge, T.J. and Schiebe, F. R. (1995): Large area mapping of soil moisture using the ESTAR passive microwave radiometer in Washita'92, Remote Sensing of Environment, 53: 27-37. Jackson, T. J., Le Vine, D. M., Hsu, A. Y., Oldak, A., Starks, P. J., Swift, C. T., Isham, J., and Haken, M. (1999): Soil moisture mapping at regional scales using microwave radiometry: the Southern Great Plains hydrology experiment, IEEE Trans. on Geoscience and Remote Sensing. 37:2136-2151. Jackson, T. J., Gasiewski, A., Oldak, A., Klein, M., Njoku, E. G., Yevgrafov, A., Christiani, S., Bindlish, R., (2001) Soil Moisture Retrieval Using the C-Band Polarimetric Scanning Radiometer During the Southern Great Plains 1999 Experiment, submitted to IEEE Trans. on Geoscience and Remote Sensing. Le Vine, D. M., Jackson, T. J., Swift, C. T., Haken, M. and Bidwell, S: (2001) ESTAR measurements during the Southern Great Plains experiment (SGP99), IEEE Trans. on Geoscience and Remote Sensing. 39:1680-1685. Piepmeier J. R. and Gasiewski, A. J. (2001): High-resolution passive microwave polarimetric mapping of ocean surface wind vector fields, IEEE Trans. on Geoscience and Remote Sensing, 39:606-622. Last updated: February 28, 2008 12:36:11 GMT Page Author: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov Web Curator: -- Website Curator: Stephen W Berrick NASA official: Steve Kempler, GES DISC Manager -- Steven.J.Kempler@nasa.gov | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Soil Moisture
PSR/C Data (ASCII) Online