SGP97 Scanning Low Frequency Microwave Radiometer (SLFMR)

Overview The Data The Files Formats The Science Basic Instrument Description Processing Steps Beam Position Correction SLFMR and ESTAR Bias Correction Data Access and Contacts FTP Site Points of Contact

Overview

The results of studies with L band radiometers on truck (Wang et al. 1982 and 1983), aircraft (Schmugge et al. 1992, Jackson et al. 1995 and 1999), and even spacecraft platforms (Eagleman and Lin, 1976) have shown that a strong relationship exists between the microwave brightness temperature and the surface soil moisture. A recurring question is how does microwave brightness temperature scale from relatively small ground and aircraft footprints (1 - 1000 m) that we are able to control and verify to the coarse resolution expected from satellite missions (10 - 50 km)?

During SGP97, an aircraft mission was conducted to address some aspects of scaling. A push broom L band microwave radiometer (SLFMR) was flown at several altitudes over test sites to collect data with different spatial resolutions. Multiple lines were used at the lower altitudes to provide coverage of the same area flown at higher altitudes. Data collected required several processing steps. These included beam position angle, day to day variations, and cross calibration with the L band Electronically Scanned Thinned Array Radiometer (ESTAR) which was also used in SGP97.

The Data

The SLFMR provides L band brightness temperature data. Six days of coverage were obtained with the SLFMR between June 29 and July 4, 1997. The following table summarizes the coverage.

Flightlines were distributed over the SGP97 region and were intended for different purposes. These are described in the following table.

For the ER area, Lines 1 through 4, Lines 5 and 6, and Line 7 provide a sequence of increasing altitude and decreasing resolution coverage of the same area on the ground. Line 7 was not flown on June 29^th, instead two higher altitude lines (16 and 17) were used. Three dates of coverage were obtained for ER.

LW was mapped on two dates using Lines 10 through 12. These were all high altitude lines and do not provide any multiple resolution coverage. This data set was intended to provide observations of conditions typical of LW.

CF was flown on one day using two altitudes. Lines 13 and 14 were low altitude and Line 15 was high altitude.

Line 8 was a 100 km transect between the ER and LW areas that was flown every day except July 4^th. This line matched a portion of one of the ESTAR flightlines. Line 9 covered Lake Ellsworth and was flown every day. Lines 18 and 19 were collected to match flightlines covered by a surface flux measurement aircraft and did not include ground truth observations.

The Files

Formats

Two types of data files are provided, the original data files and the fully processed. Each contains brightness temperature and georeferencing information. Files are all ASCII text.

Original Data File Format

Original data are the files provided by Quadrant Engineering. Each file is one coverage of a flightline. Each file consists of scan lines. For each scan line there is a sequence of six beam position observations. These are sequenced left to right along track. A record contains the following variables.

Latitude Latitude N in degrees of the beam position center Longitude Longitude W in degrees of the beam position center Salt Salinity (not predicted) TS Surface temperature (not measured) TBorig Brightness temperature in degrees Kelvin of the beam position B Beam number (2 through 7) GPS,S Original GPS variable GPS,W Original GPS variable GPS,O Original GPS variable GPS,A Original GPS variable C-TB Raw TB IR1 Infrared Channel 1 (not working) IR2 Infrared Channel 2 (not working) Tant Temperature of L band antenna in degrees C Tbtl Temperature of L band component degrees C Tnss Temperature of L band component in degrees C Tdkl Temperature of L band component degrees C Time Time in hours UTC

Processed Data File Format

As noted above, the original data may not be fully calibrated and contains artifacts that make it difficult to use. Therefore, these data were processed to normalize for beam position angle (essentially making all beam positions nadir viewing) and were cross calibrated with the ESTAR data. Details on these techniques can be found in Jackson (2000). During this processing UTM geolocation coordinates were added to the file. These processed data files contain the following data.

B Beam number (2 through 7) Latitude Latitude N in degrees of the beam position center Longitude Longitude W in degrees of the beam position center Easting UTM location in m Northing UTM location in m TBorig Original brightness temperature in degrees Kelvin of the beam position Time Time in hours UTC TBcorr Corrected brightness temperature in degrees Kelvin of the beam position

Note that the brightness temperature data collected over the water (Line 9) were not corrected. The processed files are the same as described above except that they do not include Tbcorr.

Name and Directory Information

Data are provided in two subdirectories, orig for the original data and proc for the processed. Within each of these directories all data for a particular day are in a subdirectory (June29, June30, July1, July2, July3 and July4). The file naming used for the original files is that provided by Quadrant Engineering. For clarity the date and flightline number are provided in the following table. The processed data files are named as follows

Lnnmmdd.txt

where

nn = flightline number
mm = month
dd = day

Original data file name Processed data file name Date Line m29j1153.dat L010629.txt 629 1 m29j1161.dat L020629.txt 629 2 m29j1170.dat L030629.txt 629 3 m29j1178.dat L040629.txt 629 4 m29j1189.dat L050629.txt 629 5 m29j1198.dat L060629.txt 629 6 m29j1234.dat L160629.txt 629 16 m29j1242.dat L170629.txt 629 17 m29j1257.dat L080629.txt 629 8 m29j1292.dat L090629.txt 629 9 m30j1010.dat L080630.txt 630 8 m30j1049.dat L100630.txt 630 10 m30j1059.dat L110630.txt 630 11 m30j1069.dat L120630.txt 630 12 m30j1083.dat L090630.txt 630 9 m30j1130.dat L190630.txt 630 18 m01j0986.dat L130701.txt 701 13 m01j0996.dat L140701.txt 701 14 m01j1024.dat L150701.txt 701 15 m01j1044.dat L180701.txt 701 18 m01j1059.dat L080701.txt 701 8 m01j1088.dat L090701.txt 701 9 m02j0960.dat L010702.txt 702 1 m02j0966.dat L020702.txt 702 2 m02j0974.dat L030702.txt 702 3 m02j0984.dat L040702.txt 702 4 m02j0999.dat L050702.txt 702 5 m02j1008.dat L060702.txt 702 6 m02j1021.dat L070702.txt 702 7 m02j1031.dat L080702.txt 702 8 m02j1061.dat L090702.txt 702 9 m03j1011.dat L010703.txt 703 1 m03j1018.dat L020703.txt 703 2 m03j1026.dat L030703.txt 703 3 m03j1033.dat L040703.txt 703 4 m03j1047.dat L050703.txt 703 5 m03j1054.dat L060703.txt 703 6 m03j1069.dat L070703.txt 703 7 m03j1092.dat L190703.txt 703 19 m03j1118.dat L080703.txt 703 8 m03j1150.dat L090703.txt 703 9 m04j1152.dat L110704.txt 704 11 m04j1162.dat L120704.txt 704 12 m04j1172.dat L100704.txt 704 10 m04j1182.dat L090704.txt 704 9

The Science

Basic Instrument Description

The scanning low frequency microwave radiometer (SLFMR) was designed and built for NOAA to measure ocean surface salinity from a small-engine aircraft by Quadrant Engineering, Inc.(Miller et al. 1998) This is a 1.4 GHz L Band microwave radiometer operating with V polarization with its own GPS receiver. In SGP97 it was flown on a Piper Navajo Chieftain aircraft operated by the Provincial Remote Sensing Office, Canada.

The SLFMR has an electronically steered antenna beam and is capable of viewing any of six footprints across the flight track (+/- 7, 21, and 37 degrees from nadir). Footprint size is nominally 0.3 of the altitude (3 dB is 15 degrees FOV). The total swath covered is approximately 2 times the altitude. Since this instrument was designed for salinity mapping the sensitivity and thermal resolution are high.

Components of the system were housed in a thermally controlled and aerodynamically shaped enclosure measuring 0.2 m high by 1 m wide by 1.5 m long and weighing 52 kg (115 lbs). SLFMR was flown in conjunction with the CASI instrument.

Steps In Processing SLFMR Data

The SLFMR data were preprocessed by Quadrant Engineering. They performed a two-point calibration using clear sky and microwave absorber for the two targets. The product of this step was a set of individual flightlines with geo-referencing for each individual beam position T_B.

Beam Position Correction: In order to effectively use push broom radiometer data for mapping and analysis, it is required that the effects of varying beam position angles be accounted for though a normalization procedure. It is well known that over a homogeneous bare soil target that viewing angle affects the T_B (Ulaby et al. 1986). This angular variation can be described by the Fresnel equations. In previous studies using similar instrument designs (Schmugge et al. 1992 and Jackson et al. 1995 and 1999) it has been shown that this correction can be developed on a flightline basis over mixed land covers. This procedure normalizes the data to an equivalent nadir T_B.

The first step in the processing is to assume that the deviation between beam positions is due to the Fresnel effect and calibration errors for individual beam positions. Next, it is specified that for a given day and flightline that the Fresnel effect for a BP is constant for the range of soil moisture and vegetation present.

There are some circumstances when using a limited data set for this correction, say a single flightline, can lead to errors. This can occur when there is an anomaly in a particular BP that is not present in the others (such as a small water body). The longer a line is and the more homogeneous the area is, the more reliable this BP correction approach will be. It was assumed in this study that by using a daily average for all lines in an area (LW, ER, CF, and Line 8) that potential errors due to anomalies would be minimized.

For each scan line, the average of the two near nadir BPs was assumed to be the nadir T_B. The average value for the area on a day was computed for this equivalent nadir value as well as for each BP. A correction factor was computed for each BP by subtracting the nadir average from each BP average. All data from scan lines in an area on each day were then corrected using these values. During this step, the geolocation information was modified to include both latitude-longitude and UTM coordinates.

SLFMR and ESTAR Bias Correction: Next, the BP corrected Line 8 data were analyzed to compare the SLFMR and ESTAR calibrations. Even though the SLFMR operates at V polarization and the ESTAR operates at H polarization, since both data sets were normalized to nadir through the BP corrections, a direct comparison of the results of the two instruments was possible. Since the ESTAR has been well calibrated as part of SGP97 and previous studies using both observations and theory (Jackson et al. 1995 and 1999) it provides a check on the SLFMR calibration.

Line 8 was approximately 100 km long and was flown on all dates except July 4^th. Data from both instruments were processed to produce identically gridded data sets at a resolution of 0.8 km. Data for matching ESTAR and SLFMR pixels were extracted for the entire line on each date. From this analysis it was very apparent that there was a significant bias in the SLFMR observations, which were ~20 K higher than the ESTAR data. Although there were some differences in the time of coverage, any changes in target temperature would be much smaller than this level. The observed differences were attributed to the SLFMR calibration, and based upon the data distribution, a single bias correction factor was used all data collected. Comparisons of data collected over the water site and model predictions also showed the same bias. This bias correction was applied to all of the data.

Additional details on the data processing can be found in Jackson (2000).

Data Access and Contacts

FTP Site The SLFMR data is in the following GES DISC ftp site:

The SLFMR data ftp site

Points of Contact

The Principal Investigator for the SLFMR instrument is

Tom Jackson
USDA ARS Hydrology Lab
Beltsville, MD 20705 USA
Bldg. 007, Rm. 104, BARC-West

E-mail: tjackson@hydrolab.arsusda.gov
Voice: 301-504­8511
Fax: 301-504­8931

For more information regarding GES DISC data, contact:

Hydrology Data Support Team
Goddard Earth Sciences
Data and Information Sevices Center (GES DISC)
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771

E-mail: hydrology-disc@listserv.gsfc.nasa.gov
Voice: 301-614-5165
Fax: 301-614-5268

References

Eagleman, J. R. and W. C. Lin (1976) Remote sensing of soil moisture by a 21-cm passive radiometer. J. of Geophysical Research, 81:3660-3666.

Jackson T. J. (2000) Multiple resolution analysis of L band brightness temperature for soil moisture. Submitted to IEEE Trans. on Geoscience and Remote Sensing.

Jackson, T. J., D. M. Le Vine, C. T. Swift, and T. J. Schmugge (1995) Large scale mapping of soil moisture using the ESTAR passive microwave radiometer. Remote Sensing of Environment, 53:27-37.

Jackson, T. J., D. M. Le Vine, A. Y. Hsu, A. Oldak, P. J. Starks, C. T. Swift, J. Isham and M. Haken (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.

Miller, J. L., M. A. Goodberlet, and J. B. Zaitzeff (1998) Airborne salinity mapper makes debut in coastal zone. EOS Trans. American Geophysical Union, 79 (14):175-177.

Schmugge, T. J., T. J. Jackson, W. P. Kustas, and J. R. Wang (1992) Passive microwave remote sensing of soil moisture: results from HAPEX, FIFE and MONSOON 90. ISPRS J. of Photogrammetry and Remote Sensing, 47:127-143.

Ulaby, F. T., R. K. Moore, and A. K. Fung (1986) Microwave remote sensing: active and passive, Vol. III, from theory to application. Artech House, Dedham, MA.

Wang, J. R., J. McMurtrey, E. T. Engman, T. J. Jackson, and T. J. Schmugge (1982) Radiometric measurements over bare and vegetated fields at 1.4 GHz and 5 GHz frequencies. Remote Sensing of Environment, 12:295-311.

Wang, J. R., P. E. O'Neill, T. J. Jackson, and E. T. Engman (1983) Multifrequency measurement of thermal microwave emission from soils: the effects of soil texture and surface roughness. IEEE Trans. on Geoscience and Remote Sensing, GE-21:44-55.

Last update:Mon Mar 15 10:06:09 EST 1999 Page Author: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov Web Curator: -- Stephen W Berrick NASA official: Steven Kempler, DAAC Manager -- Steven.J.Kempler@nasa.gov