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In 1997, the Canadian RADARSAT-1 satellite was rotated in orbit, so that its synthetic aperture radar (SAR) antenna looked south towards Antarctica. This permitted the first high-resolution mapping of the entire continent of Antarctica. In only eighteen days, the satellite acquired a complete coverage of radar image swaths as part of the first Antarctic Mapping Mission (AMM-1). Swath images have been assembled into an image mosaic depicting the entire continent at 25 m resolution. The mosaic provides a detailed look at ice sheet morphology, rock outcrops, research infrastructure, the coastline, and other features of Antarctica, as well as representing calibrated radar backscatter data which may provide insight into climate processes affecting the upper few meters of snow cover.
Data are available from both the Alaska Satellite Facility (ASF) and the National Snow and Ice Data Center (NSIDC), and are offered in a variety of formats. A series of approximately 90 image tiles covers the entire continent at 25 m resolution, and single mosaic images derived from these tiles cover the continent at resolutions ranging from 125 m to 1 km. The single images provide good detail in convenient image formats. The tile products preserve the highest resolution of spatial detail, and retain the quantitative measure of backscatter intensity, but are necessarily large files that require assembly by the user.
The RAMP AMM-1 SAR Image Mosaic of Antarctica is a product of the National Aeronautics and Space Administration (NASA) Pathfinder Project "RADARSAT-1 Antarctic Mapping Project (RAMP)," led by Dr. Ken Jezek of the Byrd Polar Research Center at Ohio State University. RAMP is a joint effort of NASA and the Canadian Space Agency (CSA), which designed and operates the RADARSAT-1 satellite. NASA launched the Canadian RADARSAT-1 satellite in exchange for the right to access data on a pro rata basis. Canadian partners also include the Canadian Centre for Remote Sensing (CCRS) and RADARSAT International (RSI). U.S. partners include Vexcel Corporation, NASA's Alaska Satellite Facility (ASF), and NASA's Jet Propulsion Laboratory (JPL). Additional support was provided by NASA's Goddard Space Flight Center (GSFC), the Environmental Research Institute of Michigan (ERIM), the National Science Foundation (NSF), the National Imagery and Mapping Agency (NIMA), and the National Snow and Ice Data Center (NSIDC).
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.
Jezek, K., and RAMP Product Team. 2002. RAMP AMM-1 SAR Image Mosaic of Antarctica. Fairbanks, AK: Alaska Satellite Facility, in association with the National Snow and Ice Data Center, Boulder, CO. Digital media.
Copyright notice: AMM-1 data are copyrighted by the Canadian Space Agency. Only NASA-approved RAMP investigators may access or use the 25 m resolution AMM-1 data. No restrictions apply to lower-resolution data, other than a requirement to cite the data as indicated above.
This collection includes images at a wide variety of resolutions, and images from both Versions 1 and 2 of the mosaic, which was updated in 2001 in order to correct a variety of image processing problems. The resulting Version 2 of the mosaic offers reduced antenna pattern banding, elimination of "ghost" features, improved georegistration, and other corrections as described in the Updates section below. Except for the 250 m resolution Version 1 image distributed on CD-ROM, NSIDC and ASF distribute only Version 2 products.
The following products are available:
RAMP AMM-1 SAR Image Mosaic of Antarctica
Dr. Kenneth C. Jezek, Principal Investigator
Department of Geological Sciences
and Byrd Polar Research Center
The Ohio State University
Columbus, OH
Dr. John C. Curlander
Vexcel Corporation
Boulder, CO
Dr. Frank Carsey
Jet Propulsion Laboratory
Pasadena, CA
Dr. Carl Wales
Alaska Satellite Facility
Geophysical Institute
University of Alaska Fairbanks
Fairbanks, AK
Dr. Roger Barry
National Snow and Ice Data Center
University of Colorado
Boulder, CO
Please direct all inquiries regarding these data to either NSIDC or ASF User Services:
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
ASF User Services
Alaska Satellite Facility
Geophysical Institute
University of Alaska Fairbanks
903 Koyukuk Drive
Fairbanks, AK USA 99775-7320
phone: +1 (907) 474-6166
fax: +1 (907) 474-2665
e-mail: uso@asf.alaska.edu
The RAMP AMM-1 SAR mosaic contains a great deal of information about flow features, surface undulations, crevasses, melt features, snow grain size, and accumulation rates.
The mosaic is an excellent basemap for field work, which will aid with logistical planning and safety. The mosaic is valuable for locating crevassed regions, even when these regions are covered by several meters of snow. Despite the mosaic's maximum resolution of 25 m, the high surface roughness of crevasse areas raises the backscatter well above the value for uncrevassed snow.
The high resolution and unique coverage of these data will allow for new scientific investigation of a variety of remotely-sensed features. For example, flow features or flowstripes are readily visible in the image mosaic, and may be mapped to determine the drainage paths and extents of outlet glaciers. The 'undulation field' (i.e., the 1 to 20 km scale relief caused by the flow of thick ice over irregular bedrock) is also shown in detail. However, its appearance in these radar backscatter data results from a combination of the actual surface shape and variations in snow characteristics. In many areas, the effect of snow structure on radar backscatter exceeds the effect of surface shape.
Additionally, field camps and features such as runways and heavily traveled snowmobile tracks can be identified, providing an opportunity to locate former field sites and to check image geolocation accuracy.
Please see Jezek (1999) for further description of features revealed in the mosaic. The ASF SAR Research Bibliography lists sources for more information regarding general applications of SAR data.
The interactions between radar signals and the ground surface depend upon many factors, including the density and dielectric properties of surface materials, vegetation cover, surface roughness at the scale of the signal's wavelength, topographic variations and the instrument's look angle, and signal polarization. The resolution of the image product is especially affected by signal strength, chirp pulse length and bandwidth, return signal integration time, and the time between pulse transmissions.
For more information, see the following documents:
For general information about the mathematical derivations and theories behind SAR processing algorithms, see Coert Olmsted's Alaska Satellite Facility Scientific SAR User's Guide (PDF file) or other SAR manuals listed in the references section. For a description of the specific algorithms used in ASF's SAR processor, see the ASF SAR Processing Algorithm Document (coming soon to ASF's Web site).
1. Processing raw data to create swaths
The ASF SAR Processing Algorithm Document (currently unavailable online) describes processing of the raw signal data.
2. Generating image tiles from swaths
Tiles were created from blocks at the Byrd Polar Research Center (The Ohio State University) using the RADARSAT Antarctic Mapping System (RAMS), developed by Vexcel Corporation. Swath data were converted to georegistered blocks, and blocks to tiles, in the following steps:
Block Processing Stage:
Tile Processing Stage:
The resulting 25 m resolution 16 bit tiles are available to approved users only. NSIDC's RAMP Basics page offers a description of the tiling scheme and other graphical information about blocks and tiling.
3. Creating the 125 m continent-wide mosaic image from multiple image tiles
The first step in creating the mosaic was to convert the RAMS output -- 100 m resolution 16 bit data -- into 8 bit data, in order to resolve disk space issues and processing time problems associated with 16 bit data. The following equation was used to convert 16 bit RAMS output into 8 bit unsigned integer:
DN = 7 * [10 * log(RN2) - 40]
where RN represents the 16 bit RAMS DN number and DN represents the 8 bit DN number.
Next, the 8 bit generic binary data were geocoded in a conversion from ERDAS Imagine to ARC/INFO GRID files. ARC/INFO GRID files were then merged to create the mosaic.
The bilinear interpolation method was utilized in an ARC/INFO environment to convert 100 m resolution data into 125 m data. The final mosaic image is available in generic binary format.
The images in this data collection are visual representations of variations across Antarctica in radar backscatter qualities, which can be described by the variable termed "sigma-naught," the radar backscatter coefficient.
Sigma-naught can be defined as "a quantification of the ability of an object to scatter the incident microwave radiation back toward the radar instrument." Publications regarding SAR data often refer to sigma-naught, and may provide alternative definitions. Terms such as radar cross-section or radar brightness are quite similar.
ASF defines sigma-naught as:
10 * log{a2 * [d - (a1 * n(r))] + a3}
where:
d = pixel intensity (data number)
a1 = noise scaling
a2 = linear conversion
a3 = offset
n(r) = noise as a function of range
See the section of this document entitled Collection Contents, above.
Products are currently available as follows:
Request data using NSIDC's online RAMP mosaic data order form or by contacting ASF User Services (uso@asf.alaska.edu).
A browse tool, which requires a Windows98 platform, is included on the 250 m CD-ROM.
Each 25 m tile is packaged with a variety of software tools. These C programs contain documentation on compiling and linking in their headers. A Makefile is also provided to help users build and link the software. A more detailed explanation of each program can be found in the 25 m tile README file, but brief descriptions are provided below:
GEOMAP.c - Converts latitude and longitude to x, y map coordinates
MAP2GEO.c - Converts x, y map coordinates to latitude, longitude
TILE2MAP.c - Converts RAMS tile name, line (col), and sample (row) to RAMS map x, y coordinates
MAP2TILE.c - Gives the sub-tile name, column, and row with a specified map x,y coordinate is contained
MERGEMAP.c - Generates a mosaic of SAR image sub-tiles within an area specified by center point and size. To generate an image for the entire tile the center point can be found in the file "file name"
GETSIG0.c - Prints the "original" pixel value in power for a pixel at image location (x,y) in map coordinates
RAMP images are archived at the Alaska Satellite Facility in Fairbanks, Alaska, and at the National Snow and Ice Data Center in Boulder, Colorado. For information please contact either NSIDC or ASF User Services:
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
ASF User Services
Alaska Satellite Facility
Geophysical Institute
University of Alaska Fairbanks
903 Koyukuk Drive
Fairbanks, AK USA 99775-7320
phone: +1 (907) 474-6166
fax: +1 (907) 474-2665
e-mail: uso@asf.alaska.edu
Data cover the entire continent of Antarctica, outlying islands immediately adjacent to the coast, and an approximately 100 km wide band of sea ice, icebergs, and landfast ice surrounding the coast. Sea ice coverage was increased slightly for Version 2 of the mosaic to provide a view of typical early spring sea ice and fast-ice extent and interaction with coastal features.
See the mosaic overview available from NSIDC's RAMP Basics page.
Images in this collection have resolutions (pixel size equivalents) ranging from 25 m to 1 km.
All images use the following:
polar stereographic map projection
WGS84 ellipsoid
longitude of central meridian: 0
latitude of true scale: -71
false easting (meters): 0
false northing (meters): 0
Data were collected between 9 September and 22 October 1997.
The 25 m image tiles preserve a true quantitative measure of backscatter which may be directly related to sigma-naught. For the other products, each pixel's intensity qualitatively represents its radar backscatter intensity, but actual backscatter values have been arbitrarily adjusted to improve mosaic image quality.
Variables affecting radar backscatter include surface roughness, the surface material's dielectric properties, and the geometry between the spacecraft and target. For more information, see the SAR FAQ or the SAR Theory/Image Interpretation document (PDF file).
Backscatter intensities in decibels (dB) are converted to gray-scale values between 0 and 255 for all but the 25 m images. Values range from 0 to 216 for the 25 m images. Sigma-naught values are preserved quantitatively within, and therefore can be reconstructed from, the 25 m resolution image data. Sigma-naught values are not preserved in the 125 m mosaic or the other image products derived from the 125 m data.
Version 1 of the RAMP mosaic is affected by a number of image processing problems. Version 2 benefits from the following error corrections and other updates:
The effects of automatic gain control (AGC) used in initial data gathering by the satellite remain in the Version 2 images. In collecting the data, a region of each swath is sampled for intensity, so that the gain of the processed data may be set optimally for increased contrast over the surface to be imaged. Within swaths that include abrupt changes in surface type (e.g., near the coast or near rock outcrops), this initial sample may include only water or rock outcrop, and thus the gain is set for these surfaces rather than for ice. The section of the swath is processed with gains that are not optimal for the ice sheet mosaic. The affected regions of the ice sheet show up as slightly darker blocks or corners within the mosaic.
Consult the ASF calibration group's Anomalies Matrix for a description of sources of error in raw, uncalibrated data.
As mentioned in Updates above, the automatic gain control (AGC) effect has not been addressed. The resulting anomalies appear as "seams" in affected coastal areas of both Versions 1 and 2. Known AGC problem areas are highlighted in some of the 25 m tile browse images.
In September through November of 2000, RADARSAT-1 collected a new set of SAR swath data covering Antarctica in an effort referred to as the Modified Antarctic Mapping Mission (MAMM, or AMM-2). All data are in north-looking mode, and so only regions north of about 79.8 degrees north latitude are covered. Nevertheless this represents a substantial fraction of the Antarctic ice sheet, and coverage includes the entire coastline and outlet glacier mouths. The main objective of this effort is to gather ice velocity data through interferometric SAR processing (e.g., Jezek). In addition, several image mosaics of the coastal region will probably be created. In general, these will be similar in content, appearance, structure, and application to the the RAMP AMM-1 mosaic. The mosaics may be compared to each other to reveal changes due to ice calving, crevassing, and flow since AMM-1.
RADARSAT-1 is a polar-orbiting, sun-synchronous advanced Earth observation satellite developed by the Canadian Space Agency (CSA) to monitor environmental change. NASA launched RADARSAT-1 in exchange for access to the satellite on a pro rata basis through its Alaska Satellite Facility (ASF). The Canadian Space Agency's RADARSAT page and the Canadian Centre for Remote Sensing's RADARSAT page offer general information about the RADARSAT satellite and its SAR sensor.
ASF's RADARSAT-1 Satellite document (PDF file) describes the satellite in detail, and ASF's RADARSAT-1 Left Looking RAMP SAR Images documentation (PDF file) provides information about the use of RADARSAT-1 to collect the source data for the RAMP mosaic. In addition, the following online documents provide relevant information:
RADARSAT-1 carries an advanced radar sensor called synthetic aperture radar (SAR). RADARSAT-1's SAR sensor provides its own microwave illumination and thus will operate day or night, regardless of weather conditions. Radar pulses are transmitted and the targets' radar backscatter received by the same antenna.
By sending out rapid radar pulses while orbiting overhead, the SAR is able through signal processing to simulate a large multi-antenna array to achieve high image resolution. The 15 m x 1.5 m rectangular antenna points to the side to enhance terrain variations and for technical signal processing reasons. The antenna generally "looks" to the right (north) except during the Antarctic mode, when the satellite is rotated so that the antenna is left-looking. This SAR instrument has many different beam modes which allow it to image the Earth at a variety of incidence angles and swath widths.
The following parameters describe the RADARSAT-1 SAR sensor:
Frequency: 5.3 GHz (C-Band)
Wavelength: 5.66 cm
Polarization: HH
RF Bandwidth: 11.6, 17.3, or 30.0 MHz
Pulse Repetition Frequency: 1200-1400 Hz
Transmitter Peak Power: 5 kW
Transmitter Avg Power: 300 W
Tape Recorders: 2 high speed
(10 minutes capacity)
Available SAR
Use per Orbit: 28 minutes
Radar Data Rate: 77-105 Mbps
Tape Playback Data Rate: 85 Mbps
Sample Word Size: 4 bits each I and Q
Range Chirp
Chirp Type: Linear FM down chirp
Chirp Rate/Transmit BW/
Sampling Rate: -279.300 KHz/u-sec / 11.731 MHz / 12.927 MHz
-416.200 KHz/u-sec / 17.480 MHz / 18.467 MHz
-721.400 KHz/u-sec / 30.299 MHz / 32.317 MHz
Resolution Bandwidth: 11.583 MHz / 17.282 MHz / 30.002 MHz
Transmit Pulse Width: 42.0 u-sec
For more information consult the following online documents:
Bicknell, T. 1992. Alaska Satellite Facility SAR Processor System User's Guide To Products. NASA, JPL. JPL D-9362.
Bicknell, T. and D. Cuddy. 1995. ASF Product Specification, Version 1.0. NASA, JPL. JPL D-13122, Volume 1.0.
Carande, R.E. 1988. Alaska Satellite Facility SAR Processor System Functional Design Document. NASA, JPL. JPL D-4922.
Carande, R.E., P. Jennex, and A. Schlutsmeyer. 1988. Alaska Satellite Facility SAR Processor System, Alaska SAR Processor Software Specifications Document. NASA, JPL. JPL D-5364, Volume 2.
EOSDIS Information Management System Users Manual. 1994. Hughes STX Corporation, EOSDIS IMS, NASA.
Fitch, J.P. 1988. Synthetic Aperture Radar. Springer-Verlag. New York.
Jezek, K.C. 1999. Glaciological properties of the Antarctic ice sheet from RADARSAT-1 synthetic aperture radar imagery. Annals of Glaciology 29: 286-290.
Jezek, K.C. 2002. RADARSAT-1 Antarctic mapping project: change detection and surface velocity campaign. Annals of Glaciology 34: 263 - 268.
McCandless, S.W. Jr. and S.A. Mango. 1990. The Theory, Design and Application of Space Based Synthetic Aperture Radar. University of Alaska Fairbanks, Geophysical Institute, Alaska Satellite Facility.
Olmsted, C. 1993. Alaska Satellite Facility Scientific SAR User's Guide (PDF file). University of Alaska Fairbanks, Geophysical Institite. ASF-SD-003.
Robnett, T. 1991. Alaska SAR Processor Principles of Operation. NASA, JPL.
Sabins, F.F. Jr. 1978. Remote Sensing Principles and Interpretation, Second Edition. W. H. Freeman & Company, New York.
Schlutsmeyer, A. 1991. Alaska SAR Processor Programmer's Technical Manual. NASA, JPL.
Special Issue - RADARSAT. 1993. Canadian Journal of Remote Sensing 19(4), ISSN 0703-8992.
Synthetic Aperture Radar Data Product Format Standards. 1989. CEOS-SAR-CCT, Issue 2, Revision 0.
Tabel 1 describes the acronyms and abbreviations used in this document.
| Acronym | Description |
|---|---|
| AGC | Automatic Gain Control |
| AMM-1 | Antarctic Mapping Mission 1 |
| AMM-2 | Antarctic Mapping Mission 2 |
| ASF | Alaska Satellite Facility |
| CCRS | Canadian Centre for Remote Sensing |
| CSA | Candian Space Agency |
| DEM | Digital Elevation Model |
| DLT | Digital Linear Tape |
| EOSDIS | Earth Observing System Data and Information System |
| ERIM | Environmental Research Institute of Michigan |
| FAQ | Frequently Asked Questions |
| FTP | File Transfer Protocol |
| GSFC | Goddard Space Flight Center |
| JPL | Jet Propulsion Laboratory |
| MAMM | Modified Antarctic Mapping Mission |
| NASA | National Aeronautics and Space Administration |
| NIMA | National Imagery and Mapping Agency |
| NSF | National Science Foundation |
| NSIDC | National Snow and Ice Data Center |
| RAMP | RADARSAT-1 Antarctic Mapping Project |
| RAMS | RADARSAT Antarctic Mapping System |
| RSI | RADARSAT International |
| SAR | Synthetic Aperture Radar |
April 2002
http://nsidc.org/data/docs/daac/nsidc0103_ramp_mosaic.gd.html