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Nimbus-7 SMMR Polar Gridded Radiances and Sea Ice Concentrations

Summary

This data set contains gridded brightness temperatures and sea ice concentrations for both polar regions. It spans the period from October 1978 through August 1987, when the Nimbus-7 SMMR scanner was turned off. The data were collected every other day at 6.60, 10.69, 18.00, 21.00 and 37.00 GHz. Due to spacecraft power limitations, the scanner operated only on alternate days. There are usually at least foutreen days of coverage per month, although major data gaps occur during the month of August. In August 1982, the following days are missing for both polar regions: 4th, 8th, and 16th; in August 1984, the 13th through the 23rd are missing for both polar regions; and in August 1985, the 6th through 8th are missing for the Southern Hemisphere.

Brightness temperature and sea ice concentration grids have 25 x 25 km grid elements in polar stereographic projection. Volume 7 contains all SMMR sea ice concentrations for both polar regions, plus five months of brightness temperature grids for the North polar region.

Brightness temperature grids are stored as 16-bit integers, one day of brightness temperature data is 0.27 mbytes for the North polar region, 0.21 mbytes for the South. Ice grids are stored as 8-bit integers. Files contain 136192 bytes and 104912 bytes for the North and South respectively. The NASA Team Algorithm (Cavalieri et al. 1984, Gloersen and Cavalieri 1986) was used to calculate ice concentrations from the brightness temperatures. Data were produced by Dr. P. Gloersen, NASA/GSFC, Oceans and Ice Branch, and are available via FTP.

Notice to Data Users: SMMR TCT tapes comprised the input data set, unlike the SMMR Digital Media product distributed by NSIDC in 1979.

Table of Contents

1. Title
2. Investigator
3. Introduction
4. Theory of Measurements
5. Equipment
6. Procedure
7. Observations
8. Data Granularity
9. Data Description
10. Data Manipulations
11. Errors
12. Notes
13. Application of the Date Set
14. Date Set Plans
15. References
16. Related Software
17. Data Access
18. Output Products and Availability
19. Glossary of Acronyms

1. Title

Data Set Identification: Nimbus-7 SMMR Polar Radiances and Arctic and Antarctic Sea Ice Concentrations

Revision Date: 28 February 1994

2. Investigator

Investigator's Name and Title

Per Gloersen
Oceans and Ice Branch
NASA Goddard Space Flight Center, Code 971
Greenbelt, MD 20771

Contact Information

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

3. Introduction

The Nimbus-7 SMMR Polar Radiances and Arctic and Antarctic Sea Ice Concentrations series is the most extensive set of SMMR grids for the polar regions to be released to date. This third SMMR series presents the entire SMMR time series for North and South polar regions (brightness temperatures and derived sea ice concentrations) in a single-channel file format. The data source for this SMMR time series is the SMMR TCT tape data set.

The following information concerning CD-ROMs is retained for historic purposes.

The two earlier CD-ROM SMMR data sets were in different formats, had different data sources, and served different purposes. The first CD-ROM, Nimbus-7 SMMR Brightness Temperature Grids, Northern Hemisphere, Volume 1, was issued by NSIDC in 1989. The disc contained grids for 29 October 1978 to 31 January 1980, presented in files containing interleaved values from all channels. The data source was the time series described in Polar Microwave Brightness Temperatures from Nimbus-7 SMMR: Time Series of Daily and Monthly Maps from 1978 to 1987 (Comiso and Zwally 1989), which used the SMMR CELL tapes as input.

In late 1990 the SMMR Polar Data Sampler CD-ROM was issued by Dr. P. Gloersen at NASA Goddard Space Flight Center, Oceans and Ice Branch. This disc presented SMMR data in both interleaved and single-channel grids, with information to guide users in voting for a preferred final format for this "real" SMMR time series, then under development by Dr. Gloersen.

This series provides the complete SMMR sensor time series brightness temperature and derived sea ice concentration data, gridded to the SSM/I polar stereographic grids for the North and South polar regions.

4. Theory of Measurements

See Data Processing Description.

5. Equipment

Please see Platform and Sensor Description and History. Please also see the SMMR instrument description document.

6. Procedure

7. Observations

Data Notes

The SMMR sensor was operated ever other day to conserve power. Therefore, data were collected on alternating days, and there are occasional gaps in the data. Usually at least 14 days of coverage occurred per month although time gaps in the SMMR data of varying durations occurred during the mission. The table below summarizes the total instrument "on" and "off" times each year, with percentage estimates of missing data during "on" time. The tabulated values were estimated from data times recorded on the input antenna temperature tapes. The high percentage of data missing during 1987 is due to the presence of several large data gaps in the 8- to 20-hour range. At this stage in the mission, the Nimbus-7 spacecraft began to exhibit power supply degradation, and the instrument on-off cycling modes were changed to conserve power and to focus on priority science objectives between instruments.

          Table 4: Nimbus-7 SMMR Operations Summary
          ------------------------------------------------------------------------
          Time Period              "Off" Time   "On" Time    % Data Missing
                                   (days)       (days)       (During "On" Time)

          10/25/78 - 11/16/78         0           22.04         7.51
          11/17/78 - 12/31/78        24.32        20.68         4.17
          1979                      187.14       177.86         3.14
          1980                      186.69       179.31         0.98
          1981                      186.84       178.16         0.69
          1982                      187.90       177.10         0.87
          1983                      180.61       184.39         1.66           
          1984                      188.56       177.44         1.32
          1985                      187.85       177.15         0.64
          SOP*                       62.57        20.43         *
          1986 (excl. SOP)          147.77       134.23         0.50
          1987                      101.58       130.42        17.00
          TOTAL                    1612.71      1608.33         2.60

*From 2 April 1986 to 23 June 1986, a special operation was under way during
which the SMMR was switched off more frequently. During this special operation
period (SOP), data from the Northern and Southern Hemispheres were collected on
alternate days. 

8. Data Granularity

SMMR brightness temperatures and sea ice concentrations are gridded in polar stereographic projection with grid elements of 25 X 25 km.

9. Data Description

Spatial Characteristics

Spatial Coverage Map

Arctic Coverage Map

Antarctic Coverage Map

Projection

See Data Content and Structure.

Grid Description

The maps are constructed by projecting points on the Earth's surface onto a plane tangent to the surface at one of the poles, with the vertex of the projection being the other pole (Parkinson et al. 1987).

Data are displayed in half-degree grids. The maps have the following specifications:

The poles represent the origin of the grids, although they are not at the centers of the grids. The (x,y) is (154.5, 234.5) for the North Pole and (158.5, 174.5) for the South Pole, measured from the upper left-hand corner. The northern grid is 304 pixels by 448 pixels. The southern grid is 316 pixels by 332 pixels.

Please see the following explanation for more details.

Temporal Characteristics

Temporal Coverage

The SMMR sensor operated 25 October 1978 through 20 August 1987. The sensor was placed in an alternate-day operating pattern on 19 November 1978 due to spacecraft power limitations. The SMMR provided complete global coverage ever six days. Polar regions (poleward of 72 degrees) have complete coverage for each day the sensor was recording data.

Temporal Coverage Map

Temporal coverage maps are not available for these data.

Data Source

See History and Data Source.

Format

Radiances are in 0.1 Kelvins, stored as 2-byte integers, with the least significant byte (lsb) first (lower address) and msb second (higher address). Thus, a value of 1577 represents a brightness temperature of 157.7 kelvins. Each record describes a grid row. The fixed-length records that comprise the Northern Hemisphere SMMR grids are 608 bytes (2 bytes * 304); in the south, the records are 632 bytes.

Unix workstation and Macintosh users should swap bytes in the integers before processing the data.

A land mask is built into the ice concentration maps, so no single-byte land mask overlay is provided. The 2-byte radiance land mask will not work. Single-byte lats/lons must be overlaid with 'ltlnov_n.con' or 'ltlnov_s.con'; however, note that the byte-value for the lats/lons is 251 rather than the 1 used in the 'ltlnovrl.ntb' files. The byte assignments for the ice concentrations are as follows:

000 Missing data
001 - 009 Not used
010 - 235 Ice concentration from 0 - 100% in 0.4444...% steps; thus, nine steps = 4%.
236 - 250 Not used
251 Latitude circles and longitude lines (when added)
252 Political boundaries
253 Coastlines
254 Land mask
255 Missing data

10. Data Manipulations

Formulae

Derivation Techniques and Algorithms

See:

Data Processing Sequence

See Data Processing History.

11. Errors

Sources of Error

Warning to Users of 10 GHz Brightness Temperatures:

The 10.69 GHz brightness temperatures included in this data set have not been corrected for instrument drift or errors dependent on ecliptic angle, nor have empty grid elements within the swaths been filled by interpolation. The 10.69 GHz brightness temperatures were included by popular request of those responding to the questionnaire accompanying the SMMR Polar Data Sampler CD-ROM. The drift patterns of the 10.7 GHz channels differ from the other SMMR channels in that the warm reference drifts more than the averaged oceanic brightness temperatures. A scheme for correcting this behavior has not yet been developed.

In addition, a corrupt sea ice concentration file has been discovered on volume 7 of this series. The file is: 860318n.con. There are no plans to replace this file at this time.

See also Effects of SMMR Antenna Patterns on Mapping Data for a note on the possible appearance of false impressions of sea ice within 50 to 100 km of coastal boundaries.

12. Notes

See Land Masks for SMMR Grids for notes on the brightness temperature land masks and coastline overlay grids.

Limitations of the Data

The SMMR sensor was operated every other day to conserve power. Therefore, data were collected on alternating days and there are occasional gaps in the data. Usually there are at least 14 days of coverage per month except for the following cases:

13. Application of the Data Set

Information is not available.

14. Data Set Plans

Information is not available.

15. References

See Selected References on SMMR.

16. Related Software

Software Description

Though NSIDC no longer distributes these data on CD-ROM, this documentation is retained for users who have previously acquired the data via CD-ROM. The following Web page provides more information regarding these data on CD-ROM:

CD-ROM Content and Structure

17. Data Access

These data are available via FTP.

18. Output Products and Availability

For a list of other related sea ice products, please visit Sea Ice Products at NSIDC. This site offers a complete summary of sea ice data derived from passive microwave sensors and other sources, and is useful for users who want to compare characteristics of various sea ice products to understand their similarities and differences. This site also provides links to tools for passive microwave data and a list of other sea ice resources.

19. Glossary of Acronyms

See the acronym list.

Further information options available from NSIDC