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Data Set Documentation

SEASAT and GEOSAT Altimetry Data for the Antarctic and Greenland Ice Sheets

Summary

Note: This data set is now on FTP so references to CD-ROM are historic and no longer applicable.

The SEASAT and GEOSAT Altimetry Data for the Antarctic and Greenland Ice Sheets data set contains surface elevations of the Antarctic and Greenland ice sheets derived from SEASAT and GEOSAT radar altimetry data. The SEASAT data were collected for a continuous 90 days in 1978, at latitudes between 72 degrees South and 72 degrees North. GEOSAT was launched in 1985 and placed in a nearly identical orbit to SEASAT, also at latitudes of between 72 degrees South and 72 degrees North. The orbit was designed to provide high-density measurements over the Earth's surface, at a maximum grid spacing of 2.7 kilometers at the equator and much denser spacing over polar ice sheets. Data were acquired between April 1985 and September 1986. Software to extract and browse subsets of these data is included. The software also allows the user to view contours created from the gridded data and groundtracks of the full-rate data.

For more information about the data set, please contact NSIDC User Services.

Table of Contents

1. Data Set Overview
2. Investigators
3. Theory of Measurements
4. Equipment
5. Data Acquisition Methods
6. Observations
7. Data Description
8. Data Organization
9. Data Manipulations
10. Errors
11. Notes
12. Application of the Data Set
13. Future Modifications and Plans
14. Software
15. Data Access
16. Output Products and Availability
17. References
18. Document Information

1. Data Set Overview

This data set contains surface elevations of the Antarctic and Greenland ice sheets derived from SEASAT and GEOSAT radar altimetry data.

Data Set Identification

SEASAT and GEOSAT Altimetry Data for the Antarctic and Greenland Ice Sheets

Data Set Introduction

This data set contains surface elevations of the Antarctic and Greenland ice sheets derived from SEASAT and GEOSAT radar altimetry data. The SEASAT data were collected for a continuous 90 days in 1978, at latitudes of between 72 degrees South and 72 degrees North. GEOSAT was launched in 1985 and placed in a nearly identical orbit to SEASAT, also at latitudes of between 72 degrees South and 72 degrees North. Data from GEOSAT were acquired between April 1985 and September 1986.

Objective/Purpose

The SEASAT/GEOSAT altimetry data were collected to create surface elevation maps of the Greenland and Antarctic ice sheets (below 72°). Ice sheet surface topography is needed for determining ice dynamics (direction and magnitude of ice flow). Ice elevation is required for improved estimates of changes in ice sheet mass balance.

Summary of Parameters

The data set consists of radar ice altimetry data collected at latitudes of between 72 degrees South and 72 degrees North. Data from SEASAT were acquired for a continuous 90 days in 1978; and data from GEOSAT, between April 1985 and September 1986.

Related Data Sets

2. Investigators

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. Theory of Measurements

This section is not applicable.

4. Equipment

Sensor/Instrument Description

Please see radar altimeter description.

Collection Environment

Satellite

Source/Platform

Please see Seasat and Geosat instrument descriptions.

5. Data Acquisition Methods

Initially acquired by the Johns Hopkins APL (Applied Physics Lab) satellite tracking facility, the raw altimetry satellite data from SEASAT and GEOSAT were passed on to NASA, via the U.S. Navy. NASA developed slope correction routines for the higher slopes over the ice sheets, relative to ocean surfaces. The data are height profile (Level 3) data and gridded height (Level 4) data provided by the Oceans and Ice branch of the Laboratory for Hydrospheric Physics of Goddard Space Flight Center. Elevations from the full data rate (i.e., one measurement every 662.5 m) are provided in georeferenced databases. These elevations are relative to the OSU91A geoid. Gridded elevations at 10-kilometer and 20-kilometer spacing are provided in the gridded data sets created from the GEOSAT and SEASAT data, respectively.

6. Observations

This section is not applicable.

7. Data Description

Spatial Characteristics

Although designed for data collection over oceans, SEASAT collected over 600,000 altimeter measurements from the continental Antarctic and Greenland ice sheets. Over sloping and undulating surfaces or surfaces with variable reflective characteristics, SEASAT altimeter radar pulse measurements accelerated faster than the response capability of the altimeter tracking circuit, necessitating a retracking correction for each range value. For GEOSAT, the earliest return altimeter signals came from ocean wave crests. Its radar altimeter was better designed to track over undulating surfaces. During its mission, GEOSAT provided unprecedented ground coverage with a maximum groundtrack grid spacing of 2.7 kilometers at the equator and much denser spacing over the polar ice sheets.

Spatial Coverage

SEASAT and GEOSAT covered the latitudes between 72 degrees South and 72 degrees North. (Specifically, this data set contains surface elevations of the Antarctic and Greenland ice sheets.)

Spatial Coverage Map

Greenland Coverage
Spatial Coverage Map - Greenland

Antarctic Coverage (Partial)
Spatial Coverage Map - Antarctica (Partial)

Spatial Resolution

Gridded elevations at a spacing of 10 kilometers are available for GEOSAT data and at a spacing of 20 km for SEASAT data.

Projection

The SEASAT and GEOSAT grids provide the surface topography of the ice sheets relative to the OSU91A geoid. Grid points are on a polar stereographic projection.

Grid Description

The SEASAT and GEOSAT data were gridded independently of each other. Gridded elevations are derived from the data by a weighted fitting of a biquadratic function to the elevation data that fall within a certain radius of the grid location. Where the data distribution is sparse, a bilinear function is used.

Temporal Characteristics

Temporal Coverage

SEASAT: June 28, 1978 to October 10, 1978 GEOSAT Geodetic Mission (GM): March 12, 1985 to September 1986

Temporal Coverage Map

A temporal coverage map is not included in this document.

Data Characteristics

Parameter/Variable

Surface elevations. Please see Variables.

Variable Description/Definition

The heights of the Earth's surface, measured vertically from space.

Unit of Measurement

[meters] [m]

Data Source

Radar altimeter

Data Range

For the SEASAT database:

Hsc = The height of the spacecraft above the IUGG 1980 Geodetic Reference ellipsoid (Moritz 1980) calculated from the GEM-T2 orbits
Halt = The altimeter-measured range
dHret = The retracking correction
dHcg = The center of gravity correction to the range
dHion = The ionospheric delay correction to the surface height
dHtrop = The tropospheric delay correction to the surface height
dHtide = The solid tide value

For the GEOSAT database, range is the same, except:

dHion = 0; no correction has been applied due to the ionosphere delay
(The range of this correction is approximately 2-3 cm)
Hsc   Based on the navy precision orbits Smith et al. 1987
dHslp = Slope correction

Sample Data Record

723
003 9  9999.9 1954 04 20 0000 -9 166 9999 86.60 182.80     3850    13000 99 9 99 018 ??????? -9 -9 -9 -9 -9 -9 -9 -9 -9 -9     Old latlon 86.60 182.85 E
99    3850 -9 1  600.00 -9 1 -32.10 -9 1 -99.9 0 210 -9 1   5.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??
99    4000 -9 1  587.00 -9 1 -33.30 -9 1 -99.9 0 208 -9 1   5.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??
99    5000 -9 1  509.00 -9 1 -40.80 -9 1 -99.9 0 204 -9 1   7.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??
99    5120 -9 1  500.00 -9 1 -41.50 -9 1 -99.9 0 204 -9 1   7.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??
99    5820 -9 1  448.00 -9 1 -48.90 -9 1 -99.9 0 215 -9 1   6.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??
99    6000 -9 1  439.00 -9 1 -48.90 -9 1 -99.9 0 217 -9 1   6.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??
99    6620 -9 1  400.00 -9 1 -48.90 -9 1 -99.9 0 207 -9 1   6.0 -9 1 9 99.9 -9 0 -99 ?????????????? ??

8. Data Organization

Data Granularity

A granule is the smallest aggregation of data that is independently managed (i.e., described, inventoried, retrievable). Granules may be managed as logical granules and/or physical granules. The data in this data set represent satellite altimeter measurements of 1.6-km in diameter retrieved at 0.66-km intervals along the satellite tracks.

Data Format

The SEASAT and GEOSAT Altimetry Data for the Antarctic and Greenland Ice Sheets is in Unix-compressed raw (.dat) format. ASCII tables of elevation data can also be accessed from software included on the CD-ROM.

9. Data Manipulations

Formulae

The surface elevation in the SEASAT database, Hdb, was calculated using the following:

Hdb = Hsc - Halt - dHret - dHcg + dHion + dHtrop - dHtide

The surface elevation in the GEOSAT database was calculated in a similar manner, except as stated in Derivation Techniques and Algorithms.

Derivation Techniques and Algorithms

For the GEOSAT database, dHion = 0. No correction was applied due to ionosphere delay. The range of this correction is approximately 2-3 cm.

Hsc is based on the Navy precision orbits (Smith et al. 1987) and referenced to the WGS-84 ellipsoid.

A value for the slope correction, dHslp, is written on each database record, but has not been applied to Hdb. This correction accounts for the fact that the altimeter measurement is to the closest point within the radar beam, which over sloping surfaces is not the subsatellite point (Brenner et al. 1983). When dHslp is undefined, a value of -999999999 is placed in that field. To obtain a slope-corrected surface elevation, Hcor, use the following:

Hcor = Hdb - dHslp

Data Processing Sequence

Processing Steps

SEASAT and GEOSAT topographical data were gridded independently of one another. The surface topography of the ice sheets is given relative to the OSU91A geoid. The gridding procedure is described in Zwally et al. 1990.

Gridded elevations are derived from the data by a weighted fitting of a biquadratic function to the elevation data that fall within a certain radius of the grid location. Where the data distribution is sparse, a bilinear function is used. Additional corrections are applied to the range measurements to account for atmospheric effects and satellite geometry. The elevation data without the slope correction are used and the resulting gridded heights are slope-corrected using the GEOSAT GM grids in an iterative manner. The retracking procedure is described in Zwally et al. 1983.

Calculations

Special Corrections/Adjustments

Please see Derivation Techniques and Algorithms.

10. Errors

Sources of Error

Precision of surface elevations derived from SEASAT and GEOSAT altimetry measurements is directly proportional to the precision of the missions' orbits. NASA has been calculating more precise orbits using improved gravity models and the full complement of leased GEOSAT tracking data. Preliminary results show that the radial orbit errors which map directly into the elevation should reduce from 50- to 70-cm rms to under 20-cm rms. Updates are planned as better orbits for existing data sets are calculated and more data becomes available.

A significant source of error in the absolute elevations derived from the radar altimeters is the lack of accurate data for the Earth geoid in the Antarctic region. However, this has no effect on relative elevations and the mass balance and ice dynamics interpretations that are derived from them.

11. Notes

Limitations of the Data

At the highest latitudes, the ground track spacing is very dense. This spacing becomes greater (data points are more sparse) at lower latitudes, so there is less cross-track information available. The surface topography estimates are more smoothed at these lower latitudes as a result.

12. Application of the Data Set

This data set provides an accurate topographical map of the Greenland and Antarctic ice sheets against which future variations in surface characteristics can be detected. The collection of surface elevation data from the Antarctic and Greenland ice sheets contributes to the study of ice dynamics and to possible detection of changes in global ice volume.

13. Future Modifications and Plans

Please see Sources of Error.

14. Software

Software Description

Software to extract and browse subsets of these data is included on one CD-ROM. Elevations from the full data rate (662.5 m along track) are provided in ASCII format in the georeferenced databases. Gridded elevations at 10-km and 20-km spacing are provided in the gridded data sets created from the GEOSAT and SEASAT data, respectively. Software, referred to as the Ice Altimetry System (IAS), has been supplied to extract and browse subsets of these data. The IAS software also allows the user to view pregenerated contours created from the gridded data and groundtracks of the full-rate data.

The extracted subsets are written as ASCII files that can be read and processed by general graphics or data analysis packages or by custom user programs. The binary formats of the georeferenced databases and grids are provided with sample software for users who wish to directly read them.

Software Access

To order a CD-ROM, please contact NSIDC User Services.

15. Data Access

Access the data via FTP.

Data Center Identification

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

16. Output Products and Availability

The SEASAT and GEOSAT Altimetry Data for the Antarctic and Greenland Ice Sheets is available via FTP. The data set contains the continental ice sheet elevations with all appropriate corrections applied, at the full data rate, in a georeferenced database. Gridded data sets with 20-km and 10-km spacing for SEASAT and GEOSAT, respectively, are also provided for users who do not require the detail of the full-rate data.

17. References

Brenner, A. C., R. A. Bindschadler, R. H. Thomas, and H. J. Zwally. 1983. Slope-induced errors in radar altimetry over continental ice sheets. Journal of Geophysical Research 88.

Frain, William E., Michael H. Barbagallo, and Raymond J. Harvey. 1987. The design and operation of Geosat. Johns Hopkins APL Technical Digest 8(2): 184-9.

MacArthur, John L., Paul C. Marth, Jr., and Joseph G. Wall. 1987. The Geosat radar altimeter. Johns Hopkins APL Technical Digest 8(2): 176-81.

Moritz, H. 1980. Geodetic reference system. Bulletin of Geodesy 54.

National Snow and Ice Data Center. (n.d.). Ice Altimetry CD-ROM Users' Manual. Boulder, Colorado: NSIDC Distributed Active Archive Center, University of Colorado at Boulder.

Zwally, H. Jay, and R. A. Bindschadler. 1983. Surface elevation contours of Greenland and Antarctic ice sheets. Journal of Geophysical Research 88(C3): 1589-96.

Zwally, H. J., A. C. Brenner, J. A. Major, T. V. Martin, and R. A. Bindschadler. 1990. Satellite radar altimetry over ice: Volume 1-Processing and corrections of SEASAT data over Greenland. NASA Reference Publication 1: 1233.

Zwally, H. Jay, Judy A. Major, Anita C. Brenner, and Robert A. Bindschadler. 1987. Ice measurements by GEOSAT radar altimetry. Johns Hopkins APL Technical Digest 8(2): 251-4.

Zwally, H. Jay, Judith A. Major, Anita C. Brenner, Robert A. Bindschadler, and Thomas V. Martin. 1990. Satellite radar altimetry over ice: Volume 2-Users' Guide for Greenland Elevation Data from SEASAT. NASA Reference Publication 2: 1233.

18. Document Information

Revision Date: February 5, 1998
Review Date: February 5, 1998
Document ID: NSIDC-0053

Citation:

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.

Zwally, J. and A. Brenner. 1998. Seasat and GEOSAT altimetry for the Antarctic and Greenland Ice Sheets. Boulder, CO, USA: National Snow and Ice Data Center. CD-ROM.

Document Curator: NSIDC Writers
Document URL: http://nsidc.org/data/docs/daac/nsidc0053_seasat_geosat.gd.html