Signs in a Clear Sky

It strikes with the swift, clawed fury of a pouncing cat--and jet pilots call it by that name. CAT (for clear air turbulence) can swat a jetliner down a mile in a minute flat, paste passengers to the ceiling, and rip the wings from light planes. Many CAT victims go uncounted because up to now CAT has been invisible.

Clear air turbulence often occurs where two air masses, moving in opposite directions, grind together. Unlike storm fronts, which present a large, moist target for regular storm-tracking radars, this abrupt change of wind direction, or "wind shear," usually goes unremarked by electronics. Last year RCA technicians tracking swift Army missiles on ultrahigh-frequency (above 5,000 megacycles) C-band radar noticed that they were receiving considerable "backscatter"--unexpected, and apparently unexplainable, echoes--during a clear-sky exercise. They wondered if they were on the track of CAT.

In cooperation with U.S. Army meteorologists at Fort Monmouth, N.J., they began watching on C-band while balloons climbed through regions of clear air turbulence and reported by radio what was happening to them. Radar readings matched the balloon reports.

By adding a low-noise amplifier to their radar, RCA engineers discovered that they could track CAT even better; they followed its path with a clear, rapidly wiggling line on their radarscopes. Last week RCA had two modified C-band sets at work--one in Moorestown, N.J., and another on the DAMP (for Downrange Anti-Missile Measurement Program) ship in the South Atlantic. Once they are certain they have cornered CAT, avoiding its dangerous attack will be a simple matter for the careful pilot.

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