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DMSP SSM/I Pathfinder Daily EASE-Grid Brightness Temperatures

Summary

The Special Sensor Microwave/Imager (SSM/I) Level-3 Equal-Area Scalable Earth-Grid (EASE-Grid) Brightness Temperature data set is collected as part of the NOAA/NASA Pathfinder Program. The data set consists of gridded data in three equal-area projections: Northern Hemisphere, Southern Hemisphere, and full global. The data gridding technique maximizes the radiometric integrity of the original brightness temperature values, maintains high spatial and temporal precision, and involves no averaging of original swath data.

The spatial coverage is global and begins 09 July 1987; processing is ongoing. The spatial resolution is 25 km for all channels; the 85 GHz channel is also provided at a 12.5 km resolution. There are 18 brightness temperature files per day for a given projection and two corresponding time files. Data are contained in flat binary files.

Data are available via FTP as processing is completed. Users can order temporal or spatial subsets of the data with the Graphical Interface for Subsetting, Mapping, and Ordering (GISMO) Web-based tool. Users may also order historical data on CD-ROM, while supplies last.

Citing These Data

To broaden awareness of our services, NSIDC requests that you acknowledge the use of data sets distributed by NSIDC. Please refer to the citation below for the suggested form, or contact NSIDC User Services for further information. We also request that you send us one reprint of any publication that cites the use of data received from our Center. This helps us to determine the level of use of the data we distribute. Thank you.

The following example shows how to cite the use of this data set in a publication. List the principal investigators, year of data set release, data set title, dates of data you used (for example, June to September 2001), publishers (NSIDC), and digital media.

Armstrong, R. L., K. W. Knowles, M. J. Brodzik, and M. A. Hardman. 1994, updated current year. DMSP SSM/I Pathfinder Daily EASE-Grid Brightness Temperatures, [list dates of data used]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Overview Table

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator

Richard L. Armstrong
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
(303) 492-6199

Technical Contact

Acknowledgements

Special thanks to the following:

• Dr. Tony England, Dr. John Galantowicz, and Ed Kim for their ongoing research efforts; for consistent support and input during all phases of this project, including prototyping and development; and for producing the Antenna Pattern Correction coefficients used in the interpolation procedure.
• Molly Hardman for software development of the first revisions of the processing software.
• Dr. Ted Habermann for the development of the FREEFORM and makeHDF tool set.
• Ray Kokaly for technical support and comments.
• Ken Knowles for technical support, gridding and overlay software support, and advice and comments.
• Mark Ohrenschall for support with the FREEFORM and makeHDF tool set.
• To members of the product team at the NSIDC DAAC including Renea Ericson, Rachel Hauser, David Hoogstrate, Tracy Thrasher-Hybl, Xiaoming Li, Mike Machowski, Alex Machado, John Maurer, Heidi Schumacher, Annette Varani, I-Pin Wang, Jason Wolfe, Fran Coloma, Dave Korn, Michelle Holm, Jonathan Kovarik, Peter Gibbons, and Ann Windnagel, whose constant attention to detail and professional energies have made possible the continuing availability of this high-quality data set.
• Annette Varani for documentation support, developing the EASE-Grid Sampler area of the Equinox Sampler, and much patience involved in the packaging artwork

2. Detailed Data Description

Format

Brightness Temperature Files

Brightness temperature data are contained in flat binary files (little-endian) with one grid per file consisting of 2-byte integer arrays of brightness temperatures in tenths of kelvins. Note: Subsetted data ordered through the GISMO tool prior to July 2005 are big-endian; subsetted data ordered through GISMO after July 2005 are little-endian.

Each brightness temperature file represents gridded data for a single channel and polarization; they are derived from either ascending or descending orbits (for example, 37 GHz, horizontal, ascending) for one day. There are 18 brightness temperature files per day for each projection.

Time Files

Time data are contained in 1-byte, unsigned integer arrays consisting of Coordinated Universal Time (UTC) in tenths of hours. Each time file represents the corresponding time of the swath sample used for the interpolation of the given grid cell, for either ascending or descending orbits for that day. There are two time files per day (ascending and descending passes) for a given projection, both at a 25 km resolution.

File and Directory Structure

The directory structure differs as a function of distribution media: FTP or CD-ROM.

FTP

Data are on the FTP site in the ssmi directory. Within the ssmi directory, there are three subdirectories, one for each projection: global, north, and south. Each of these directories is further subdivided into directories labeled for each year from 1987 to the most current year's processing. Figure 1 displays the directory structure.

Figure 1. FTP Directory Structure

CD-ROM

The directory structure of the CD-ROM is described in the File Naming Convention section of this document.

CD-ROM ERROR ALERT, PLEASE READ

File Naming Convention

The file naming convention differs as a function of distribution media (FTP or CD-ROM).

FTP

Brightness temperature files on the FTP server are named according to the following convention and as described in Table 1:

EASE-Fxx-zzyyyydddp.ccc.gz

Where:

Time files on the FTP server are named according to the following convention and as described in Table 2:

EASE-Fxx-zzyyyydddp.tim.gz

Where:

CD-ROM

Brightness temperature files on the CD-ROM are named according to the following convention and as described in Table 3. Note: The file naming convention includes the directory path.

yyyy/proj/Dddd_ddd/rdddpccc.GZ

Where:

Time files on CD-ROM are named according to the following convention and as described in Table 4. Note: The file naming convention includes the directory path:

yyyy/proj/Dddd_ddd/LdddpTIM.GZ

Where:

File Size

Gzipped brightness temperature data files range in size from 25 KB to 3.4 MB, and gzipped time files range in size from 1.1 KB to 36 KB.

Spatial Coverage

These data files are provided in three different equal-area, spatial coverages: Northern Hemisphere azimuthal, Southern Hemisphere azimuthal, and global cylindrical. Please see the Grid Extent Table on the EASE-Grid: A Versatile Set of Equal-Area Projections and Grids Web page for specific latitude and longitude values. Figure 2 shows a map of the three different coverages.

Figure 2. Left two images: Northern and Southern Hemispheres (Lambert's equal-area, azimuthal). Right image: global (cylindrical, equal-area).

Spatial Resolution

The spatial resolution for the 19 GHz, 22 GHz, and 37 GHz channels is 25 km. There are two different spatial resolutions for the 85 GHz channel: 25 km and 12.5 km.

Projection/Grid Description

The SSM/I EASE-Grids are a set of three equal-area projections: two azimuthal equal-area projections, one for the Northern and one for the Southern Hemisphere; and a global cylindrical equal-area projection. Please see the EASE-Grid: A Versatile Set of Equal-Area Projections and Grids for more information on the EASE-Grid.

Temporal Coverage

Coverage begins 09 July 1987 and is ongoing. See the SSM/I Data Availability Web page for specific dates.

The goal of the SSM/I Pathfinder product team is to produce a continuous time series of SSM/I data using a single consistent processing and interpolation scheme. Data are made available on the FTP site as processing is completed. All data are available via FTP, and historical data through 2004 are also available on CD-ROM. For a complete list of missing dates, see the SSM/I Data Availability Web page. A brief list of some of the larger gaps in the data are explained below:

Missing Data

85 GHz channel April 2008 to present: Beginning with data in the late spring of 2008, the 85 GHz horizontal data and some Southern Hemisphere 85 GHz vertical data files contain zeros for the brightness temperatures. This means that either no brightness temperatures were measured or the brightness temperatures failed quality control procedures from Remote Sensing Systems that NSIDC uses in our preprocessing. The affected periods of 85 GHz horizontal files are shown in Table 5.

Alaska and Canadian Prairies 1994 to May 1995: Substantial amounts of swath data over Alaska and the Canadian Prairies are missing beginning early in 1994 until May 1995. During this period, the data tape recorder on the DMSP-F11 failed. As a result, it was necessary to download data to ground stations more frequently than usual. Data download and acquisition could not occur simultaneously, consequently data gaps exist in the EASE-Grid data for Alaska and the Canadian Prairies from early 1994 until data acquisition by the DMSP-F13.

85 GHz channel 01 February 1989 to 31 December 1991: There are usually 18 brightness temperature files per day for a given projection, except from 01 February 1989 through 31 December 1991. For this period, no data exist for the 85 GHz channel due to degradation of this channel after heating cycles during Northern Hemisphere winters that resulted in increased solar illumination on the SSM/I instrument (Wentz 1992). Only 10 brightness temperature files are available per day during these dates, and all files are in the 25 km resolution grid.

Temporal Resolution

The data are daily, separated by ascending and descending passes.

Parameter or Variable

The parameter of this data set is brightness temperature.

Parameter Description

Theoretically, brightness temperature is the effective temperature of a blackbody radiating the same amount of energy per unit area at the same wavelengths as the observed body. Empirically, brightness temperature is the apparent radiant temperature of a non-blackbody determined by measurement with an optical pyrometer or radiometer. The brightness temperature (T_b) at a given wavelength (λ) is the product of the physical temperature (T_p) and the emissivity (ε) at the given wavelength of the surface viewed by the radiometer. Refer to Equation 1.

Equation 1 is the Rayleigh-Jean approximation of Plank's law for the passive microwave region of the electromagnetic spectrum. It is an approximation and does not take into account effects of the atmosphere on the microwave radiation.

Parameter Range

Brightness temperature data values are scaled by 10; divide the stored values by 10 to get kelvins. Values range from 550 (representing 55.0 K) to 3200 (representing 320.0 K); missing data are indicated by the value 0.

Time values range from 0 (representing 0000 UTC or midnight) to 239 hours (representing 2354 UTC or 23.9 hours); missing data are indicated by the value 255.

Latitude and longitude values vary depending upon the grid used. Values are in decimal degrees scaled by 100000; divide the stored values by 100000 to get actual values. Latitude values range from -9000000 to 9000000, and longitude values range from -18000000 to 18000000. Missing data values are indicated by the value 1431655765. Table 6 summarizes these data ranges.

Sample Browse Image

Figure 3 shows a sample browse image of this data set.

Figure 3. Sample Browse Image for 01 January 2008 for the Northern Hemisphere, 25 km, 37 GHz Data

Error Sources

For information about errors in this data set, please read the Error Alert.

Measurement Error for Parameters

Geolocation errors in input Remote Sensing Systems (RSS) swath data are no more than 10 km, "although there may be exceptions" (Sharon Tremble, RSS, e-mail to M. J. Brodzik, 18 January 1996). Additional error introduced by nearest-neighbor interpolation from the over-sampled array is approximately 6 km for the 25 km grids and 3 km for the 12.5 km grids.

Quality Assessment

Each brightness temperature and time file is visually inspected by data center operators before being archived and distributed.

3. Data Access and Tools

Data Access

Full time series of the data are available via FTP as processing is completed. See the SSM/I Data Availability Web page for specific dates by platform. After January 2005, data are no longer distributed via CD-ROM. Instead, data are made available on our public FTP site as the data are produced. Users may order historical data on CD-ROM while supplies last.

Users can also access data via the Graphical Interface for Subsetting, Mapping, and Ordering (GISMO) to select specific temporal and spatial subsets of gridded data from NSIDC, including DMSP SSM/I Pathfinder Daily EASE-Grid Brightness Temperatures. Subsetted data from GISMO ordered prior to July 2005 have a big-endian byte order; GISMO data ordered after July 2005 are little-endian. Users can request subsetted data by FTP. The GISMO was designed to allow users to order spatial and temporal subsets of data. It is not an appropriate tool for ordering entire data sets or entire grids for long time series. Users who wish to order a full time series, or several years' worth of SSM/I EASE-Grid data, should contact NSIDC User Services. Large orders will be fulfilled as resources allow.

Volume

The approximate yearly volume of this data is 6.2 GB.

Software and Tools

Software and tools differ as a function of distribution media: FTP or CD-ROM.

FTP

Geolocation tools for this data set are available via NSIDC's EASE-Grid Geolocation Tools Web page. IDL tools for reading and displaying these data are available by request from NSIDC User Services.

CD-ROM

A TOOLS directory accompanies the data on CD-ROM that includes gzip utilities, routines for IDL users to display and manipulate images, and C and FORTRAN routines for use in converting grid row/column coordinates to latitude and longitude. The TOOLS directory contains three subdirectories: GZIP, IDL, and SOURCE. A description of each subdirectory follows.

Note: Due to the file naming convention, some file names will exist on at least three separate CD-ROMs, one for each projection and more for separate years (for example: L213A37H.GZ). If you plan to use data for the same dates from more than one projection, reproduce the year and the NORTH-SOUTH-GLOBL directory structure on your machine; or change your local filenames so that they are unique.

GZIP Directory

The /TOOLS/GZIP/ directory contains four subdirectories: MSDOS, SGI, SOURCE, and SUN. The SUN and SGI directories contain copies of the latest available GZIP Unix source code, one for Sun platforms and one for SGI platforms. The MSDOS directory contains a self-extracting archive file for DOS (PC) systems. The copyright and distribution information are in the file /TOOLS/GZIP/README.GNU.

Note: You may also obtain GZIP directly over the internet from GNU's GZIP Web site.

Sun/SGI platforms GZIP Installation

• Users with IDL:
1. Set the environment variable EASE_GZIP to the location of the executable on the CD-ROM. The IDL EASE Tools will do the rest.
    
• Users without IDL:
  1. If you do not already have gzip, copy the executable version you need to a directory on your machine.
      
  2. Rename the files using lower case characters.
      
  3. Add its location to your appropriate path environment variable.

Note: For help on using gzip, enter the following at the command prompt:

gzip -h

PC platforms GZIP Installation

If you do not already have GZIP, copy and execute the file /TOOLS/GZIP/MSDOS/GZIP*.EXE on your machine. It will create a set of files and documentation for you to use for installation on your platform.

Other Unix platforms GZIP Installation

If you do not already have gzip, follow these steps to install it:

1. Copy the tar file /TOOLS/GZIP/SOURCE/GZIP.TAR to your machine.
   
    
2. Enter the following at the command prompt:
   
   tar xvf GZIP.TAR
   
   This will expand the file and it contents into the directory gzip-x.x.x.
    
3. Follow the instructions in the INSTALL file.

Using GZIP (Users with IDL)

Once gzip/gunzip are installed, users with IDL need only set the environment variable EASE_GZIP to the location of the executable on the local file system. The IDL EASE Tools will do the rest.

Using GZIP (Users without IDL)

Once you have obtained a copy of gzip (which includes gunzip after installation), follow these steps to begin uncompressing the .GZ files:

1. Copy the files you want from the CD-ROM to a directory on your own system.
    
2. Uncompress any single file or multiple files with the .GZ extension by entering the following at the command prompt:
   
   gunzip -v /your_path/DATAFILE.GZ
   
   where your_path is the directory path to the .GZ files saved on your local machine, and DATAFILE.GZ is the file you want to uncompress. The -v option stands for verbose output.
   
   Note: To uncompress multiple files, use shell wildcards for pattern matching. Be sure to calculate the necessary disk space for the files you are uncompressing.

IDL Directory

Users who currently have IDL should read the file /TOOLS/IDL/README.IDL, in addition to reading the information in this file. Unix users with IDL can now read data directly into IDL from the CD-ROM, performing gunzipping and byte-swapping on the fly, using the latest version of the ssmi_read procedure (included in ssmi.pro beginning with revision 3.1 of that file), included in the IDL EASE tools.

The IDL directory contains routines for IDL users to display and manipulate images in IDL. These routines contain several updates to the Prototype Version 1.0 tools, including the new grid definitions. Prototype versions of the tools should be replaced with these new ones.

Note to IDL users: the IDL tools included on the CD-ROMs will uncompress gzipped files on the fly on Unix platforms. If you plan to use IDL to read and manipulate these data, consider using this capability to bypass the need to copy and uncompress data to your hard drive via GET_EASE_DATA.CSH. See the SOURCE Directory section below for more information about GET_EASE_DATA.CSH.

SOURCE Directory

The SOURCE directory contains C and FORTRAN subroutines for the family of EASE-Grids to convert between grid row/column coordinates and latitude/longitude coordinates. The SOURCE directory also contains a file called GET_EASE_DATA.CSH, that is an example of a C Shell script that may be used and/or tailored to the user's system to gunzip and byte swap a number of data files at one time.

The /TOOLS/SOURCE/GET_EASE_DATA.CSH script takes two input arguments:

• A pattern string to determine the names of the files that are to be extracted from the CD-ROM, for example, /CDROM/1979/NORTH/D001_005/L002/37H*
   
• A destination directory to which the uncompressed files are to be written.

Each uncompressed data filename will consist of the filename base from the CD-ROM, with no extension. For example, entering the following command at the command prompt:

get_ease_data.csh '/CDROM/1979/NORTH/D001_005/L002/37H*' ./

will uncompress to the files:

./L002A37H
./L002D37H

which will be write-protected in the destination directory. This script may need some adjustment on different platforms, but may prove to be a useful tool during the tedious process of extracting compressed data from the CD-ROM. The user should verify, before running GET_EASE_DATA.CSH, that enough local disk space exists for the uncompressed versions of data being extracted.

Related Data Collections

The following list of related data collections is available from NSIDC.

• TOVS Pathfinder Path-P Daily and Monthly Polar Atmospheric Grids
• AVHRR Polar 1 km Level 1b Data Set
• DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures
• Global Monthly EASE-Grid Snow Water Equivalent Climatology
• Nimbus-7 SMMR Pathfinder Daily EASE-Grid Brightness Temperatures
• Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent
• Near-Real-Time DMSP SSM/I-SSMIS Pathfinder Daily EASE-Grid Brightness Temperatures

To access Level 2 SSM/I brightness temperatures, please visit Remote Sensing Systems data pages.

4. Data Acquisition and Processing

Sensor or Instrument Description

These data were acquired using the SSM/I instrument on the DMSP F08, F11, and F13 platforms. For more information about the SSM/I instrument, please see NSIDC's SSM/I Instrument Description Web page.

Theory of Measurements

On the DMSP platform, the SSM/I instrument measures passive microwave radiances. For a detailed description on how SSM/I obtains its measurements, please see NSIDC's SSM/I Instrument Description Web page for information.

Data Source

The source for the raw antenna temperature and brightness temperature data for this data set is Remote Sensing Systems (RSS), Santa Rosa, California (Wentz 1991).

Data Acquisition Methods

For a detailed discussion of the theory behind the acquisition methods used here, please refer to the following articles:

• Poe, G. A. 1990. Optimum Interpolation of Imaging Microwave Radiometer Data. IEEE Transactions on Geoscience and Remote Sensing 28(5):800-810.
• Galantowicz, J. F. and A. W. England. 1991. The Michigan Earth Grid: Description, Registration Method for SSM/I Data and Derivative Map Projections. Technical Report 027396-2-T. Radiation Laboratory Dept. of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor.

Derivation Techniques and Algorithms

The input orbital brightness temperature data for this product was ingested via the RSS software programs DECODE and QUAL1 (Wentz 1993). Switches were used to include the following options:

1. Along-scan bias corrections were turned on.
    
2. Sensor intercalibrations (adjusting non-F8 SSM/I antenna temperatures to correspond to F8 SSM/I antenna temperatures) were turned off.
    
3. Data values during times that RSS has determined to be "periods of erroneous data," and identified in RSS files BADLOC.D08, BADLOC.D11, BADLOC.D13, were discarded prior to gridding and interpolation.
    
4. Groups of scans that RSS identified as corrupted by calibration error problems between 09 October 1990 and 29 August 1992 and identified in RSS files BADCAL.D08 and BADCAL.D11, were discarded prior to gridding and interpolation.
    
5. After 29 August 1992 calibration errors detected in the RSS DECODE processing were discarded prior to gridding and interpolation.

Over the course of a day, points at mid-to-high latitudes will be observed from multiple orbits. For a given grid cell location, only observations from a single orbit were used in the Backus-Gilbert interpolation. In order to ensure the most consistent local observation time at each location, we chose the sample from the orbit whose local time was closest to the equatorial crossing times shown in Table 7. Actual equatorial crossing times do change slowly over the life of sun-synchronous satellites such as the DMSPs. The choice of these particular times was based on nominal crossing times at launch.

The antenna pattern coefficients used in the Backus-Gilbert interpolation were provided to NSIDC by John Galantowicz (Galantowicz 1995, Appendix C). The interpolated brightness temperatures represent optimally filtered data, that is, they represent what the sensor would have measured had it been directed at the center of the fixed grid cell. Depending on one's purposes, the Backus-Gilbert method can be tuned to enhance resolution, or reduce noise, but both cannot be achieved simultaneously. Galantowicz chose to generate coefficients that would minimize noise (ones that produced a pattern with lowest relative side-lobes). See Figure 4.

"The 19 GHz pattern has the largest footprint of the four SSM/I frequencies, and the interpolated patterns of the other channels are able to fit it without high side-lobes. If the 19 GHz pattern were interpolated to a significantly smaller desired pattern--that is, either the 37 GHz or 85 GHz pattern--then the best achievable interpolated pattern would be distorted and have high side-lobe levels." (Galantowicz 1995)

Figure 4. An example frequency response of Backus-Gilbert interpolation, as 3-D shaded relief (left) and the profile at y=0 (right). The side-lobes are the bumps outside the intervals x = ± 1/2 and y = ± 1/2. The decision to "tune" the EASE-Grid SSM/I brightness temperatures minimizes the side-lobes in the interpolation. Images courtesy of K. Knowles, NSIDC.

The resulting EASE-Grid brightness temperatures for all channels in the 25 km grids represent the effective field of view (EFOV) at -3 db of the 19 GHz vertically polarized channel, and the EASE-Grid brightness temperatures for 85 GHz channels in the 12.5 km grids represent the EFOV of the 85 GHz vertically polarized channel. Users of the SSM/I EASE-Grid data can make inter-channel comparisons and develop geophysical algorithms based on the assumption that the gridded data represent the brightness of the same geographical area. For further details, refer to Galantowicz (1995), Galantowicz and England (1991), and Poe (1990).

Processing Steps

The NOAA/NASA Pathfinder Program is designed to provide scientists with time series of global-scale remote sensing data ahead of the EOS satellite launches. The Pathfinder concept involves careful reprocessing of existing data sets and then making them readily available as high quality products for global change research. Since the polar regions hold special significance for global change research, the Polar Pathfinders have established a cooperation to maximize the scientific potential of polar data.

Binary data arrays contain spatially interpolated data. The data gridding technique maximizes the radiometric integrity of the original brightness temperature values, maintains high spatial and temporal precision, and involves no averaging of original swath data. Backus-Gilbert optimal interpolation is used to artificially increase (16 times) the density of brightness temperature measurements in the satellite swath reference frame (sample interval of 25 km for 19, 22, and 37 GHz, and 12.5 km for 85 GHz). This process uses actual antenna patterns to create the over-sampled array, and the net effect is as if the additional samples had been made by the satellite radiometer itself (the beam patterns and spatial resolutions of the interpolated data approximate those of the original samples). This method is based on the earlier work of Galantowicz and England (1991) and Poe (1990). The brightness temperature for a given EASE-Grid cell is obtained from the over-sampled array by the nearest neighbor method.

The Backus-Gilbert technique also allows resolution enhancement but with a noise penalty. To avoid the noise penalty, the brightness temperatures for all channels in the 25 km grids were interpolated to the effective field of view (EFOV) at -3 db of the 19 GHz vertically polarized channel. Brightness temperatures for 85 GHz channels in the 12.5 km grids were interpolated to the EFOV of the 85 GHz vertically polarized channel. Please see Derivation Techniques and Algorithms section of this document for further details.

As the first step in the processing of Pathfinder data, a common Benchmark Period (April 1987 to November 1988) was chosen to facilitate the analysis and comparison of individual Pathfinder data products. Note that the SSM/I Benchmark Period is somewhat shorter (August 1987 to November 1988) than the Pathfinder Benchmark Period because the data record started with the launch of the DMSP-F8 satellite in July 1987.

Processing of the data has progressed over the course of several years. The goal of the product team has been to produce a consistent time series of continuous, gridded SSM/I data. However, over the course of the operational processing we have chosen at certain times to change some parts of the processing code. We have only made such changes after careful consideration of the consequences to the time series and then, only when the changes will result in an overall better quality time series. The following changes have been made.

1. We determined through visual inspections of processed data that some data for 28 June 1989 were mislocated. As a result, we added the following line to our BADLOC.D08 file in order to eliminate the erroneous orbital data:
   1989 179  8.0 1989 179  8.2
    
2. Due to an error in the processing code, time data files for the F8 data (August 1987 through December 1991) contain times that were inadvertently truncated to tenths of hours, instead of rounded to the nearest tenth of an hour. This problem was corrected beginning with data for January 1992.
    
3. The treatment of out of bounds input data changed between the F8 and F11 data. For F8 data, input brightness temperatures that were out of bounds (defined to be less than 55.0 K or greater than 320. 0 K) were replaced with the average of the two closest in bound brightness temperatures along the same scan. Beginning with F11 data (January 1992), we eliminated this treatment and set the out of bounds value to an invalid brightness temperature. The Backus-Gilbert interpolation for any EASE-Grid pixel was only performed if both of the following conditions were true:
    
   1. The interpolation coefficient corresponding to any missing brightness temperature must have been smaller than a missing coefficient threshold of 0.0005.
       
   2. The sum of the interpolation coefficients corresponding to valid brightness temperatures must have been within 1.0000 ± 0.0005 (which is the sum of all 16 coefficients).
       
4. There are no 85 GHz data for 01 February 1989 through 31 December 1991; see related note.

5. References and Related Publications

Based on the recommendations of the SSM/I Products Working Team (SPWT), the point of departure for the EASE-Grid interpolation from swath coordinates to earth-gridded coordinates is the methodology of Galantowicz and England (1991) which is based on earlier work by Stogryn (1978) and by Poe (1990). Those interested in the details of the interpolation method used in the EASE-Grid should consult the references listed below.

Brodzik, M. J. 1997. "EASE-Grid: A Versatile Set of Equal-Area Projections and Grids." NSIDC, Boulder, Colorado USA. http://nsidc.org/data/ease/ease_grid.html.

Galantowicz, J. F., A. W. England. 1991. The Michigan Earth Grid: Description, Registration Method for SSM/I Data, and Derivative Map Projections. Radiation Laboratory, Department of Electrical Engineering and Computer Science, Technical Report 027396-2-T. University of Michigan, Ann Arbor, Michigan USA.

Galantowicz, J. F. 1995. Microwave Radiometry of Snow-Covered Grasslands for the Estimation of Land-Atmosphere Energy and Moisture Fluxes. PhD Thesis, Department of Electrical Engineering and Computer Science and Department of Atmospheric, Oceanic, and Space Sciences. University of Michigan, Ann Arbor, Michigan USA.

Knowles, K. W. 1993. "Points, Pixels, Grids, and Cells -- A Mapping and Gridding Primer." NSIDC, Boulder, Colorado USA. http://cires.colorado.edu/~knowlesk/ppgc.html

Poe, G. A. 1990. Optimum Interpolation of Imaging Microwave Radiometer Data. IEEE Transactions Geoscience & Remote Sensing GE-28:800-810.

Stogryn, A. 1978. Estimates of Brightness Temperatures from Scanning Radiometer Data. IEEE Transactions Antennas & Propagation AP-26:720-726.

Wentz, F. J. 1991. User's Manual SSM/I Antenna Temperature Tapes, Revision 1. Remote Sensing Systems Technical Report 120191. Santa Rosa, California USA.

Wentz, F. J. 1992. Final Report, Production of SSM/I Data Sets. Remote Sensing Systems Technical Report 090192. Santa Rosa, California USA.

Wentz, F. J. 1993. User's Manual SSM/I Antenna Temperature Tapes, Revision 2. Remote Sensing Systems Technical Report 120193. Santa Rosa, California USA.

Table 8 lists related documents available on NSIDC's Web site.

6. Document Information

Acronyms

Table 9 lists the acronyms used in this document.

Document Creation Date

May 1998

Document Revision Date

January 2008
July 2007
January 2007
November 2005
August 2004

Document URL

http://nsidc.org/data/docs/daac/nsidc0032_ssmi_ease_tbs.gd.html