Doppler Reckoning

When oldtime ships' officers could not see the sun or stars, they fell back on dead reckoning. By recording the ship's direction and its motion through the water, they tried to keep track of its position. The system did not work very well, chiefly because of crude instruments and because the effect of ocean currents was often unknown. But if a ship could have measured accurately its motion across the solid ocean bottom instead of the fluid surface, dead reckoning would have brought it to any harbor through the thickest fog.

The Navy's automatic navigator AN/ANP-67, which was publicly described in detail for the first time last week, uses "ocean-bottom" dead reckoning for airplane navigation. Developed by Ryan Aeronautical Co., the "67" is a 200-lb. black box packed with electronic apparatus that transmits to the ground two narrow beams of microwaves. When the waves hit the ground, a small part of their energy is reflected back to the transmitter. If the transmitter is moving, as in an airplane, the frequency of the waves is changed slightly by the Doppler effect.*The amount of the change, which can be measured with gnat-hair accuracy, gives the speed of the airplane.

The airplane is usually drifting sideways under the influence of side winds, but the 67 determines the drift and allows for it. It notes the heading from the airplane's compass. Then a computer figures automatically the latitude and longitude, the ground traveled and the deviation, if any, from the planned course. This information is displayed on dials, and it can also be fed electrically into the airplane's controls.

The pilot has little to do. He tells the 67 the position of the take-off point (in latitude and longitude, from his chart) and the course to his objective. Then he flies the airplane so that the course-error needle reads zero. It will take him any place in the world. It will even tell him when a slight change of course or altitude has found a more favorable wind. A common experience for Navy pilots flying with the 67 is to take off from San Diego, navigate across the continent by watching a single needle, and come down through a cloud deck to find the East Coast destination right in front of them.

-*Sound or radio waves reflected from an approaching object have higher frequency; reflected from a receding object, they have lower frequency.

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