. A NASA radar instrument has been successfully
used to measure some of the fastest moving and most inaccessible glaciers
in the world -- in Chile's huge, remote Patagonia ice fields -- demonstrating
a technique that could produce more accurate predictions of glacial response
to climate change and corresponding sea level changes. This image, produced
with interferometric measurements made by the Spaceborne Imaging Radar-C
and X-band Synthetic Aperture Radar (SIR-C/X-SAR) flown on the Space Shuttle
last fall, has provided the first detailed measurements of the mass and
motion of the San Rafael Glacier. Very few measurements have been made
of the Patagonian ice fields, which are the world's largest mid-latitude
ice masses and account for more than 60 percent of the Southern Hemisphere's
glacial area outside of Antarctica. These features make the area essential
for climatologists attempting to understand the response of glaciers on
a global scale to changes in climate, but the region's inaccessibility
and inhospitable climate have made it nearly impossible for scientists
to study its glacial topography, meteorology and changes over time. Currently,
topographic data exist for only a few glaciers while no data exist for
the vast interior of the ice fields. Velocity has been measured on only
five of the more than 100 glaciers, and the data consist of only a few
single-point measurements. The interferometry performed by the SIR-C/X-SAR was used to generate both a digital elevation
model of the glaciers and a map of their ice motion on a pixel-per-pixel
basis at very high resolution for the first time. The data were acquired
from nearly the same position in space on October 9, 10 and 11, 1994, at
L- band frequency (24-cm wavelength), vertically transmitted and received
polarization, as the Space Shuttle Endeavor flew over several Patagonian
outlet glaciers of the San Rafael Laguna. The area shown in these two images
is 50 kilometers by 30 kilometers (30 miles by 18 miles) in size and is
centered at 46.6 degrees south latitude, 73.8 degrees west longitude. North
is toward the upper right. The top image is a digital elevation model of
the scene, where color and saturation represent terrain height (between
0 meters and 2,000 meters or up to 6,500 feet) and brightness represents
radar backscatter. Low elevations are shown in blue and high elevations
are shown in pink. The digital elevation map of the glacier surface has
a horizontal resolution of 15 meters (50 feet) and a vertical resolution
of 10 meters (30 feet). High-resolution maps like these acquired over several
years would allow scientists to calculate directly long-term changes in
the mass of the glacier. The bottom image is a map of ice motion parallel
to the radar look direction only, which is from the top of the image. Purple
indicates ice motion away from the radar at more than 6 centimeters per
day; dark blue is ice motion toward or away at less than 6 cm per day;
light blue is motion toward the radar of 6 cm to 20 cm (about 2 to 8 inches)
per day; green is motion toward the radar of 20 cm to 45 cm (about 8 to
18 inches) per day; yellow is 45 cm to 85 cm (about 18 to 33 inches) per
day; orange is 85 cm to 180 cm (about 33 to 71 inches) per day; red is
greater than 180 cm (71 inches) per day. The velocity estimates are accurate
to within 5 millimeters per day. The largest velocities are recorded on
the San Rafael Glacier in agreement with previous work. Other outlet glaciers
exhibit ice velocities of less than 1 meter per day. Several kilometers
before its terminus, (left of center) the velocity of the San Rafael Glacier
exceeds 10 meters (32 feet) per day, and ice motion cannot be estimated
from the data. There, a revisit time interval of less than 12 hours would
have been necessary to estimate ice motion from interferometry data. The
results however demonstrate that the radar interferometry technique permits
the monitoring of glacier characteristics unattainable by any other means.
Spaceborne Imaging Radar-C and X-Band Synthetic
Aperture Radar (SIR-C/X-SAR) is part of NASA's Mission to Planet Earth.
The radars illuminate Earth with microwaves allowing detailed observations
at any time, regardless of weather or sunlight conditions. SIR-C/X-SAR
uses three microwave wavelengths: L-band (24 cm), C-band (6 cm) and X-band
(3 cm). The multi-frequency data will be used by the international scientific
community to better understand the global environment and how it is changing.
The SIR-C/X-SAR data, complemented by aircraft and ground studies, will
give scientists clearer insights into those environmental changes which
are caused by nature and those changes which are induced by human activity.
SIR-C was developed by NASA's Jet Propulsion Laboratory. X-SAR was developed
by the Dornier and Alenia Spazio companies for the German space agency,
Deutsche Agentur fuer Raumfahrtangelegenheiten (DARA), and the Italian
space agency, Agenzia Spaziale Italiana (ASI), with the Deutsche Forschungsanstalt
fuer Luft und Raumfahrt e.v.(DLR), the major partner in science, operations,
and data processing of X-SAR.
May 1, 1995
Note: Je ne me souviens plus de l'endroit où
j'ai trouvé ce texte. Si je le retrouve, je le préciserai
évidemment ou je créérai un lien sur le site concerné.
