SGP97 SSM/I Brightness Temperature Footprint Data
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Overview
Although the Special Sensor Microwave/ Imager (SSM/I) was not designed for soil moisture sensing, it is possible based on theory to extract soil moisture information under some conditions. The limiting feature of the SSM/I for soil moisture related studies is that the frequencies are quite high and are significantly affected by vegetation. However, other features of the data such as the frequency of measurements are very good for observing time varying hydrologic variables such as soil moisture. There have been several studies in this region that have attempted to utilize SSM/I in soil moisture and related studies (Heymsfield and Fulton, 1992, Teng et al., 1993, and Jackson, 1997).
This data set was modified in May, 2001 to include data collected by the F11 satellite. Data from this satellite was not available when the original processing was performed. Only the *.lo.txt files were produced for F11.
The Data
BackgroundThe Special Sensor Microwave/Imager (SSM/I) instrument package has been available on at least one satellite since 1987. It is a conical scanning total power microwave radiometer system operating at a look angle of 53o. Table 1 summarizes the key features of the instruments. The nominal swath width is 1400 km. Data are collected at 128 locations across track on every scan at 85 GHz. Only 64 observations are made across track on every other scan at the other frequencies. Additional information is provided in Hollinger et al. (1990).
Characteristics
Source
Table 1. SSM/I Characteristics Frequency (GHz) Polarization Spatial Resolution (km) 19.4 H and V 69 x 43 22.2 V 60 x 40 37.0 H and V 37 x 28 85.5 H and V 15 x 13 Table 2 lists the characteristics of the various platforms that were in operation during SGP97. For a given satellite, coverage is possible twice a day approximately 12 hours apart on the ascending and descending passes.
Table 2. SSM/I Satellites Spacecraft Launch Date Supported Through Ascending Equatorial Crossing Time (UTC) F10 Dec. 1990 Nov. 1997 22:09 F11 Nov. 1991 Aug. 2000 19:25 F13 March 1995 present 17:43 F14 May 1997 present 20:39 The orbital period is about 102 minutes, the SSM/I's orbit the earth about 14.1 times per day. The minimum data unit, an orbit, consists of two passes. These are pole-to-pole swaths, one ascending (south to north) and one descending (north to south). The first pass of a UTC day is defined as the first complete pass of the day.
Data are generally available via the NOAA/National Environmental Satellite Data and Information Service (NESDIS). By following the search instructions, it is possible to acquire the necessary files.
Data are available as antenna temperatures (TDR format). Latitude and longitude coordinates for each pixel are included with these records. Each orbit of data is about 5 mb of data in compressed mode. When a study area is specified in the data search procedure, all orbits that cross that area are extracted. For an individual satellite, two orbits might cover parts of the study area and the extracted file will then contain about 10 mb of data. On a given day it is possible to have coverage by each of the three satellites twice a day. It is also possible to have no coverage. Between June 1 and July 30, 1997 there were 166 SSM/I satellite passes that included coverage of the SGP study area.
All of these data sets were acquired and further processed. The additional processing consisted of reducing the data set size by eliminating scans without coverage in the SGP region, converting the antenna temperatures to brightness temperatures, and reformatting the data into ASCII files with a latitude and longitude assigned to each data point.
Software is also provided through the data source that will allow the subsetting of the original data for smaller study areas and converting the data to brightness temperature. We modified this for our purposes. For each pass, all scans that included close to 100% coverage of the primary study area (34-38oN, 97-99oW) were extracted and put in new significantly smaller files. There are actually two files created, one for the low frequencies (lo) and one for the 85 GHz channels (hi). These files consist of an ASCII list of latitude, longitude, and the channel brightness temperatures for each pixel. No hi files were produced for F11.
Figure 1 shows the pixel centers for one of the subsets.
The Files
Name and Directory Information
The SSM/I footprint data files are contained in subdirectories June97 and July97 of directory path
/ftphttp://disc.sci.gsfc.nasa.gov/data/sgp97/sat_remote_sensing/ssmi/footprint_data/ssmi97_dataThe data files are named as follows;
TDmmddyysstttt.rr.txt
- where
- mm=month
- dd=day
- yy=year
- ss=satellite number
- tttt=start time of swath (hour and minute of local standard time)
- rr=frequency set lo or hi
Group File type Format Size/file # files Storage required *.lo.txt ascii 7-column tables
Cols from left:
- Latitude (oN)
- Longitude (oW)
- TB19V
- TB19H
- TB22V
- TB37V
- TB37H
~70kb 166 ~34MB *.hi.txt ascii 4-column tables
Cols from left:
- Latitude (oN)
- Longitude (oW)
- TB85V
- TB85H
~200kb 166
Data Access and Contacts
FTP SiteThe SSM/I data reside on DAAC anonymous FTP. You may access them from this document,
- or directly via FTP at
- ftp disc.gsfc.nasa.gov
- login: anonymous
- password: < your internet address >
- cd http://disc.sci.gsfc.nasa.gov/data/sgp97/sat_remote_sensing/ssmi/footprint_data/ssmi97_data
Points of Contact
- The Principal Investigator for the SGP97 SSM/I footprint data is
- Thomas J. Jackson
- USDA ARS Hydrology and Remote Sensing Lab
- Bldg. 007, Rm. 104, BARC-West
- Beltsville, MD 20705
- Internet: tjackson@hydrolab.arsusda.gov
- 301-504-8511 (voice)
- For information about or assistance in using DAAC data, contact
- Hydrology Data Support Team
- EOS Distributed Active Archive Center(DAAC)
- Code 610.2
- NASA Goddard Space Flight Center
- Greenbelt, Maryland 20771
- Internet: hydrology-disc@listserv.gsfc.nasa.gov
- 301-614-5165 (voice)
- 301-614-5268 (fax)
