Sea Surveillance

Mary Keller, a remote sensing scientist in APL’s Space Exploration Sector, has more than 40 years of experience in microwave remote sensing. She developed algorithms to characterize sea ice during the summer melt while at the U.S. Naval Research Laboratory in Washington, D.C., and monitored ice year-round in both the Arctic and Antarctic for the National Ice Center in Maryland. She says the big unknown about the Arctic right now is how thick the sea ice is, an attribute critical to the climate in general.

“The older the sea ice and [the] thicker it is, the more it acts as Earth’s refrigerator,” Keller explained.

 The thick, bright-white ice reflecting sunlight back into space keeps the planet cool. But as ice begins to melt, more of the dark, heat-absorbing surface of the ocean is exposed, creating a feedback loop that accelerates the melting process. The result is the release of thick and dangerous ice chunks into the paths of ships, infrastructure and coastlines.

For years, scientists have made efforts to measure sea ice thickness. 

Both the European Space Agency’s CryoSat-2 (a delay-Doppler radar altimeter) and NASA’s ICESat and ICESat-2 laser altimeters infer sea ice thickness by measuring the height of the ice above sea level. As with ice cubes and icebergs, generally, one-tenth of the ice sits above water surface. The rest remains below.

But none of these altimeters, Keller said, has the power of a method called radar sounding.

Like synthetic aperture radar (SAR) and laser altimetry, radar sounding works by sending rapid pulses of electromagnetic waves from antennas and measuring how long it takes the pulses to reflect back. But the method uses low microwave frequencies that can penetrate deep into a substance, including miles of ice. The result is a cross-sectional image of the ice with direct measurements of its thickness, volume and strength.

In 2018, Keller with her colleague Josh Cahill and a team of APL scientists and engineers proposed building such a sounder for sea ice. It would be part of an ice-monitoring infrastructure, with a sounder on a satellite working in conjunction with others on an airplane or uncrewed aerial vehicles to capture more localized, higher-resolution measurements closer to the ground. They could even provide data for forecasting where ice is headed. Once interfaced with SAR data from other satellites, they could provide complete coverage over a desired region of the Arctic. They called the project Walrus.

The idea of using radar sounding wasn’t new.

In fact, sounders have been used to explore features on the Moon and the polar ice caps on Mars. NASA’s Europa Clipper mission will even use one to peer beneath the ice on Jupiter’s ice-covered moon Europa, which is suspected to have a global (and potentially life-bearing) ocean below the ice.

The main problem with using it to study sea ice on Earth, Keller said, is the salt. Salt rapidly absorbs microwave signals. Even at optimum frequencies for peering into sea ice, which Keller and her team calculated is around 600-900 megahertz, you can see through just 7 feet (2 meters) of saline ice. By comparison, NASA’s Pathfinder Advanced Radar Ice Sounder that APL built and operated in the mid-2000s could see through nearly 2 miles (3 kilometers) of freshwater glacial ice on top of Greenland.

Given enough time (meaning years), the salt will flush out, leaving multiyear ice that is essentially frozen freshwater like that in glaciers. But as the climate changes, more and more ice in the Arctic will be first-year ice, so there’s a lot of salt to deal with.

At the ideal frequencies, the team figured they still could see to the bottom of very thick first-year ice with 12-inch (30-centimeter) resolution. “With that, we could actually get ice thickness that correlates to the different ice types,” Keller said.

They have yet to get the sounder built, though, because various efforts to get the program off the ground have stalled.

Without an ice sounder, Keller said, we risk not being able to fully understand or predict how the ice or Earth’s climate will change. It’s also dangerous for ships — if you don’t know where the thick ice is and where it’s going, you can’t confidently say where there’s a safe passage.