Summary

This README file contains information on the structure of the MSFC SSM/I Geophysical Product Data Sets, instructions for accessing the Hierarchical Data Format (HDF) library, and pertinent scientific references.

IMPORTANT NOTICE: Due to the Navy's activation of a radiation/calibration beacon on the DMSP-F15 platform, the SSM/I 22V channel has become corrupted. This beacon was started on August 14, 2006. The corrupted 22V will affect all geophysical products which use it. This means the GHRC's IWV, CLW, and OWS (for F-15 only) are likely unusable, and will be discontinued from the time this beacon was activated. We're sorry for this inconvenience.

Table of Contents

• 1. Introduction
• 2. Processing Steps
• 3. File Naming Convention and Contents
• 4. Geophysical Product Files
  • 4.1 Integrated Water Vapor
  • 4.2 Cloud Liquid Water
  • 4.3 Oceanic Wind Speed
• 5. File Access
  • 5.1 Sample Program to Read Files
• 6. HDF Library and Tools
• 7. References
• Appendix A: Metadata Objects
• 8. Contact Information

1. Introduction

This README file contains information on the Special Sensor Microwave Imager (SSM/I) Geophysical Products produced, using Wentz's SSM/I Benchmark Pathfinder Algorithm, at the Global Hydrology Resource Center (GHRC). From the brightness temperature swath data, the GHRC generates three geophysical products at swath and gridded resolutions. These three oceanic geophysical products are produced with data from SSM/I instruments aboard the Defense Meteorological Satellite Program (DMSP) F-13, F-14, and F-15 satellites. The parameters (except windspeed) are measured between the ocean surface and the top of the atmosphere. Passes are produced at the resolution of the high frequency (85 GHz) and low frequency (37,22, & 19 GHz) channels, while the gridded data is produced at 0.5 degree resolution. Browse images of the gridded files are also created. The three products are:

• 4.1 Integrated Water Vapor (IWV)
• 4.2 Cloud Liquid Water (CLW)
• 4.3 Oceanic Wind Speed (OWS)

Since May 1995, the SSM/I data source has been the Fleet Numerical Meteorology and Oceanography Center (FNMOC). Data is obtained from FNMOC and processed at the GHRC within hours of its reception. Each day, full resolution swath or "pass" brightness temperatures (Tb's) and reduced resolution "gridded" data sets are generated and stored in HDF files. Browse images of the gridded files are also created in both HDF raster8 and GIF formats. HDF represents the Hierarchical Data Format, the data format standard for NASA’s Earth Observing System Data and Information System (EOS-DIS).

 2. Processing Steps

 2.1 Data Acquisition

The source of the GHRC's SSM/I data is the Navy's Fleet Numerical Meteorology and Oceanography Center (FNMOC). FNMOC is the Department of Defense's (DOD) center of expertise for DMSP passive microwave data processing. The DOD has an agreement with the National Oceanographic and Atmospheric Administration (NOAA) under which they share data via the Shared Processing Network (SPN). FNMOC generates SSM/I antenna temperature files known as Temperature Data Records (TDR's) and sends them to NOAA/NESDIS. NESDIS performs minimal file unpacking and makes the data available to the GHRC. Files are usually available within 1 hour of data acquisition by FNMOC.

2.2 Data Quality Control

Most of the quality control for these products is done as the brightness temperatures are being produced. See the README file for the MSFC Brightness Temperature data for more details.  Each day the GHRC performs quality assurance checks on the raw SSM/I antenna temperatures from FNMOC. These are to verify that:

• the scan start times are accurate and dependable
• previously recorded data is not used
• poorly geolocated scans are flagged
• bad lat/lon values and associated data are flagged
• brightness temperatures calculated from bad calibration are flagged

The purpose of these QC steps is to produce a dependable, stable brightness temperature data set and to ensure that only "good" brightness temperatures are used in calculating the geophysical products.

2.3 Data Processing-Swath Data

Swath files are classified as either ascending (from the south pole to the north pole) or descending (from the north pole to the south pole). After the above listed QC steps are performed, brightness temperature (Tb) and geolocation data are read sequentially for each swath file for an entire day. Three geophysical products are generated from each of the MSFC SSM/I brightness temperature ascending and descending swath data files.

2.3.1 Gridded Data

For each day, all of the swath files containing ascending swaths are averaged into a 0.5 x 0.5 degree global grid (720 x 360). The same is separately done for all descending swaths. The global grids are centered on the Greenwich meridian. Each grid box value is the mean of the geophysical product values located within the half degree box centered at every xx.25 and xx.75 degrees. Only valid (positive) geophysical product values are used. Table 1 lists some representative grid points and their geographic extents.

Table 1

2.3.2 Browse Images (HDF,GIF)

Browse files are created from daily gridded HDF products files. Two formats are used; GIF and HDF raster8. For each day the HDF raster8 file contains an image for both the ascending and descending orbits (for each product). For each day there are two GIF files created for each product, one for the ascending passes and one for the descending passes.

Both the HDF raster8 image and the GIF images are created from the HDF gridded data files. The images have been annotated. Also, the coastal boundaries have been color coded.

3. File Naming Convention and Contents

There are 4 types of files produced for each day for each geophysical product (IWV, CLW, and OWS). The 4 types are swath HDF data file, gridded HDF data file, HDF raster8 image file, and GIF image file. The HDF raster8 image file and the GIF image file are both created from the gridded HDF data file. Below is the naming convention and contents for each type of file. The HDF type files have been compressed using "gzip." To uncompress the files use Unix "gunzip”.  There are several Windows and Mac software packages that handle gzipped files.

3.1 Swath HDF Data File

For each day, the swath data files (all ascending and all descending files) are created for each of the 3 geophysical products from the MSFC SMM/I brightness temperature swath files.

3.1.1 File Naming Convention

 The swath file naming convention is:

For example, the file "f13_iwva_05008_06D.hdf.gz" contains F13 integrated water vapor (algorithm version a) data from the 5th swath (descending) of day 05008. The corresponding geolocation data for this swath is in "f13_ln_05008_06D.hdf.gz."

3.1.2 File Content

The HDF swath files contain data grouped in structures called "objects". Table 2 lists the objects in the HDF swath files. The dimension 'N' represents the number of B-scans in the swath, normally 785 - 808, and can be retrieved with a HDF library call. All objects are filled with missing scans where needed. HDF labels are provided with each object.
 

Table 2

In the Spacecraft Position Vector array, missing scans are flagged with -999.0. Since the values are floating point, no scaling is done. Times are in seconds (to the nearest half second), latitudes range from 0.00 to 180.00 degrees, longitudes are positive east from -180.00 to 180.00 degrees, and altitudes are in whole meters.

The two-line element set (object #5) contains a two-line formatted group of ephemeris values closest in time to 1200 UTC of the day being processed. The GHRC retrieves two-line element sets daily. The McIDAS Navigation directory (object #6) is for use with McIDAS software.

3.2 Gridded HDF Data File

For each day there is one HDF gridded data file created for each of the 3 geophysical products. Each file contains two objects – the ascending swath grid and the descending swath grid.

3.2.1 File Naming Convention

The file naming convention for the gridded HDF files is:

For example, the file "f14_owsa_04219_dayAD.hdf.gz" contains gridded F14 oceanic wind speeds (algorithm version a) for day 04219. The 'AD' in the file name represents the fact that the ascending and descending swaths are computed separately and placed in different objects.

3.2.2 File Content

Table 3 lists the objects in the HDF files and their contents.

Table 3

where ppp is the geophysical product code (iwv, ows, or clw) and V is the algorithm version letter.

3.2.3 Data Values

The geophysical product data in the gridded HDF files are stored as real*4. Table 4 shows possible values.

Table 4

3.3 Browse files (HDF and GIF)

Both the HDF raster8 image and the GIF images are created from the HDF gridded data files. The images have been annotated. Also, the coastal boundaries have been color coded.

3.3.1 File Naming Conventions

3.3.1.1 HDF Raster8 Images

The file naming conventions are:

HDF Raster8 files are in the HDF 8-bit raster image format. They each contain two 8-bit raster images, two palettes (the same), and a file description (metadata).

3.3.1.2 GIF Images

The file naming conventions are:

4. Geophysical Product Files

The GHRC generates three geophysical products at swath and gridded resolutions. These three oceanic geophysical products are produced, using Wentz's SSM/I Benchmark Pathfinder Algorithm, from data recorded by SSM/I measurements aboard the DMSP F-13, 14, & 15 satellites. The parameters are measured between the ocean's surface and the top of the atmosphere. The three products are:

• 4.1 Integrated Water Vapor (IWV)
• 4.2 Cloud Liquid Water (CLW)
• 4.3 Oceanic Wind Speed (OWS)

4.1 Integrated Water Vapor (IWV)

Integrated water vapor is defined as the total amount of non-precipitating water vapor from the ocean surface to the top of the atmosphere. Where results are not attainable the FNMOC surface type (negated) is assigned. For the Integrated Water Vapor ocean product, the brightness temperature (Tb) data is read from the HDF Tb swath files and is then converted back to antenna temperatures (Ta) with the along-scan bias and satellite inter-calibration included. The Ta's are modified to include Wentz' radiative transfer model offsets.

4.1.1 Integrated Water Vapor Algorithm Information

4.1.2 Integrated Water Vapor Data Values

The integrated water vapor data in the HDF files are stored as real*4, ranging from -33.0 to 10.0. Negative values represent flagged data while positive values represent valid data. Table 5 shows all possible values.

Table 5

4.2 Cloud Liquid Water (CLW)

Cloud liquid water is defined as the total amount of non-precipitating liquid water from the ocean surface to the top of the atmosphere. Where results are not attainable the FNMOC surface type (negated) is assigned. Note, since the FNMOC surface type code for land is zero, land codes are included with vegetated land codes and have a value of -1. For the Cloud Liquid Water ocean product, the brightness temperature (Tb) data is read from the HDF Tb swath files and is then converted to antenna temperatures (Ta's) with the along-scan bias and satellite inter-calibration included. The Ta's are modified to include Wentz' radiative transfer model offsets.

4.2.1 Cloud Liquid Water Algorithm Information

4.2.2 Cloud Liquid Water Data Values

The cloud liquid water data in the HDF files are real*4, ranging from -33.0 to 1000.0. Negative values represent flagged data while positive values represent valid data. Table 6 shows all possible values.

Table 6

4.3 Oceanic Wind Speed (OWS)

Oceanic wind speed is defined as the wind speed just above the ocean surface. Where results are not attainable the FNMOC surface type (negated) is assigned. Note, since the FNMOC surface type code for land is zero, land codes are included with vegetated land codes and have a value of -1. For the Oceanic Wind Speed ocean product, the brightness temperature (Tb) data is read from the HDF Tb swath files and then is converted to antenna temperatures (Ta's) with the along-scan bias and satellite inter-calibration included. The Ta's are modified to include Wentz' radiative transfer model offsets.

4.3.1 Oceanic Wind Speed Algorithm Information

4.3.2 Oceanic Wind Speed Data Values

The oceanic wind speed data in the HDF files are real*4, ranging from -33.0 to 40.0. Negative values represent flagged data while positive values represent valid data. Table 7 shows all possible values.

Table 7   

5.Data and Browse Access

SSM/I data and browse products processed by the GHRC are available for online ordering via the Global Hydrology Resource Center. (click "Dataset List", then "DISCOVER") Additionally, the most recent two years of SSM/I products are available for immediate download via our Data Pool at http://datapool.nsstc.nasa.gov/.

Note: The SSM/I products and Navigational data on our Data Pool are not compressed, nor are they stored together in the same directory as on our GHRC anonymous FTP server. Please refer to the Data Pool Navigational Documentation found here: http://moby.itsc.uah.edu/ftpdata/doc/ssmi_tb/readme_1st.html for directions on how to locate all the necessary data products. The Data Pool has the additional capabilities of subsetting, packaging, and compressing the data when you use the preferred "shopping cart" method of data ordering.

5.1 Sample Program to Read Files

Two sample FORTRAN program have been provided to read swath and gridded data. read_ssmi_px.f is for reading geophysical product swath files while read_ssmi_px_grid.f is for reading geophysical product grid files. They will read a file and fill arrays containing various data and metadata. The HDF software library is required (see below).

6. HDF Library and Tools

6.1 Hierarchical Data Format

HDF is a library and platform independent data format for the storage and exchange of scientific data. It includes Fortran and C calling interfaces, and utilities for analyzing and converting HDF data files. HDF is developed and supported by the National Center for Supercomputing Applications (NCSA) and is available in the public domain (http://hdf.ncsa.uiuc.edu/). HDF stands for Hierarchical Data Format. It is a multi-object file format for the transfer of graphical and numerical data between machines. HDF is a portable file format. HDF files can be shared across platforms. An HDF file created on one computer, say a Silicon Graphics (SGI) workstation, can be read on another system, say IBM PC, without modification.

6.2 How to Obtain the HDF Library

The HDF library and tools can be down loaded from the World Wide Web at: http://hdf.ncsa.uiuc.edu/. Follow the instructions given there.

6.3 Visualization Software

The HDF files were created using HDF version 4.1, release 3. Numerous useful visualization tools can be found at http://hdf.ncsa.uiuc.edu/hdftools.html. The GIF images may be viewed by most popular GIF image viewers.

7. References

Hollinger, J., et al., DMSP Special Sensor Microwave / Imager Calibration / Validation Final Report Volume I, Naval Research Laboratory, Washington, D. C.,20 July 1989.

 Wentz, Frank J., Measurement of Oceanic Wind Vector Using Satellite Microwave Radiometers, IEEE Transactions on Geoscience and Remote Sensing, Vol. 30, No. 5, 5 September 1992.

Wentz, Frank J., User's Manual SSM/I Antenna Temperature Tapes Revision 1, RSS Technical Report 120191, Dec. 1, 1991, Remote Sensing Systems, Santa Rosa, CA.

Wentz, Frank J., User's Manual SSM/I Antenna Temperature Tapes Revision 2, RSS Technical Report 120193, Dec. 1, 1993, Remote Sensing Systems, Santa Rosa, CA.

Appendix A: Metadata Objects

A.1 Grid Metadata Object

The metadata object of each brightness temperature global grid HDF file contains a 512 X 31 (4-byte integer) word array. The metadata arrays from each swath contained in that daily grid are included in "columns" 1 through 29 while "columns" 30 and 31 contain the metadata arrays for the daily grid (ascending and descending swaths respectively). The definitions for each element of those last two metadata arrays are defined here.

Use of the word "scan" refers to 85 GHz resolution. (I.e., half of an A-B scan-pair)
 

A.2 Swath Metadata Object

The metadata object of each geophysical product swath HDF file contains a 512 (4-byte integer) word array. The definitions for each element of that array is defined here.
 

8. Contact Information

Data can be ordered and questions addressed at http://ghrc.nsstc.nasa.gov/.

To order this data or for further information, please contact: Global Hydrology Resource Center User Services 320 Sparkman Drive Huntsville, AL 35805 Phone: 256-961-7932 E-mail: ghrc@eos.nasa.gov

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NASA Information Contact: Michael Goodman, Global Hydrology and Climate Center GHRC Web Curator: GHRC Web Team Last update: Tuesday, 17-Oct-2006 14:17:04 CDT If you have trouble viewing or navigating this page, please contact GHRC User Services. U.S. Government Compliance report.