Easy Accuracy at Sea

Deep below deck, with all the mindless certainty of a Ouija board, a marking pen moved by steel fingers glided across a nautical chart of Narragansett Bay. As he followed the pen's thin red line, a Navy lieutenant, cut off from any view of the water, telephoned commands to the bridge. At each command, the helmsman altered course, and the 65-ft. test ship Alan threaded neatly among islands and inlets. Each change in direction and speed was instantly recorded by the moving pen.

Delicate Marriage. The easy accuracy of the Raytheon navigator that the A Ian was demonstrating for the Navy last week masked a delicate marriage of intricate techniques: the sonar sound-detection systems that have been used by submarines and sub detectors since World War II, and the more advanced electronic navigation devices that have recently come into use aboard high-speed aircraft. Mounted beneath the Alan's hull are four small pairs of sound projectors and receivers. A gyrocompass keeps them constantly aimed toward the cardinal points of the compass as powerful beams of sound are caromed off the ocean floor and picked up again.

The noise that comes back from the bottom is changed in frequency by the movement of the ship. This easily detected frequency shift is the celebrated Doppler effect, and a computer translates the change into speed-and-direction instructions for the automatic marking pen. A single dial adjusts the navigator to the scale of any standard marine chart. And last week's sea trial found the new Doppler sonar accurate within a startling 20 yds.

Man Overboard. Now that his newest brainchild has proved such a prodigy, Sonar Engineer Edwin Turner, 64, plans to deliver two prototypes to the Navy for further trials and then retire. He stresses that Doppler sonar is a supplement, not a replacement for radar and other modern navigational aids. It can function properly only in well-charted waters or far at sea, where the course picked out by its pen is not likely to run into unexpected obstacles. The Navy already has a built-in need for such a device on many of its ships, and along the world's coastlines, where the bulk of merchant shipping still plies its way, the new navigator may soon prove indispensable. Though the first commercial models may cost upwards of $10,000, the price is expected eventually to come within pocketbook range of the well-heeled amateur skipper.

In the most dramatic test of the new navigator last week, a sailor-sized, life-jacketed dummy nicknamed "Oscar" was pitched off the stern. At the shout "Man overboard!", the lieutenant in the hold marked the chart and began barking commands. When the red line had curved back on itself, there was Oscar, 10 yds. to port, in more danger of being run down than drowned.

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