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

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Armstrong, R. L., K. W. Knowles, M. J. Brodzik, and M. A. Hardman. 1994, updated current year. DMSP SSM/I-SSMIS Pathfinder Daily EASE-Grid Brightness Temperatures. Version 2.0. Boulder, Colorado USA: National Snow and Ice Data Center.

Overview

Platforms	DMSP-F08, -F11, -F13, -F17
Sensors	SSM/I, SSMIS
Spatial Coverage	Northern Hemisphere, Southern Hemisphere, global
Spatial Resolution	25 km for all channels (19, 22, 37, and 85 GHz), plus 12.5 km for 85 and 91 GHz
Temporal Coverage	09 July 1987 - current processing
Temporal Resolution	Daily
Parameter	Brightness temperatures (0.1 K precision)
Data Format	Brightness Temperature Files: Flat binary, 2-byte unsigned, compressed (Gzip) Time Files: Flat binary, 1-byte unsigned, compressed (Gzip)
Metadata Access	View Metadata Record
Current Version	V2.0
Data Access	FTP | Polaris Note: The data format information in this document represents the data in its native format as it is archived at NSIDC. If you have downloaded the data using Polaris, please consult the 00README file located in the tar file for information on the data format operations that were performed on this data set.

1. Contacts and Acknowledgments

Investigator

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

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

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, Ann Windnagel, Donna Scott, and Karla LeFevre, 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

Summary

The 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 from the Special Sensor Microwave/Imager (SSM/I) and the Special Sensor Microwave Imager/Sounder (SSMIS) 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 and 91 GHz channels are 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 and are available via FTP as processing is completed.

Format

File and Directory Structure

File Naming Convention

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 and Resolution

Temporal Coverage and Resolution

Coverage begins 09 July 1987 and is ongoing. See the SSM/I-SSMIS 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. For a complete list of missing dates, see the SSM/I-SSMIS Data Availability Web page. A brief list of some of the larger gaps in the data are explained in Table 3.

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 3.

Table 3. 85 GHz 2008 Missing Data

Region	Pass	Polarization	Day of Year (ddd)
Northern	Ascending	Horizontal	128
Northern	Descending	Horizontal	118 - 128
Southern	Ascending	Horizontal	123 - 127
Southern	Descending	Horizontal	114 - 116, 118, 120, 123 - 182
Southern	Descending	Vertical	117, 121
Global	Ascending	Horizontal	128 - 182
Global	Descending	Horizontal	117 - 127, 129 - 182
Global	Descending	Vertical	173, 175, 176, 179-182

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 SSM/I began in May 1995.

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

Error Sources

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

Data are available via FTP.

See the SSM/I-SSMIS Data Availability Web page for specific dates by platform. From January 2005 forward, data are no longer distributed via CD-ROM. Instead, data are made available on our public FTP site as processing is completed.

Volume

The approximate yearly volume of this data is 6.2 GB.

Software and Tools

Geolocation tools for this data set are available via the EASE-Grid Data Geolocation Tools Web page.

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, as well as the SSMIS instrument on the DMSP-F17 platform. For more information about the SSM/I instrument, please refer to the SSM/I Instrument Description Web page. For more information about the SSMIS instrument, refer to the SSMIS Instrument Description Web page.

Theory of Measurements

The SSM/I and SSMIS instruments measure passive microwave radiances. For a detailed description of how SSM/I obtains its measurements, please see NSIDC's SSM/I Instrument Description Web page for information. For more information about the SSMIS instrument, please see the SSMIS Instrument Description Web page.

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 1993).

Version History

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

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.

Processing Step Change for the DMSP-F17 SSMIS Sensor

Beginning with processing for the DMSP-F17 SSMIS sensor, EASE-Grid brightness temperature fields are gridded using an inverse distance squared method instead of the Backus-Gilbert interpolation that had been used for the earlier sensors. The Backus-Gilbert method requires analysis of the antenna pattern of the sensor to derive weighting coefficients, yet the required analysis has not yet been performed for SSMIS. Instead, the inverse distance squared method performs a weighted average based on a 2x2 km spatial kernel.

Since the two interpolation methods differ, there is a difference in the brightness temperature fields. NSIDC has investigated the difference and has found that most differences are within +/- 1 K, and the vast majority of grid cells have differences within 2 K. However, in regions with steep brightness temperature gradients, the differences can be upwards of 20 K. These regions include:

• coast lines,
• edges of swaths where overlap between swaths occurs, which is most noticeable near the poles (poleward of 60 degrees latitude), and,
• to a lesser degree, in mountainous regions.

These differences may be positive or negative and are one or two grid cells wide.

Users who make use of this extended EASE-Grid brightness temperature time series should be cautious using SSMIS data, particularly in the high difference regions noted above. NSIDC will investigate options for providing fully consistent EASE-Grid brightness temperatures as resources allow.

5. References and Related Publications

Related Data Collections

6. Document Information

Acronyms and Abbreviations

Table 8 lists the acronyms used in this document.

Table 8. Acronyms

Acronym	Description
DAAC	Distributed Active Archive Center
DMSP	Defense Meteorological Satellite Program
EASE-Grid	Equal-Area Scalable Earth-Grid
EOS	Earth Observing System
FAQ	Frequently Asked Question
FTP	File Transfer Protocol
GISMO	Graphical Interface for Subsetting, Mapping, and Ordering
NASA	National Aeronautics and Space Administration
NSIDC	National Snow and Ice Data Center
SSM/I	Special Sensor Microwave/Imager
Tb	Brightness Temperature
URL	Uniform Resource Locator
UTC	Coordinated Universal Time

Document Creation Date

May 1998

Document Revision Date

April 2011
December 2009
January 2008
July 2007
January 2007
November 2005
August 2004

Document URL

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