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.
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."
Author: Ian Sample
New Scientist issue: 19 May 2001
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