Sentry Against Crawlers

The first radars of World War II could detect invading aircraft (giving the R.A.F. a big advantage in the Battle of Britain), but they were not much good on smaller targets. Modern radar is vastly more sophisticated, and a wondrous new refinement is an eye developed by the Army Signal Corps in collaboration with Hazeltine Corp. It can stare through darkness or fog at a terrain of tangled scrub and tell if a man is crawling through it two miles away; it can look at a walking human six miles away and tell whether its target is male or female.

The Combat Surveillance Radar AN/TPS-25 (called Tipsy 25 by the G.I.s) is easily mobile, depends on the Doppler effect, which detects slight movements toward or away from the instrument because of the change in frequency of radio waves reflected from moving objects. When set up on the front line, Tipsy 25 is trained toward the direction of probable enemy approach. It covers an angle of about 30DEG, and if anything is moving there, the operator hears a crackling sound like radio static. He then narrows his beam and focuses on the suspected object. When he pinpoints it. he hears a characteristic sound, which is simultaneously displayed as a wave pattern on an oscilloscope.

A vehicle (tank, jeep or train) twelve miles away is easy to identify. A tank sounds very much like the clanking of its tracks. A wheeled vehicle makes a whine that increases in pitch as its speed increases. A man walking toward the radar sounds like "ump-ump-ump,"--each "ump" being Tipsy's reaction to the relatively fast movement of his legs as he takes a step. A woman's skirt has no effect, but she moves her arms differently and swings her hips more, so the radar sound that comes from her has more frills, lacking the plain solidity of the male "ump-ump-ump."

A crawling man does not usually move as fast as 1 m.p.h., the lowest speed that Tipsy 25 detects, but movements of his arms and legs exceed the speed limit. So they give a characteristic sound and warn the radar sentry that in the darkness somewhere two miles away, someone is crawling who presumably means no good.

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