Get lost in the woods and a cell phone in your pocket can help camping buddies find you. Drive into a ditch and GPS in your car lets emergency crews pinpoint the crash site. But when a transcontinental flight is above the middle of the ocean, no one on the ground can see exactly where it is — in the air, or worse, in the water.

The disappearance of Air France Flight 477 and its 228 passengers over the Atlantic Ocean this week has critics of radar-based air traffic control calling on the U.S. and other countries to hasten the move to GPS-based networks that promise to precisely track all planes. Current radars are obsolete more than 200 miles from land.

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