RADARSAT-1 Antarctic Mapping Project
Carried aloft by a NASA rocket launched from Vandenburg Air Force Base on November
4, 1995, the Canadian Radarsat-1 is equipped with a C-band Synthetic Aperture
Radar (SAR) capable of acquiring high resolution (25 m) images of Earth's
surface day or night and under all weather conditions. Along
with the attributes familiar to researchers working with SAR data from
the European Space Agency's Earth Remote Sensing Satellite and the Japanese
Earth Resources Satellite, RADARSAT-1 has enhanced flexibility to collect
data using a variety of swath widths, incidence angles and resolutions. Most
importantly, for scientists interested in Antarctica, RADARSAT-1 can be
maneuvered in orbit to rotate the normally right-looking SAR to a left-looking
mode. This 'Antarctic Mode'
provides capability for the first time of nearly instantaneous, high-resolution
views of the entirety of Antarctica. The
first Antarctic Mapping Mission (AMM-1) began on September 9, 1997 and
was successfully concluded on October 20, 1997. The
second, AMM-2 began on September 3, 2000 and was successfully concluded
on November 17, 2000. For both projects,
The Ohio State University provides overall project and scientific direction
as well as producing final products. The
Alaska SAR Facility is responsible for data acquisition and processing
signal data to images. The
Jet Propulsion Laboratory is responsible for developing acquisition strategies
and for data processing system development. Vexcel
Corporation is responsible for developing the RADARSAT Antarctic Mapping
System used by OSU to create final products. The
project is advised by a science team composed of researchers from around
the world.
(CSA illustration of the RADARSAT-1 Satellite
above. The SAR antenna shown
in gold beneath the satellite is 15 m long and 1.5 m wide.)
Antarctic Mapping Mission -1
The Goal
Antarctica is the coldest,
windiest and on average, highest continent on Earth. Shrouded
in darkness during the austral winter and often obscured from view by persistent
cloud cover, Antarctica has remained one of the most poorly mapped parts
of our planet. That situation
changed in 1997 when RADARSAT-1 began to scan Antarctica from space. The
goal of the project was to create the first, high resolution, radar image
of the continent. The resulting
map was intended to serve as a benchmark for gauging future changes in
the polar ice sheet, to understand more about the behavior of the glacier
and its interaction with the polar atmosphere and coastal ocean, and to
simply expand our ability to explore the vast, remote, and often beautiful,
southernmost continent.
The Approach
The first Antarctic Imaging
Campaign (AIC-1) was made possible by the unique capabilities of RADARSAT-1 including
an electronically steerable antenna array that provided a range of selectable
beam pointing angles. This
capability was essential for maximizing the range of the acquisition swaths
away from the satellite nadir track. The
satellite also can maneuver in orbit enabling it to change the look direction
of the SAR. These two capabilities
permitted acquisitions to the Earth's South Pole and represent technical
abilities afforded by no other civilian spaceborne radar.
AIC-1
acquisitions began seven days earlier than the anticipated start of the
nominal acquisition plan. The
early data constituted an important contingency against anomalies encountered
later in the mission. Nominal
acquisitions started on schedule shortly after noon Eastern Standard Time
on September 26. The nominal plan
was designed to obtain complete mapping coverage within 18 days. The
nominal plan proceeded nearly flawlessly through completion on October
14. It was executed in parallel with
acquisition plans for other RADARSAT-1 users and with CSA's Background
Mission. An additional opportunity
was realized because of the early start on September 19. Radar
data collected after the conclusion of the nominal mission were acquired
exactly 24 days after the beginning of the early start data. This schedule
repositioned the spacecraft to within a few hundred meters of its position
24 days earlier. Consequently
the data are suitable for interferometric analysis - a demonstrated technique
for estimating ice sheet surface displacement. Exact
repeat data collections started on October 14 and continued through October
20.
Preparations to return the satellite to normal
operations began on October 20. Arctic
mode operations resumed on October 23. Acquisitions
for customers resumed on October 26. This
occurred 9 days ahead of the planned schedule.
The Mosaic
The
RAMP mosaic, shown here at reduced ( click
here for a 5 km version ) resolution is truly a new view of Antarctica. We
observe several new and exciting features about Antarctica from the mosaic. First,
there are large-scale spatial variations in radar brightness. The
bright portion of Marie Byrd Land and the eastern sector of the Ross Ice
Shelf probably represent the region where significant melting and refreezing
occurred during an early 1990's melt event. Most
of the coastal areas and much of the Antarctic Peninsula appear bright
also because of summer melt. But
unlike Greenland, where most of the large-scale brightness patterns are
associated with firn melt facies, the remaining, strong variations in radar
brightness are poorly understood.
Many of the thousands of kilometer long curvilinear features across East Antarctica appear to
follow ice divides separating the large catchment areas. Ice
divides derived from topographic information and overlaid onto the mosaic
demonstrate the good but difficult to explain correlation between ice divides
and radar brightness patterns.
On an intermediate scale, the East Antarctic Ice Sheet appears to be very 'rough'. The
texturing is probably due to the flow of the ice sheet over a rough glacier
bed. Textures are particularly
strong paralleling the flanks of the Transantarctic, Pensacola and Shackelton
Mountains and extending deep into adjacent portions of the East Antarctic
Plateau. Long linear patterns
are strongly suggestive of subglacial geology and may indicate that the
ice sheet in this area is resting on relatively resistant basement rocks. The
texture changes abruptly across the northernmost section of the Wilkes
Subglacial Basin located in George V Land. There
the imagery shows remarkable, subtle rounded shapes similar in appearance
to the signature of subglacial lakes such as Lake Vostok.
Most intriguing are ice stream and ice stream-like features of the East Antarctic
Ice Streams in Queen Maud Land partly described in previous research using
optical imagery. Ice streams
are made visible by the intense crevassing along the shear margins where
chaotic surface roughness results in a strong radar echo. Complexities
within the interior of the ice streams are revealed by radar flow stripes
that probably originate from subtle variations in topography. Slessor
Glacier is located on the northeastern margin of the Filchner Ice Shelf. The
upper reaches of the glacier are funnel-shaped with the interior of the
funnel punctuated by patches of crevasses. The
ice stream is about 450 km long from the grounding line to the upstream
area that seems to be characterized by several long scars. The
scars are probably shear margins but it is not possible to deduce whether
they are recently initiated or relict ice stream flow.
An enormous ice stream,
reaching at least 800 km into East Antarctica, feeds Recovery Glacier. It
too is fed by a funnel-shaped catchment. Down-glacier,
crevasses cascade across the ice stream at several locations suggesting
strong variations in basal topography modulates the flow. The
confluence of a thin, elongated, 280 km-long tributary ice stream with
Recovery Glacier is located approximately 250 km from the constriction
where Recovery Glacier enters the Filchner Ice Shelf. The
central body of the pipe-like tributary is crevasse free indicating that
shear stresses are concentrated only at the margins. The tributary is an enigma in that there is little evidence for ice flow
into the tributary from the adjacent ice sheet and there is little if any
indication as to the source of ice from the up-glacier catchment region. A
less active pipe-like tributary merges with Recovery Glacier just upstream
of the grounding line. The
uppermost portion of that 300 km long tributary is dark and featureless,
similar to the eastern companion. Down-glacier,
the tributary surface is similarly mottled to the adjacent ice sheet.
Using RADARSAT SAR imagery obtained during the 1997 Antarctic Mapping
Mission, ice velocity vectors were obtained over the East Antarctic Ice
Streams (see figure above). The upstream velocity of the Recovery Glacier
is about 100 meters/year (light blue areas). Near the grounding line there
is a local peak velocity of about 900 meters/year (yellow and red areas).
Antarctic Mapping Mission - 2
Why AMM-2?
Glaciers and ice sheets
move under the load of their own weight. They
spread and thin in a fashion dictated by their thickness, the material
properties of ice, and the environmental conditions on the glacier surface,
sides and bottom. Measurements
of Antarctic Ice Sheet surface motion are of keen interest to geoscientists. The
rate and direction of motion reveals important information about the forces
acting on the glacier, provides knowledge about the rate at which ice is
pouring into the coastal seas, and enables scientists to predict how the
ice sheet might respond to changing global climate.
Glacier motion has intrigued scientists and lay people for over a century. In
1880, the American author Mark Twain wrote of his experiences with Alpine
glacier motion in his book, A Tramp Abroad , "I
was aware that the movement of glaciers is an established fact; so I resolved
to take passage for Zermatt on the great Gorner Glacier. The
next thing was how to get down the glacier comfortably. I
marched the Expedition down the steep and tedious mule-path and took up
as good a position as I could upon the middle of the glacier--because Baedeker
said the middle part travels the fastest. I waited and waited, but the glacier did not move. Night
was coming on, the darkness began to gather--still we did not budge." While
Twain was disappointed in the outcome of his idea, his approach was essentially
correct. Since the International
Geophysical Year of 1957-58 and before, scientists have placed markers
on the ice sheet and then, using a variety of navigation techniques from
sun shots to GPS, have measured and re-measured their positions to calculate
motion. More recently, scientists
have used high-resolution satellite images to track the position of crevasses
carried along with the glacier to compute surface motion. But
all of these approaches are time consuming and result in patchy estimates
of the surface velocity field.
During the early 1990's, researchers at the Jet Propulsion Laboratory showed that synthetic aperture
radar (SAR) offered a revolutionary new technique for estimating the surface
motion of glaciers. Here, the
SAR is operated as an interferometer. That
is, the distance from the SAR to a point on the surface is computed by
measuring the relative number of radar-wave cycles needed to span the distance
between the radar and the surface. Later,
another measurement is made from a slightly different position and the
numbers of cycles is computed again. The
difference in the number of cycles is used to estimate displacement to
about one quarter of a radar-wave cycle (just a few centimeters for RADARSAT-1!). The
demonstration of this technique for SARs in general and the demonstration
during AMM-1 that the technique worked for RADARSAT-1 in particular were
the impetus for the Antarctic Mapping Mission -2 (AMM-2) which occurred
during the fall of 2000 as part of a joint Canadian Space Agency and U.S.
National Aeronautics and Space Agency project to map the Southern Continent.
AMM-2 had two primary science objectives. The first
was to re-map the perimeter of the continent and the majority of Antarctica's
fast moving glaciers. Intuitively,
these are the areas that are most likely to have experienced change over
the 3 years that followed after the first mission - and as discussed below,
we have results demonstrating that this objective is a success. The
second MAMM objective was more ambitious, that being to obtain as much
surface velocity data on the ice sheet as possible.
The New Approach
Acquisitions were planned
in two ways to reach this goal. First,
data were acquired so that, where possible, the position of structures
on the glacier could be compared between the 1997 and 2000 data sets so
as to measure point velocities. Second,
and the real challenge of MAMM, was to acquire interferometric data so
as to estimate velocity fields. The
second approach required the use of RADARSAT-1 fine and standard beams,
and the unprecedented control of the spacecraft orbit and attitude. As
the mission unfolded, CSA spacecraft engineers demonstrated a remarkable
ability to navigate the satellite in the manner dictated by the science
requirements. The outcome of
this combined effort are extraordinary observations of glacier motion captured
over three, 24 day, RADARSAT cycles.
MAMM
acquired data from about 80 degrees South Latitude to the Antarctic Coast. Interferometric
SAR calculations required that this area be imaged 6 times during the mission
(three times in descending orbit mode and three times in ascending orbit
modes). The number and orientation
of acquisitions then enable scientists to measure two components of the
surface velocity vector and to remove the effects of surface topography. The
MAMM data are being processed into images by the Alaska SAR Facility. The
Ohio State University will convert the images into maps and velocity fields
using a system being developed by Vexcel Corporation. Once
complete (about 1 year for the first mosaic and about 3 years for the velocity
field maps) the products will be made available to the international science
community.
Some Early, AMM-2 Results
Even over just a three year period, the Antarctic Ice Sheet can change appreciably. Some
of the most dramatic changes
on the continent are occurring along the Antarctic Peninsula imaged in
2000 by RADARSAT-1. The black line
on the image to the left is the coastline measured using RADARSAT-1 data
acquired in 1997. The results document
the continued retreat of the northern Larsen Ice Shelf - over 30 kilometers
(18.6 miles) in just 3 years. The astonishing retreat of the Larsen Ice
Shelf is dramatically captured in this image and seems to add credence
to a 1978 prediction that collapse of the Antarctic Peninsula Ice Shelves
would a precursor signal to more global scale consequences of greenhouse
gas warming. But this is not the whole story. We also observe
places where the ice sheet is advancing, such as the Amery Ice Shelf. The
Antarctic Ice Sheet is huge, and this is the first time we have the data
to study and compare ice sheet behavior around the entire continent. These
data will help us determine whether the local changes we see represent
expected episodic behavior or whether they represent regional trends driven
by changing climate.
Lambert Glacier and Amery Ice Shelf(1997-lambert)
This mosaic of RADARSAT SAR images is centered on the Lambert Glacier, one of the largest and longest
of Antarctica's glaciers, draining about 900,000 kilometers2
of East Antarctica. Several
smaller ice streams on the eastern half of the image, channeled by numerous
exposed mountains including the Mawson Escarpment to the south, merge into
the Lambert which broadens as it eventually flows into the ocean forming
the Amery Ice Shelf. The Lambert
has clearly visible surface flowlines that extend hundreds of kilometers
into the interior. In the center
section, isolated features on the ice shelf that appear bright in the radar
image are likely due to past occurrences of surface meltwater which accumulated
into small lakes and troughs. This
mosaic was derived from RADARSAT imagery obtained during the 1997 Antarctic
Mapping Mission and shows an area approximately 900 kilometers by 675 kilometers
(560 by 415 miles). The Lambert Glacier
is centered at approximately 72 degrees south and 67.5 degrees east.
Using RADARSAT SAR imagery
obtained during the 2000 Antarctic Mapping Mission, ice velocity vectors
were obtained over the Lambert Glacier. The
areas of no motion (yellow) are either exposed land or stationary
ice. The smaller confluent
glaciers have generally low velocities (green, 100-300 meters per year)
which gradually increase as they flow down the rapidly changing continental
slope into the upper reaches of the faster flowing Lambert Glacier (click
on ant-flyover-animation). Most of the Lambert itself has velocities between
400-800 meters per year, with a slight slowing in the middle section. As
the glacier extends across the Amery Ice Shelf, velocities increase up
to 1000-1200 meters per year as the ice sheet spreads out and thins. Only a handful of in
situ velocity measurements
have been previously reported of this huge glacier system. While
the in situ and radar-derived measurements appear to be qualitatively similar,
the extent and accuracy of the new measurements are unprecedented and provide
quantitative baselines for future comparisons. The
ice velocities are obtained from pairs of images obtained 24 days apart,
using a technique called radar interferometry. This technique enables a
highly precise alignment of image pairs that provides accurate measurements
of topography as well as surfaces that have changed or moved over the short
time interval, including glaciers.
This RADARSAT SAR image
mosaic of the Amery Ice Shelf is obtained from the 2000 Antarctic Mapping
Mission. In general, the shelf
is overall brighter than the Lambert Glacier due to previous occurrences
of surface melt, which alters
the ice crystals sufficiently to produce higher radar returns. The
blue coastline is derived from the 1997 Antarctic RADARSAT mosaic, indicating
that the ice shelf edge has extended seaward about 5 kilometers (3 miles)
over the three year interval. The
yellow coastline dates from the mid-1970's, indicating that the Amery has
been in a period of general advancement, having moved about 25 kilometers
(15.5 miles) over approximately a 25 year period. Note
that the coastal areas adjacent to the Amery Ice Shelf on both sides show
little change in extent. The
two Antarctic mosaics provide highly accurate baselines needed for future
comparisons. This image is
centered at approximately 69 degrees south and 72.5 degrees east, covering
an area about 115 by 165 kilometers (70 by 100 miles).
The Shirase Glacier shows
a significant retreat in extent between the 1997 and 2000 Antarctic Mapping
Missions. The Shirase, located
in the Indian Ocean sector of Antarctica called Enderby Land (70 degrees
south, 37 degrees east), is shown in the lower central portion of the image
(right) at the head of the embayment. This
image covers an area of about 375 kilometers (240 miles) by 240 kilometers
(150 miles). As the blue coastline from the 1997 RADARSAT mosaic indicates,
the floating tongue of the Shirase extended about 12 kilometers (7.5 miles)
further northward from the current 2000 position. However,
earlier coastline maps show that the extent of the Shirase is extremely
variable.
A more detailed view of the Shirase
Glacier (left) where in addition to the 1997 coastline (blue), coastlines
are also shown from the mid-1970's (yellow) and1962 (green). In
1962 the Shirase was at its furthest extent, then retreated about 60 kilometers
(38 miles) in the mid-1970's to a position that nearly matches the 2000
position. In fact, previous
in situ measurements indicate that the Shirase was at one time one of the
fastest advancing glaciers in Antarctica. In comparison, adjacent areas
to the northwest show current positions similar to both 1962 and 1997 coastlines,
with significant retreat periods in the mid-1970's. The advance and retreat
of the Shirase is shown in the animation (click on shirase-tran). Imagery
for this animation is derived from 1962 recently declassified reconnaissance
data, optical imagery from Landsat obtained in 1988, plus radar imagery
from the 1997 and 2000 Antarctic Mapping Missions. The
Shirase drains a basin of about 165,000 kilometers2 that extends
some 500 kilometers inland from the coast. Most
of the image area offshore of the blue 1997 coastline is composed of a
mixture of bright-appearing calved icebergs and darker-appearing sea ice.
The Antarctica Peninsula
is the furthest north extension of the Antarctic continent and thus is
exposed to regional and slightly warmer climate conditions than the greater
continent. The broad Larsen
Ice Shelf (left) lies to the east, extending into the Weddell Sea, and
smaller ice shelves including the Wordie and George VI are in the southwest
corner. Most of the peninsula
is shown in this mosaic from the 2000 Antarctic Mapping Mission. The blue
coastline is derived from the 1997 Antarctic Mapping Mission.
Focussing on the northern end
of the Larsen Ice Shelf (right), earlier coastlines are plotted from the
1997 Antarctic Mapping Mission (blue), 1992 based on SAR imagery from the
European Space Agency's ERS (red, over Larsen Ice Shelf only), and the mid-1960's
(yellow). The northern Larsen Ice
Shelf has been retreating since the mid-1970s, with major collapses in
1990s, including 1995, shown dramatically here. The
southern Larsen Ice Shelf (not shown) was advancing during this same time period
until a major collapse also occurred in 1995.
The Wordie Ice Shelf (left) has
also been undergoing continued disintegration since the mid-1970's (yellow),
with significant retreat during the 1990's through 2000 (where red is the1992
coastline and blue is the 1997).
However, closer examination of
the northern and southern Larsen Shelves shows that some glaciers have
experienced advancement since 1997. In
the animation of the Larsen Ice Shelf using SAR imagery from 1992, 1997,
and 2000 (click on larsen-trans), most of the northern shelf is retreating. Close
examination of the southern Larsen Ice Shelf (lower right corner) and a
small glacier tucked south of the Sobral Peninsula (south of the large
island (Ross) in the upper part of imagery) advancement is clearly detected. These
advancements may indicate early rebuilding of the overall extent of the
Larsen Ice Shelf. These climatic-induced
shelf retreats are related to rising air temperatures that in turn warm
the surrounding oceans. Warming
in both the air and ocean underlying the ice shelves leads to increased
fracturing and eventually calving of the ice shelf fronts into icebergs.
The 1995 Larsen calving events are due to anomalously warm summer temperatures
in the early 1990s. The warming
noted in the Antarctica Peninsula, as measured from several research stations
located there, is not sufficient to affect the thicker and more extensive
West Antarctic Ice Shelves to the south on the main continent. The
two RADARSAT mosaics from the1997 and 2000 Antarctic Imaging Campaigns
provide highly accurate snapshots of this rapidly changing region of the
greater Antarctic Continent.
The Ross Ice Shelf, in
the Pacific Ocean sector, is one of the two great ice shelves of Antarctica,
along with the Ronne-Filchner Ice Shelf which is located in the Atlantic
Ocean sector. The Ross Ice
Shelf extends hundreds of kilometers off the coastal margin, draining numerous
glaciers that extend through the western Transantarctic Mountains as well
as extensive glaciers in the plateau region to the north and east of the
mountain range. The principle
research stations of the United States (McMurdo) and New Zealand (Scott)
lie on the westernmost margin of the Ross Shelf. For
most of the 1900's the Ross Ice Shelf has been slowly advancing. A major
calving event occurred in 1987 in the eastern sector, removing about 100
years of advance (5100 kilometers2) in that sector. However,
in March 2000 a significant calving event occurred in the western sector
(78 degrees South, 170 degrees west), when the ice front was at its furthermost
extent in the last 150 years and a calving event had not occurred in over
90 years. This calving formed
a gigantic iceberg (termed B-15) which was approximately 11,000 kilometers2
and covered an area about 290 kilometers (180 miles) by 15 kilometers (9
miles). Subsequently, several
smaller icebergs have calved in adjacent areas and the large iceberg (B-15) has
itself broken into smaller pieces and been swept equatorward. More
recently, a large iceberg on the far west of the Ross Ice Shelf, near McMurdo
station, also recently calved. The RADARSAT animation (click on ross_ice_shelf_loop)
shows widening of a fracture in the westernmost portion of the Ross Ice
Shelf (approximately 78 degrees South and 178 degrees East) between 1997
and 2000, covering an area approximately 225 kilometers by 140 kilometers
(140 by 85 miles). This broadening
suggests that this section of the Ross Ice Shelf, almost the last remaining
chunk of the western shelf that did not calve off in 2000, may also calve
off in the near future.
The End?
RAMP is demonstrating the technical capability to acquire nearly instantaneous
high-resolution microwave imagery of the entire Antarctic Continent. The
technical achievement is being followed by an unfolding scientific examination
that is revealing the glaciology and geology of Antarctica in unexpected
detail. As importantly, the
acquisitions provide an important benchmark for gauging and understanding
future changes in the Antarctic.
As this new century begins to unfold, it is interesting to reflect on
the fact that we now possess the ability to regularly observe the entirety
of our world with unprecedented detail and across a wide portion of the
electromagnetic spectrum. The
RADARSAT program, building on a scientific, engineering and political heritage
going back to the early days of the Corona missions, is demonstration of
that ability and of the requisite international commitment necessary to
achieve such a goal. In turn,
that ability levies a responsibility on the science community to forcefully
argue for regular acquisition of such information in a fashion that is
accessible and understandable to anyone interested in the results and pondering
their implications.
So is this the end of our looks at Antarctica? We hope not!
Additional Links
www.asf.alaska.edu
www.vexcel.com
www.space.gc.ca
www.nasa.gov
http://photojournal.jpl.nasa.gov/
ASPEN Automed Mission Planner for MAMM
For further information about RAMP contact:
Kenneth C. Jezek, Professor
Byrd Polar Research Center The Ohio State University 1090 Carmack Road
Columbus, OH 43210, 614.292.7973 jezek.1@osu.edu
