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DMSP SSM/I-SSMIS Pathfinder Daily EASE-Grid Brightness Temperatures


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

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

Armstrong, R., K. Knowles, M. Brodzik, and M. A. Hardman. 1994, updated current year.. DMSP SSM/I-SSMIS Pathfinder Daily EASE-Grid Brightness Temperatures. Version 2. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the 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

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.


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 (721x721) of brightness temperatures in tenths of kelvins.

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.

Beginning with F17 files, time data are contained in 2-byte, signed integer arrays (721x721). Time data are minutes since 00:00 Coordinated Universal Time (UTC), or midnight, of the date of the enclosing file. 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 25 km resolution.

File and Directory Structure

Data on the FTP site are divided into three subdirectories, 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.

Directory structure

Figure 1. FTP Directory Structure

File Naming Convention

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

EASE-Fxx-zzyyyydddp-vV.ccc.gz

where:

Table 1. Naming Convention for Brightness Temperature Files on FTP
Variable Description
EASE Indicates EASE-Grid
xx DMSP platform ID (08, 11, 13, or 17)
zz EASE-Grid ID (NL, NH, SL, SH, ML, MH)
  N: Northern Hemisphere
  S: Southern Hemisphere
  M: full global
  L: 25 km resolution
  H: 12.5 km resolution, for 85 and 91 GHz only
yyyy 4-digit year
ddd 3-digit day of year
p Direction of pass (A: ascending, D: descending)
vV Data version number (example: v2)
ccc Channel (GHz) and polarization (19H, 19V, 22V, 37H, 37V, 85H, 85V, 91H, or 91V)
gz Identifies this as a gzipped file

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

EASE-Fxx-zzyyyydddp-vV.tim.gz

where:

Table 2. Naming Convention for Time Files on FTP
Variable Description
EASE Indicates EASE-Grid
xx DMSP platform ID (08, 11, 13, or 17)
zz EASE-Grid ID (NL, SL, or ML)
  N: Northern Hemisphere
  S: Southern Hemisphere
  M: full global
  L: 25 km resolution
yyyy 4-digit year
ddd 3-digit day of year
p Direction of passes (A: ascending, D: descending)
vV Data version number (example: v2)
tim indicates the contents are time data
gz Identifies this as a gzipped file

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

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 maps of the three different coverages.

thumbnail of EASE-Grid azimuthal Northern Hemisphere coverage thumbnail of EASE-Grid azimuthal Southern Hemisphere coverage thumbnail of EASE-Grid cylindrical global coverage

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

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 and 91 GHz channels: 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.

The EASE-Grid dimensions are 721 columns by 721 rows. For more details on the EASE-Grid, please refer to the Versions page.

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

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 (Tb) at a given wavelength (λ) is the product of the physical temperature (Tp) and the emissivity (ε) at the given wavelength of the surface viewed by the radiometer. Refer to Equation 1.

Tb(λ) = ε(λ)Tp

(Equation 1)

where:

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. Beginning with F17 time files, the missing data value is the minimum 2-byte integer, or -32768. The legitimate, non-missing value of 0 is midnight of the enclosing day, and a non-missing value of 1439 is one minute prior to midnight the following day.

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 4 summarizes these data ranges.

Table 4. Data Value Range
Parameter Unit of Measurement Data Range Missing Data Value
Brightness Temperatures (Tb) Tenths of kelvins 550 (representing 55.0 K) to 3200 (representing 320.0 K) 0
Time Minutes since midnight of the date of the enclosing file
0 (representing 0000 UTC or midnight) to 239 hours (representing 2354 UTC or 23.9 hours) 255
Time (beginning w/ F17 files) Minutes since midnight of the date of the enclosing file 0 (representing 0000 UTC or midnight) to 239 hours (representing 2354 UTC or 23.9 hours) -32768
Latitude/Longitude Hundred thousandths of degrees (1 meter precision) Latitude values range from -9000000 to 9000000 (representing -90 to 90))
Longitude values range from -18000000 to 18000000 (representing -180 to 180)
1431655765

Sample Browse Image

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

Sampe Browse Image

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

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

Table 5 outlines the processing and algorithm history for this product.

Table 5. Description of Version Changes
Data
Version
Platform Temporal Range Source Data
Version
Description of Changes
V2 F17 14 Dec 2006 — most current processing date RSS V7
  • Updated version to reflect the beginning of the F17 data record
  • The source for this data set, RSS, changed from their version from V4 to V7
  • RSS V7 cross-calibrates between all SSM/I and SSMIS sensors as well as AMSR-E and WindSat, providing interconsistency of brightness temperatures from the sensors
V1 F13 03 May 1995 — 31 Dec 2007 RSS V4 N/A
V1 F11 03 Dec 1991 — 30 Sep 1995 RSS V3 N/A
V1 F8 09 Jul 1987 — 31 Dec 1991 RSS V3 N/A
V1 F8 Not available N/A Original version of data. Note: V1 was not indicated in Version 1 file names.

Intercomparison of F13 and F17 Data

NSIDC conducted an intercomparison of F13 with F17 data during the 01 January 2007 through 31 December 2007 period. The vast majority of differences are between 0.5 K and 2 K. Some larger differences, up to 10 K, are found primarily in regions of sharp gradients of brightness temperatures, along coasts and the ice edge, likely due to the changes in geolocation. Smaller biases of 0.5 K to 2 K in some channels, such as 19V, 19H, 22V, and 37H, are likely due to the cross-sensor calibration.

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 6. 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.

Table 6. Ascending and Descending Nodes by Platform
Platform Ascending Node
(Decimal Hours)
Descending Node
(Decimal Hours)
F08 6.20 18.20
F11 17.17 5.17
F13 17.58 5.58
F17 17.31 5.31

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)

bakgil_fshade.gif bakgil_fslice.gif

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.

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

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://geospatialmethods.org/documents/ppgc/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 7 lists related documents available on NSIDC's Web site.

Table 7. Related Documents
Document Description URL
All About EASE-Grid Web page that describes EASE-Grid. http://nsidc.org/data/ease/index.html
EASE-Grid: A Versatile Set of Equal-Area Projections and Grids Technical Report on EASE-Grid http://nsidc.org/data/ease/ease_grid.html
NSIDC's EASE-Grid Geolocation Tools Web page that provides descriptions of each EASE-Grid geolocation tools for the various data sets. http://nsidc.org/data/ease/tools.html
Special Sensor Microwave Imager (SSM/I) SSM/I instrument description Web page http://nsidc.org/data/docs/daac/ssmi_instrument.gd.html
Special Sensor Microwave Imager/Sounder (SSMIS) SSMIS instrument description Web page http://nsidc.org/data/docs/daac/ssmis_instrument/index.html
EASE-Grid Image Gallery Sampler image gallery of SSM/I Level 3 Brightness Temperatures. http://nsidc.org/data/docs/daac/nsidc0032_ssmi_ease_tbs/gallery/index.html
Frequently Asked Questions NOAA/NASA Pathfinder SSM/I Level 3 EASE-Grid Brightness Temperatures FAQ's http://nsidc.org/data/docs/faqs/ssmi_ease_faq.html

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
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