Current and near future satellite observing systems, while not ideal for soil moisture measurement, can provide information for some conditions. Among these is the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). Evaluating the potential of the TMI in soil moisture estimation is a logical extension of research to date. How to effectively interpret and utilize satellite microwave data that are available now or will be in the near future was a primary goal of the Southern Great Plains 1999 experiment (SGP99). The experiment involved collecting ground observations of soil moisture and related variables in conjunction with ground, aircraft and satellite microwave sensor measurements. It is possible that the TMI may improve our ability to estimate soil moisture and provide us with a bridge between SSM/I and AMSR. The lowest frequency of the TMI is about half that of the SSM/I. Another interesting feature of the TMI is its significantly higher spatial resolution (at 19 GHz the TMI has a spatial resolution four times as good as the SSM/I). TMI coverage is limited to latitudes between -38o and 38o. Fortunately, the SGP region falls in its domain and at the latitude of the SGP region it is possible to obtain coverage at least once every day. TMI and derived soil moisture data sets for SGP99 are described and provided. Additional information on the SGP99 TMI data set can be found in Jackson and Hsu (2001).
TRMM TMI Brightness Temperature
The TRMM Satellite was launched on November 27, 1997 into a near circular, non-sun-synchronous orbit of approximately 350 km altitude with an inclination of 35 degrees and a period of 91.5 minutes (15.7 orbits per day). This orbit provides extensive coverage in the tropics and allows each location to be covered at a different local time each day. This kind of sampling will enable the analysis of the diurnal cycle of precipitation, however, it may not be optimal for soil moisture retrieval.
The TMI antenna is an offset parabola which views the earth surface with a "nadir" angle of 49 degrees resulting in an incident angle of 52.8 degrees at the earth's surface. The antenna rotates about a nadir axis and draws a "circle" on the earth's surface. Only 130 degrees of the forward sector of the complete circle is used for taking data. From the TRMM orbit, this 130 degrees scanned sector produces a swath width of 758.5 km. Because the TMI is rotating while its receiver is integrating, the effective area sweeps by the antenna beam during the integration time is called the effective field of view (EFOV). The standard TMI brightness temperature data will be provided at single-sample EFOV level. More information about the TRMM sensor package is presented in the following table and in Kummerow et al. (1998).
| Channels Number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| Center Freq. (GHz) | 10.65 | 10.65 | 19.35 | 19.35 | 21.3 | 37.0 | 37.0 | 85.5 | 85.5 |
| Polarization | V | H | V | H | V | V | H | V | H |
| No. EFOVs per scan | 104 | 104 | 104 | 104 | 104 | 104 | 104 | 208 | 208 |
| No. Samples / beam width | 4 | 4 | 2 | 2 | 2 | 1 | 1 | 1 | 1 |
| Beam EFOV (km x km) | 63 x 37 | 63 x 37 | 30 x 18 | 30 x 18 | 23 x 18 | 16 x 9 | 16 x 9 | 7 x 5 | 7 x 5 |
TRMM has its own unique Science Data and Information System (TSDIS) at the Goddard Space Flight Center to process the information from the satellite. All TRMM science data products are archived and distributed to the public by the Goddard Distributed Active and Archive Center (DAAC). Most of the data were archived in HDF format. To obtain TRMM science data, one needs to go to the Goddard DAAC homepage.
Unlike airborne microwave radiometer, the footprint of TMI data at 10.7 GHz is about 63 km x 37 km (Beam EFOV). In order to generate higher resolution soil moisture images from TMI compatible to those derived from ESTAR for SGP97 and PSR/C for SGP99, the soil moisture retrieval was accomplished at footprint scale and output was gridded to create 800 m x 800 m pixels. The area for soil moisture mapping was restricted to the region covered by the aircraft mapping component of SGP99, which was similar to that mapped in SGP97.
The soil moisture retrieval algorithm used for analyzing TMI data is described in Jackson (1993) and only the 10 GHz H channel was used. For each TMI 10 GHz footprint within the SGP99 area, an ancillary data set was compiled that includes effective soil temperature, proportion of land cover types in each footprint (15 classes in total), average NDVI values for individual land cover types, and soil texture data. The effective soil temperature was computed using air temperature measured at 1.5 m and soil temperature measured at 10 cm under sod from the Oklahoma Mesonet based on the method described in Choudhury et al. (1982). Thematic Mapper (TM) data taken on July 15, 1999 were used to derive land cover types for the study area. Information about vegetation attenuation effects on microwave signal required by the algorithm was estimated from a NDVI image based on the same TM data set. The soil moisture retrieval algorithm actually retrieves the dielectric constant of the soil. A dielectric mixing model is used to convert dielectric constant to volumetric soil moisture. The dielectric mixing model used in this study was developed by Hallikainen et al. (1985). It requires soil texture as input. Soil texture data were obtained from a multi-layer soil characteristics data set for the conterminous United States (eoswww.essc.psu.edu/dbndx/tree/amer_n/us_sc/sgpr.html).
| Orbit | Date | Time (GMT) | |||
| Month | Day | Year | Hour | Minute | |
| 9270 | 7 | 8 | 99 | 17 | 49 |
| 9271 | 7 | 8 | 99 | 19 | 25 |
| 9285 | 7 | 9 | 99 | 16 | 36 |
| 9286 | 7 | 9 | 99 | 18 | 13 |
| 9301 | 7 | 10 | 99 | 17 | 0 |
| 9302 | 7 | 10 | 99 | 18 | 37 |
| 9316 | 7 | 11 | 99 | 15 | 48 |
| 9317 | 7 | 11 | 99 | 17 | 24 |
| 9332 | 7 | 12 | 99 | 16 | 12 |
| 9333 | 7 | 12 | 99 | 17 | 48 |
| 9347 | 7 | 13 | 99 | 14 | 59 |
| 9348 | 7 | 13 | 99 | 16 | 36 |
| 9363 | 7 | 14 | 99 | 15 | 23 |
| 9378 | 7 | 15 | 99 | 14 | 11 |
| 9379 | 7 | 15 | 99 | 15 | 47 |
| 9394 | 7 | 16 | 99 | 14 | 35 |
| 9409 | 7 | 17 | 99 | 13 | 23 |
| 9410 | 7 | 17 | 99 | 14 | 59 |
| 9424 | 7 | 18 | 99 | 12 | 10 |
| 9425 | 7 | 18 | 99 | 13 | 47 |
| 9440 | 7 | 19 | 99 | 12 | 34 |
| 9441 | 7 | 19 | 99 | 14 | 11 |
| 9455 | 7 | 20 | 99 | 11 | 22 |
| 9456 | 7 | 20 | 99 | 12 | 58 |
| 9471 | 7 | 21 | 99 | 11 | 46 |
File Formats: Each data take is in a separate file named 1B11.yymmdd.tttt.xxxx.5.txt where yy=year, mm=month, dd=day, tttt=start time of swath (hour and minute of local standard time), and xxxx=orbit number. (The 5 represents the version of the original TRMM data used). The files consist of the footprint observations of brightness temperature (TB) within the study region. Each data file is an ASCII 11-column table. The file format is described in the following table.
| Column No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| File Name | |||||||||||
| 1B11.yymmdd.tttt.xxxx.5.txt | Latitude (o N) | Longitude (o W) | UTM northing | UTM easting | 10 GHz V pol | 10 GHz H pol | 19 GHz V pol | 19 GHz H pol | 21 GHz V pol | 37 GHz V pol | 37 GHz H pol |
Characteristics: Soil moisture products were generated for 800 m by 800 m grid cells compatible to the ESTAR products for SGP97 and PSR/C for SGP99. Soil moisture retrieval was performed on a footprint basis and output as a grid. The area for soil moisture mapping was restricted to the region covered by the aircraft mapping component of SGP99, which was very similar to that mapped in SGP97. 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 175 pixels wide by 349 lines. The georeferencing information is presented in the following table.
| Projection | Universal Transverse Mercator Zone 14S | |
| Earth Ellipsoid | Clarke 1866 (NAD 27) | |
| Upper Left Corner | 543600 m | 4096200 m |
| Upper Right Corner | 683600 m | 4096200 m |
| Image Center | 613600 m | 3956600 m |
| Lower Left Corner | 543600 m | 3817000 m |
| Lower Right Corner | 683600 m | 3817000 m |
| Pixel Size | 800 m E | 800 m N |
| Upper Left Corner | 98o30'35.68" | 37o00'46.04" |
| Upper Right Corner | 96o56'12.52" | 36o59'45.11" |
| Image Center | 97o44'36.97" | 35o44'55.18" |
| Lower Left Corner | 98o31'30.40" | 34o29'43.65" |
| Lower Right Corner | 97o00'02.66" | 34o28'48.08" |
| Data Type | Byte | |
| File Type | Binary | |
| Dimensions | 175 columns | 349 rows |
| Units | ||
File Formats: The derived soil moisture files are named in the similar fashion to the brightness temperature data files, bhlsm.yymmdd.xxxx.raw. However, they are structured differently. Each derived soil moisture data file is an 8 bit binary image consisting of 175 pixels and 349 lines with no headers.
TRMM TMI Brightness Temperatures
Derived Soil Moisture
Or directly via FTP at
This TRMM subset was created for SGP99 users by
For information about or assistance in using DAAC data, contact
Choudhury, B. J., T. J. Schmugge, and T. Mo, A Parameterization of Effective Temperature for Microwave Emission. J.Geophys. Res., 87, 1301-1304, 1982.
Hallikainen, M. T., F. T. Ulaby, M. C. Dobson, M. A. El-Rayes, and L. K. Wu, Microwave Dielectric Behavior of Wet Soil – Part I: Empirical Models and Experimental Observations. IEEE Trans. on Geoscience and Remote Sensing, 23 (1), 25-34, 1985.
Jackson, T. J., Measuring Large Scale Surface Soil Moisture Using Passive Microwave Remote Sensing, Hydeological Processes, 7, 139-152, 1993.
Jackson, T. J. and A. Y. Hsu, Soil Moisture and TRMM Microwave Imager Relationships in the Southern Great Plains 1999 (SGP99) Experiment, IEEE Trans. on Geoscience and Remote Sensing, 39 (8)1632-1642, 2001.
Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and J. Simpson, The Tropical Rainfall Measuring Mission (TRMM) Sensor Package Journal of Atmospheric and Oceanic Technology, 15 (3), 809-817, 1998.
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