Whistle of the Missile

As rockets fly higher & higher toward empty space, it gets harder & harder to keep track of them. Telescopes have followed V28 to about 100 miles up. But the slender WAC Corporal, which rose 250 miles above White Sands, N. Mex. (TIME, March 7), was too much for a telescope or radar. What kept track of it was DOVAP (DOppler Velocity and Position), a new instrument designed for rocket-tracking by the Army's Ballistic Research Laboratories at Aberdeen Proving Ground, Md.

The Doppler Tells. Last week, Dr. Dorritt Hoffleit, 41, a dark-haired spinster astronomer of the Harvard College Observatory, who helped develop DOVAP, explained how it works. A missile to be tracked by DOVAP carries a small radio transceiver (transmitter & receiver) which receives a radio wave from the ground, doubles its frequency, and relays it back to earth. But when the doubled wave is received on the ground, it is not exactly doubled. Because of the "Doppler effect,"* its frequency is changed slightly in proportion to the speed of the missile. This variation, recorded as a sinuous line on a strip of movie film, measures how fast the missile is moving away from the receiving station. Four such records, from stations spotted about 14 miles apart around the rocket firing point, can be combined by a kind of triangulation to give the rocket's speed and position. Army Ordnance hopes that DOVAP will tell within six feet where the WAC Corporal was at every moment of its whole flight.

Silence at the Top. DOVAP's ground set has a loudspeaker which blares out the DOppler wave. As the rocket rises into the air and disappears from sight, the whistle of the missile tells listeners how it is doing. The sound gets shriller & shriller as the rocket gains speed. Then the pitch begins to fall as the rocket, its fuel exhausted, begins to slow down. The sound dies to a deep bass. Then comes silence as the rocket reaches the top of its flight. The sound starts again as the rocket begins falling. It rises to a piercing shriek and ends in sudden silence as the rocket hits the earth.

Such complex scientific measuring would not be very interesting to most women, even women scientists. But Dr. Hoffleit (her title at Harvard is "Astronomer") loves rockets because they fly high enough to seem rather like stars. Graduated from Radcliffe in 1928, she got a job at the Harvard Observatory and became an expert at determining, spectroscopically, the absolute brightness of stars. In 1943 she went to Aberdeen to work on radar. This led, by natural stages, to tracing the trajectories of high-flying rockets. "If the altitudes had been lower," Astronomer Hoffleit says simply, "I would not really have been interested in the work."

* A body moving toward a receiver "crowds" the waves together, shortens them and thus increases their frequency. Moving away, it "pulls out" the waves, reducing their frequency.

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