It happens all the time. Someone places a call to the Coast Guard station in Atlantic City, New Jersey, saying they're on a boat somewhere off the Atlantic coast and they're in trouble. So the Coast Guard sorties HH-65 rescue choppers and small cutters to pluck the unfortunate boater from danger. But when they arrive at the caller's claimed location, there's nothing there. The nation's fourth military service - and the world's 12th-largest navy - has been the victim of a prank.

Pranks are a huge drain on Coast Guard resources, explains spokesman Petty Officer Kyle Niemi. And with last week's announcement that the service is pulling its troubled eight 123-foot patrol boats from service due to hull problems, the need to husband remaining assets is even more urgent.

Fortunately, one system just now coming on line promises to cut back on responses to prank calls. Rescue 21, a radio telemetry system manufactured by General Dynamics, uses tower-mounted radio receivers to triangulate the source of a distress call. Now Coast Guardsmen at shore stations can tell if a call is coming from where the callers claims it is. And if it's not - if the call is a prank - the Coast Guard can pass info to the Federal Communications Commission or local law enforcement so they punish the offenders.

On December 5, Operations Specialist 3rd Class Tanya Berard demonstrates the system in the Atlantic City operations center. She points on a computer screen at a tiny icon shaped like a boat "floating" on a digital blue representation of the Atlantic Ocean. This boat represents the source of a strange broadcast she heard on an open channel earlier today. Clicking on a menu item, she plays a digital recording of the call - another new capability provided by Rescue 21. "Beep beep. Beep beep," sounds a male voice.

"Beep beep?" Berard echoes. She chuckles. The transmission probably means nothing. But she's tracking it just in case. If she decides there's a genuine emergency afoot, she will change the icon to one representing distress. That will bring the icon up on identical screens at operations centers in other Coast Guard stations up and down the East Coast. That way the stations can coordinate a rescue. And aircrew can drop by their operations centers, get quick briefs on the endangered boat's location and speed out for a rescue.

Atlantic City is one of the first stations to get the full Rescue 21 system. Around 40 more have or are building their systems, with full implementation by 2010.