SGP99 SSM/I Brightness Temperature Footprint Data

Overview

The Data: Background; Characteristics; Source

The Files: Name and Directory Information; Format

Data Access and Contacts: FTP Site; Points of Contact

References

Overview

Due to the relatively low levels of vegetation present in the SGP, it is possible to extract soil moisture information from SSM/I data. The limiting feature of the SSM/I for soil moisture related studies is that the frequencies are quite high. 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).

The Data

Background

The 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 53^o. 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

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

Source

Table 2 lists the characteristics of the various platforms that were in operation during SGP99. 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)
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 at the NOAA Satellite Active Archive (http://www.nesdis.noaa.gov/sat-products.html). 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 consists of 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. During the SGP99 period from July 6^th to July 23^th, there were 51 relevant SSM/I satellite overpasses. For the period of June 1 to August 31, 258 SSM/I overpasses were available.

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 (33 - 38 ^oN), converting the antenna temperatures to brightness temperatures, and reformatting the data into ASCII files. The processed data files include a latitude and longitude assigned to each data point. Only the low frequency channels, i.e., 19.4, 22.2 and 37 GHz, were processed for 1999.

The Files

Name and Directory Information

The 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 only)

Format

Group

File type

Format

Size of file

# files

*.lo.txt

Ascii

7-column tables
Cols from left: 1. Latitude (^oN) 2. Longitude (^oW)

3. TB19V

4. TB19H

5. TB22V

6. TB37V

7. TB37H

~100 Kb

258

The Principal Investigator for the SGP99 SSM/I footprint data is: Thomas J. Jackson; USDA ARS Hydrology and Remote Sensing Lab; Bldg. 007, Rm. 104, BARC-West; Beltsville, MD 20705; 301-504-8511 (voice); tjackson@hydrolab.arsusda.gov

For information about or assistance in using DAAC data, contact

Hydrology Data Support Team
GSFC DAAC, Code 610.2
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
(301) 614-5165 (voice)
(301) 614-5268 (fax)

hydrology-disc@listserv.gsfc.nasa.gov

References

Heymsfield, G. A. and R. Fulton, Modulation of SSM/I microwave soil radiances by rainfall. Remote Sensing of Environment, 29, 187-202, 1992.

Hollinger, J. P., J. L. Peirce, and G. A. Poe, SSM/I instrument evaluation, IEEE Trans. on Geoscience and Remote Sensing, 28, 781-790, 1990.

Jackson, T. J., Soil moisture estimation using SSM/I satellite data over a grass land region, Water Resources Research, 33, 1475-1484, 1997.

Teng, W. L., J. R. Wang, and P. C. Doraiswamy, Relationship between satellite microwave radiometric data, antecedent precipitation index, and regional soil moisture, Int. J. of Remote Sensing, 14, 2483-2500, 1993.

Last Updated:Thu Oct 28 09:44:26 EDT 2004

Page Author: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov
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