The sky's the limit


By Paul Marks ALMOST a century after Marconi bounced short-wave signals off the ionosphere to get the first crackly radio signal across the Atlantic, a British team has used the ionosphere to bounce digital pictures thousands of kilometres over the horizon. The trick, devised by researchers at Lancaster University and the Defence Evaluation and Research Agency in Malvern, lies in coding images in a way that makes them largely immune to the noise created by bouncing off the ionosphere. This has thwarted all previous attempts to use short-wave radio signals to carry pictures. DERA wants to use the system to transmit pictures to and from military search or reconnaissance aircraft that may be thousands of kilometres away. And they want to do it without using expensive and complex satellite-based communications systems. “The military like to have options—they don’t like all their eggs in one basket,” says Paul Arthur, principal scientist at DERA and a leader of the project. The system could also have civilian applications, such as telemedicine in remote areas. “You could transmit medical images such as X-rays from remote places like the Australian outback,” Arthur says. Short-wave signals are refracted by the ionosphere, a layer of ions created by solar radiation and extending from 60 to 1000 kilometres above the Earth’s surface. This deflects them back down into the lower atmosphere, allowing signals to travel well over the horizon. Each “hop” can take a signal up to 4000 kilometres, and by bouncing off the surface of the Earth a signal can make several hops. The refracted signal tends to be extremely noisy, however. The sources of this noise include the movement of the ionosphere itself, as well as interference from galactic radio waves, sunspot activity and fading. Nor do all paths taken by the beam arrive at the receiver at the same time, creating confusion in the received signal. “It’s just a very noisy channel,” says Arthur. If a normal picture is sent via ionospheric bounce, the noise can rip out whole chunks of it, making it useless for important communications. So the Lancaster and DERA researchers have worked out how to make digital image data resilient to errors. In a system they call Absolute Address Picture Element (APEL), the team converts a photograph into an image comprising just 16 shades of grey. To make it resilient to noise, they then split the image into four pictures, each of which has just four shades of grey (see diagram). Each of these pictures is then broken down again into the array of squares and lines that best represents the image. It is these geometric picture elements that are transmitted. Noise might well knock out a line or a square in one slice of the picture, but as long as the others get through, the picture can be reconstructed using error correction techniques. What’s more, each element can be compressed before being sent to speed up transmission times. Geoff Spells, head of frequency management at Merlin Communications in London, which transmits the BBC World Service, says that the selective fading by the ionosphere is a challenge. “Certain frequencies in a short-wave band just get knocked out,” he says. “But if you get the coding right,
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