Sailing by Satellite

With long-range navigational aids (LORAN), ships at sea can plot their locations to within one nautical mile. Under clear skies on a calm ocean, a good navigator can take an equally accurate fix with a sextant. But nothing does the job as well as the Navy's all-weather Transit satellite navigational system, which can pinpoint a ship's position to about 300 ft. Until now, Transit has been classified because it guides the Polaris missile submarine fleet, but last week the Government released it for use by any U.S. merchant ship.

Transit operates on a simple principle: once a satellite's location is known, two calculations of its range from any ship will determine position at sea. Under the Navy system, three Transit satellites circle the globe in 105-minute polar orbits at an altitude of 700 miles. Since the earth also rotates beneath them, the Transits provide round-the-world navigational checkpoints. Four Transit tracking and receiving stations in Maine, Minnesota, Hawaii and Point Mugu, Calif., track the satellites as they pass within range, then relay position data to a computer center at Point Mugu. There, projected twelve-hour paths for each satellite are calculated. The future position data is fed back to each satellite, which in turn broadcasts the information at two-minute intervals. A ship equipped with special receivers and computers can then fix its own position by measuring the shifting frequency of the Transit's beeping signals. This Doppler shift, similar to the falling pitch of a passing train whistle, gives the distance between the satellite and the ship.

Until now, only a few naval and scientific vessels used the Transit system, largely because the shipboard equipment is so expensive. Custom-built, each receiver costs between $21,000 and $35,000, compared with $5,000 to $10,000 for a LORAN rig. In addition, each ship needs a $25,000 computer. The Navy hopes that commercial manufacture will lower the unit cost, allowing more Transit use by Navy as well as merchant ships. Last week most details of the system were being turned over to interested U.S. electronics manufacturers. The company that can most efficiently simplify the system and reduce its cost will chart a market as wide as the seven seas.

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