POLAR scientists could soon be calling back to base by sending radio signals
through sea ice鈥攁llowing them to communicate over distances six times as
great as would be possible with signals sent through the air.
Radio communications at the Earth鈥檚 poles are often unreliable because
charged particles from the Sun are attracted to the poles鈥� 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.
鈥淭he ionosphere can become so energetic, it disrupts its usual nature of
reflecting radio waves,鈥� says Paul Mileski of the US Naval Undersea Warfare
Center in Newport, Rhode Island. 鈥淎t an ice camp, the radio might work one
minute and not the next鈥攊t can be very exasperating,鈥� 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. 鈥淏ecause the electrical properties of the different layers are so
different, the structure acts as a waveguide鈥攖he interfaces form
boundaries for the radio wave to bounce off,鈥� he says.
To prove the idea, Mileski laid out a simple 鈥渄ipole antenna鈥� on the sea ice.
鈥淵ou just roll out two wires, one after the other, so they鈥檙e in line with each
other and in the direction you want to talk to somebody,鈥� he says. 鈥淭hen you
connect the wires in the middle to a transceiver.鈥� Mileski then set up a similar
antenna some distance away to detect the signal
(see Figure). 鈥淎lthough the
signal is mostly trapped in the ice, you can detect it some height above the
ice,鈥� he says. After testing various lengths of wire, he found the best results
were obtained when the wavelength of the signal was 600 metres鈥攇enerated
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. 鈥淭wo camps located even 500 miles apart could
communicate using this system,鈥� Mileski says.
It could also be used to help stranded scientists, says Peter Wadhams of
Britain鈥檚 Scott Polar Research Institute in Cambridge. 鈥淎t an ice station, you
often have people going out by helicopter or on skidoos to do measurements
somewhere else,鈥� he says. 鈥淚f they get stranded and there鈥檚 a magnetic storm
going on, this could be an alternative way to make contact.鈥�