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Environment and Energy
Icy stare: electrical engineers focus satellite eyes on precious polar ice
By one estimate, about a tenth of this warming planet’s population lives
near a coast at an elevation of less than 10 meters. That’s 600 million
people who need someone to keep a sharp set of eyes trained on the earth’s
poles, where ice melting into the oceans could put their homes at risk. Stanford
electrical engineers Howard Zebker and Shadi Oveisgharan are developing precisely
such a sentry for detecting changes in polar ice masses.
“That’s the most direct impact, that if ice melts the ocean level
goes up,” says Zebker, a professor of electrical engineering and of geophysics. “But
if you get into the details, a loss of ice figures into the global climate
system in other ways as well.”
White snow and ice reflect a lot of solar radiation back into space that would
otherwise stick around to heat the planet. Meanwhile, the ice sheet in places
like Greenland is so thick and high that if it disappeared, the change in topography
could change regional air circulation.
While changes in ice cover are clearly important, they are not easy to measure.
Antarctica and the arctic are enormous, inhospitable places. Rather than trying
to drill core after core through the ice to check its depth directly, people
would be better off coming up with a way to remotely sense accumulation and
melting with satellites. That’s the research of Zebker and doctoral student
Oveisgharan. Measuring snowfall is not the most common job description among
electrical engineers, but their expertise in electromagnetic sensing technologies,
such as radar, has shown how to the job could be done.
“I don’t consider myself a glaciologist,” Oveisgharan says. “But
I’m really satisfied with relating our data and images by some [theoretical]
modeling to the accumulation rates and having an application for that.”
Good interference
Remote sensing of the earth is hardly a new idea, of course. Zebker’s
satellite radar group studies not only ice but also volcanoes, seismic
movements and even the surface of Saturn’s moon Titan. It’s never
as simple as taking mere pictures from above, however.
Throughout his career, Zebker has made great strides in improving radar remote
sensing technology, helping to pioneer a particularly precise technology called
interferometric synthetic aperture radar (InSAR). Although the instruments
fly 500 miles above the earth, InSAR yields data with astoundingly fine accuracy
on the scale of centimeters.
The reason for the precision is the interferometry involved. Interferometry
essentially is a way of detecting a minute change in a sample, such as a chunk
of ice, based on the differences in the radar waves it reflects back before
and after it has changed. The strength of the correlation between the two signals
is mathematically computed and ultimately used to build maps of large swaths
of geographic area.
To see how this would matter in measuring a sheet of ice, for instance, say
the satellite flies overhead in January and bounces a radar wave off a spot
on the sheet. The wave bounces back with a certain “phase,” or
position of its crests and troughs, based on the distance it had to travel.
Then the satellite returns on the same path in April, after the ice has slowly
crept downhill towards the sea, and bounces another wave off that location.
The longer distance the wave must travel to the now lower ice sheet is indicated
as a small shift in its phase compared to the first wave.
These phase differences occur on scales that are less than the 6 centimeter
wavelength of the radar (microwave) waves, which is why their correlations
show detail down in the centimeter range.
Sensing snow
Unfortunately, InSAR doesn’t work for measuring ice accumulation in
quite so straightforward a way. Historically there has been a jarring disparity
between what observers on the ground have seen directly in the layers of ice
cores and what satellite radar has purported to measure. Rectifying this disparity
and thereby making radar a reliable tool for measuring ice accumulation is
a tough challenge, but such problems are where PhDs are forged.
Oveisgharan’s work has been to figure out what’s really going
on when the radar waves penetrate the ice sheet, because the way the ice and
snow bounce them around and send them back has everything to do with the data
the radar gathers.
For example, snow in areas with slow accumulation tends to coalesce into relatively
large grains of ice, which reflect more “light” back, yielding
a “brighter” radar image. Researchers for a while figured that
bright images therefore meant slow accumulation. The problem, Oveisgharan has
found, is that they weren’t accounting for how the boundaries among annual
layers of snow also reflect light. Thicker layers reflect more, meaning that
fast accumulation can produce a bright signal, too. Clearly, brightness alone
is not the right way to measure accumulation.
What Oveisgharan has done is figure out a more sophisticated approach that
accounts for layer thickness and grain size in a more accurate way. To do this,
she has incorporated not only the brightness of radar images, but also applied
interferometry. After careful study and working with Zebker, she has developed
a set of calculations that can properly turn the raw radar data into much more
accurate estimates of accumulation rates.
In an article in the January 2007 issue of the journal IEEE Transactions on
Geoscience and Remote Sensing, she and Zebker show that their method yields
accumulation rate estimates that are much closer to actual data taken from
Greenland ice gauges.
“We have presented a model that can retrieve snow accumulation parameters
more accurately than existing remote sensing methods over a limited region
in Greenland,” they wrote. In solving this mystery, they have accomplished
an important mission.
To accomplish the broader mission of monitoring all the world’s polar
ice caps, more refinement and research is necessary. Oveisgharan’s model
only works in the roughly 50 percent of Greenland where the temperature never
gets warm enough for melting to occur.
But Zebker is optimistic that the advances she has made will make it easier
to turn InSAR into a global snow sentry.
“We could take what we have, in principle, and we can apply this continent-wide
to Greenland and Antarctica,” he says.
Hopefully there will still be a lot of ice left to see when the work is done.
JULY 2007
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Last Modified: June 25 2007 11:14:41 AM |
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Strategic Priorities
