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FIRING a special bullet into the ground from a helicopter could pinpoint buried landmines, say researchers in Missouri. The bullet emits a radar pulse as it grinds to a halt, offering a safe and efficient way of finding landmines. And it could help geologists surveying for oil, minerals and other buried natural resources.
Ground-penetrating radar is potentially one of the most accurate ways of finding landmines (see Technology, New Scientist, 26 April 1997, p 24). A radar transmits pulses of radio waves into the ground, and a computer analyses their reflections. But little radiation penetrates the soil-most is reflected by the ground because of the sudden change in density between the air and the soil. This means that ground radars need a large power supply to generate pulses intense enough to penetrate the ground. This makes the system bulky-a major problem since most minefields are in remote parts of the world.
To get round these limits, electrical engineers Thomas Engel and William Nunnally at the University of Missouri at Columbia, with $5 million in funding from the US Army, designed a bullet that gives out a powerful burst of radio waves from under the ground. Eventually, the researchers say, the bullet will be fired downwards from a gun mounted in a helicopter flying about 100 metres above the ground.
The bullet will produce a pulse of radio waves as it pierces the ground, and the signals reflected from any landmines within about a 15-metre radius will be detected by an antenna on the helicopter. "You don't have to be on the ground-stepping on mines-to use it," says Engel. Once the mines are located, they can be destroyed at once or their exact position noted so they can be dealt with later. Of course, a mine would explode if the bullet hits it.
The radar pulse is generated from the bullet's kinetic energy by a process known as magnetic flux compression. Inside the bullet is a solid metal cylinder, surrounded by a tightly wound coil of wire. As the bullet leaves the gun, a battery passes a current through the coil, generating a magnetic field in the cylinder.
When the bullet smashes into the ground, the sudden deceleration forces the cylinder out from inside the coil. In much the same way that moving a wire through a magnetic field generates an electrical current in the wire, the sudden movement of the metal cylinder through the magnetic field induces a large pulse of current in the coil. The coil then acts like an antenna, converting the pulse into a short burst of high frequency radiation.
The light-weight system can be fitted to any helicopter. And in tests, Engel has found their 30 millimetre bullet gives out a 4 kilowatt radar pulse-almost 10 times more power than a standard ground-penetrating radar-from 20 centimetres down. "Since the bullet is beneath the surface, it transmits more radio waves into the ground," says Engel.
New Scientist issue 24 October, page 11
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