POLAR scientists could soon be calling back to base by sending radio signals
through sea ice—allowing them to communicate over distances six times as
great as would be possible with signals sent through the air.
Radio communications at the Earth’s poles are often unreliable because
charged particles from the Sun are attracted to the poles’ strong magnetic
field, creating electrical disturbances in the atmosphere, such as the Northern
lights. These disturbances cause so much interference that they prevent people
employing the usual technique of bouncing radio signals off the ionosphere on
their way to a distant receiver.
“The ionosphere can become so energetic, it disrupts its usual nature of
reflecting radio waves,” says Paul Mileski of the US Naval Undersea Warfare
Center in Newport, Rhode Island. “At an ice camp, the radio might work one
minute and not the next—it can be very exasperating,” he says. In a recent
trip to the Arctic, Mileski found that communications became erratic about 50
miles from a base station.
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But he says an easy way to improve communications across large regions of the
Arctic is to send signals through the ice. In winter, Arctic sea ice is
typically around 3 metres thick, with the bottom fifth being highly saline.
Mileski says that the purer ice above this layer is perfect for conducting radio
waves. “Because the electrical properties of the different layers are so
different, the structure acts as a waveguide—the interfaces form
boundaries for the radio wave to bounce off,” he says.
To prove the idea, Mileski laid out a simple “dipole antenna” on the sea ice.
“You just roll out two wires, one after the other, so they’re in line with each
other and in the direction you want to talk to somebody,” he says. “Then you
connect the wires in the middle to a transceiver.” Mileski then set up a similar
antenna some distance away to detect the signal
(see Figure). “Although the
signal is mostly trapped in the ice, you can detect it some height above the
ice,” he says. After testing various lengths of wire, he found the best results
were obtained when the wavelength of the signal was 600 metres—generated
by two 150-metre lengths of wire. His 600-metre (500-kilohertz) signals
travelled 300 miles through sea ice.
Mileski says the finding could help researchers build a cheap Arctic
communications network. “Two camps located even 500 miles apart could
communicate using this system,” Mileski says.
It could also be used to help stranded scientists, says Peter Wadhams of
Britain’s Scott Polar Research Institute in Cambridge. “At an ice station, you
often have people going out by helicopter or on skidoos to do measurements
somewhere else,” he says. “If they get stranded and there’s a magnetic storm
going on, this could be an alternative way to make contact.”