Blind Sailing

With no guiding stars or radio beams to give her position, how did the U.S.S. Nautilus navigate under the Arctic icecap? The secret is inertial navigation--a new means of finding latitude and longitude wholly without external reference points. Last week it was also used in the Arctic by the U.S.S. Skate, will go in even more sophisticated form into all the Navy's nuclear submarines, some of them designed to creep deep in enemy underwaters with the Polaris missile.

Inertial navigation systems are only as old as guided missiles, which brought to a head the brewing problem of modern aerial navigation: how to get a fix at great speed while all the usual sun and star angles are constantly changing. Solution: an instrument that records and remembers earth distance and direction traveled from a known starting point. One of the best systems was developed by North American Aviation, Inc. for the Navaho missile. The Navaho was scrapped, but last February the Navy ordered a Navaho guidance system installed in Nautilus. It was aboard the sub nine weeks later--and it seems likely to change marine navigation forever.

Earth-Angled. In inertial navigation, every motion of a ship in any direction is accounted for and automatically computed to give precise distance traveled. The key instrument is an accelerometer --a container holding a weight that can move, against springs, toward one end or the other. The weight acts like a man's head that is jerked back because a cab driver starts suddenly. The weight thus measures a vehicle's thrust (acceleration), and from this information, an electronic computer can determine the vehicle's velocity. Inertial navigation uses two accelerometers, one to measure all north-south motion from the starting point, and one to measure all east-west motion.

To work properly, the accelerometer cylinders must lie at right angles to the earth's radius lines; i.e., their weights must move along tangents to the earth's circumference. Otherwise, gravity, as well as lateral movement, would affect the weights. To hold the accelerometers steady, they are hinged to platforms, stabilized by gyroscopes, which keep an unchanging relationship to the earth (the platform of the north-south instrument, for instance, is always at the same angle to the polar axis). But the accelerometers do not remain immovable. Holding their tangential position, they must slowly tip on their platforms as the ship moves. What tips them is a motor that takes its electric cues from the accelerometers' own velocity reports.

Byproduct. This gives the system an important information byproduct in addition to distance traveled. As the cylinders move to keep alignment, the angles they form with their stable gyro platforms are computed to give the ship's location in degrees of latitude and longitude. With readings for distance traveled, plus latitude and longitude, the ship's position is clear at any moment.

Chief complication is keeping the gyro platform absolutely stable and unaffected by gravity; it tends to drift. Such forces as bearing friction and the rotation of the earth itself tend to tilt the platform out of line. On the Nautilus the system apparently worked without significant drift for the full 96 hours under the ice, and eventually the Navy hopes for accuracy up to 90 days at a time.

For a final check of its inertial navigation gears, the Nautilus had one of the handiest wrinkles that submariners have developed since the snorkel--a periscope sextant. Where the ice is open, a sub equipped with this gadget can up periscope and shoot star sights without surfacing. The sextant does the work electronically. At a computer below, the navigator receives the figures on a paper tape without rising from his chair.

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