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Quirk of sea air provides video link to protected reef
24 May 2008
From New Scientist Print Edition.
Rachel Nowak, Melbourne

Shortly after the UK's Royal Navy started using radar, it found to its irritation that targets would sometimes disappear from screens thanks to a quirk of the sea air. Now the same phenomenon is being used in Australia to set up a data link allowing conditions on the Great Barrier Reef to be monitored remotely.

Evaporation ducting, as the phenomenon is called, allows radio signals to travel much further than usual. With regular line-of-sight transmission, the range of a ground-based transmitter is limited to 30 kilometres or so by the Earth's curvature. Increasing the range requires taller transmitters, which is why microwave antennas are often mounted on masts. This would be unthinkable on environmentally sensitive reefs, though, as would fibre-optic cables.

Evaporation ducting offers an alternative: channelling microwaves along the surface of the ocean and over the horizon. The technique works partly because humidity above the ocean drops off sharply with altitude. Microwaves heading skywards are refracted down as they pass through air of lower humidity, while those hitting the ocean are reflected upwards. The result is that the microwaves end up bouncing along the ocean's surface inside a so-called evaporation duct.

"It's like a waveguide or a coaxial cable," says Trevor Bird of CSIRO Information and Communication Technologies in Sydney. Evaporation ducts can also trap radar signals, which is why radar images occasionally vanish.

The technique could play a critical role in establishing the planned Great Barrier Reef Ocean Observing System (GBROOS), a network of hundreds of sensors surrounding seven locations across the 350,000-square-kilometre reef. Researchers hope the GBROOS will allow them to monitor water temperature, solar radiation, salinity, pollution and even fish behaviour in real time.

Graham Woods of James Cook University in Townsville, Queensland, who developed the system, worked out that the optimal transmitter height for conditions on the Great Barrier Reef is 5 metres, making it possible to place antennas on existing weather-monitoring towers. These are powered by specially developed generators that produce electricity from both solar and wind power. Since July 2007 a prototype transmitter has been sending data from Davies Reef, 78 kilometres offshore, to the Australian Institute of Marine Science in Townsville.

Under optimal conditions, evaporation ducts are reliable enough for a huge amount of data to be transmitted. "We've shown you can send the equivalent of 10 high-quality, real-time video streams at once - and we hope to develop systems that allow 10 times that," says Woods.

The major downside of this approach is that changes in humidity, wind speed and sea conditions can break up the evaporation ducts, which is why they aren't used routinely for communication. But Woods says the prototype link has been at least 80 per cent reliable because the ducts are relatively stable over tropical oceans.

Woods says the system could also be used to set up video monitoring targeting illegal fishing and drug smuggling.

From issue 2657 of New Scientist magazine, 24 May 2008, page 26

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