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.
