In the late 1950s the American Central Intelligence Agency began sending Lockheed U-2 ‘spy-planes’ over the Soviet Union to take intelligence photographs. The U-2s flew at 80,000ft (24,000m) to be out of range of anti-aircraft fire, but it became clear that radar was not detecting them.
These extraordinary planes were little more than jet-powered gliders built of plastic and plywood. On takeoff they jettisoned their small outrigger wheels from the ends of their wings – and they landed on their main, retractable wheels in the centre.
It was not until May 1960, after more than four years of overflights, that the Russians shot one down using new radar equipment belonging to SA-2 surface-to-air missiles. And even then the U-2 did not receive a direct hit. A missile exploded close enough to put the fragile aircraft into an uncontrolled diver, and the pilot, Gary Powers, had to eject.
The success of the U-2s led to highly classified research work in the United States, known as ‘Stealth’, to create a military aircraft that was invisible to radar.
The U-2 had gone undetected for so long because it was made of non-metallic materials which absorbed radar waves rather than reflecting them back to the radar ground station as normally happens.
The Stealth programme aimed at designing high-performance military aircraft incorporating, among other features, a minimum of metal and with the exterior clad in highly absorbent tiles. The aircraft would be almost invisible to radar and could make most radar-controlled anti-aircraft systems obsolete.
Key targets
After being developed under a blanket of secrecy, the high-tech B-2 Stealth bomber was unveiled at the Northrop Company’s manufacturing plant in Palmdale, California, in November 1988.
An audience of invited guests and journalist was kept well away from the plane – which is designed to slip through enemy radar defences without being detected and then drop up to 16 nuclear bombs on key targets.
To help achieve radar invisibility, the bomber is coated with radar-absorbent paint on its leading edge.
A similar technology of radar invisibility is used underwater to foil sonar detection. Modern submarines are coated in a thick layer of a top-secret resin which is highly absorbent acoustically, and reflects only a minute amount of the energy transmitted by sonar detectors.
Ground clutter
Another technique used by aircraft to avoid radar is to fly at very low levels where there is a great deal of ‘ground clutter’ – radar reflections given off by buildings and other objects. Low-level aircraft can go undetected by most radar systems. But the latest, most sophisticated ground-defence systems are designed to discriminate between ground clutter and hostile planes. In addition, ground clutter is partly avoided by using ‘look-down’ radar systems, which track aircraft from other aircraft flying above.
Picture Credit : Google