When Tom Hanks carved "HELP" into the sand after his FedEx plane crashed and left him alone on an island in the movie "Cast Away," we all knew it was a failed scheme.

It's not like there was a satellite up there watching for lost souls or something.

Actually, there are several satellites up there watching for lost souls, whether on land or sea. A space-based global lifeguard system has quietly helped emergency officials locate and rescue some 4,200 people on and off the coast of the United States since 1982, and nearly 13,000 worldwide.

In some cases, the signal bounced from the sky is the only SOS that gets noticed.

The crux of the system is a series of boxes mounted on a host of satellites operated by the National Oceanic and Atmospheric Administration and other agencies. NASA helped develop the system originally.

For hikers and hunters or anyone else planning to get lost or injured in faraway places, there's the PLB, or Personal Locator Beacon.

In short, these devices all transmit a signal that satellites can pick up. The satellites then relay the victim's location and in some cases other information to U.S. Mission Control Center in Suitland, Maryland, where the data is automatically processed and combined with other data. An alert is generated and sent to the rescue squad best positioned to help.

Of course, the invention of such devices can't guarantee a rescue. For instance, there is no requirement for boaters to carry emergency beacons. And most beacons lack automatic activation. That is, someone has to be alert or injury-free to turn the thing on. And even then, folks may die before rescuers arrive.

Still, any boater or pilot can get one of these devices nowadays if they choose. But for land use, unless you live in Alaska where there's a PLB pilot program, you can't have one yet.

Ajay Mehta manages the overall system, called SARSAT (Search and Rescue Satellite-Aided Tracking) for the National Oceanic and Atmospheric Administration (NOAA).

Mehta explained that the primary use of the system is for sailors and boaters in distress, be they merely adrift or capsized and about to drown in frigid waters. While boaters still rely on radios to place mayday calls, SARSAT can be a life saver when a boat is out of radio range.

"It's really a last-resort device," Mehta said.

On April 5 of this year, the 60-foot sailboat "Spinning Jenny of Lune" capsized east of Port Canaveral, Florida. The crew activated their emergency beacon, and the signal was processed by SARSAT. A Coast Guard jet was dispatched, and its crew spotted the boat riding low in the water. They dropped a pump to the five Germans struggling to keep the sailboat afloat. But the pump was not enough. The ship capsized and the crew jumped into the water.

Meanwhile, the Coast Guard had also sent two helicopters, which eventually arrived and plucked the crew from the water, delivering them to a merchant ship that had also been diverted to the scene.

Some ships capsize without as much warning, and new versions of the beacons release automatically upon sinking, float to the surface, and send a signal.

SARSAT's roots go back to 1970, when a plane carrying two U.S. congressmen crashed in a remote region of Alaska and, despite a massive search, was never found. As a reaction, Congress mandated that all aircraft in the United States carry an Emergency Locator Transmitter (the ELT). The original system did not involve satellites, but over time it grew to include ships at sea, and ultimately became unmanageable without the aid of satellites.

The test project has allowed residents of Alaska to use a personal version of the device (the PLB) since 1994 and has helped rescue more than 100 people.

"It's highly effective," said Melvin Nading, a search and rescue pilot with the Alaska State Troopers.

On June 1, a hunter ran out of gas on his way back from a camping trip near Barrow, Alaska. In a heavy fog, he became disoriented and lost. So he activated his PLB and was found by a local search and rescue squad.

Nading, who tracks all PLB signals in the state, said the system can reduce rescue time from days to hours. And it saves money. Instead of sending planes, helicopters, hikers and maybe even boats to search for someone who is somewhere up in the wilderness and didn't return home when planned, officials can send a single search and rescue team to a pinpointed location, knowing what sort of terrain to expect.

But the system does get abused. People who might be only mildly injured will sometimes push the button.

"That's the dark side," Nading said. "A person who falls down and bruises his knee and says, 'I don't feel like snow machining all the way home. I'm going to set this thing off and someone will just come get me.'"

Only rarely is a signal sent by accident.

PLBs are so popular in Alaska that they're even used in sales pitches. Alaska Snow Safaris, in Anchorage, advertises the PLB as an important safety feature in its snowmobile tours.

Mehta said his agency has petitioned the Federal Communications Commission to allow use of the system throughout the United States, but approval has not come.

"A cell phone is the best way of communicating a distress" from most remote areas, he said. "But there are still areas of the U.S. where there is no cell phone coverage, where satellite alerting may be the only option available."

While all light aircraft in the United States are required to carry some type of emergency beacon, commercial flyers are not, Mehta said. But beginning in 2002, aircraft that fly international routes over water or remote areas will be required to install the devices.

In "Cast Away," Tom Hanks was lucky enough to have recovered a life raft from his sinking plane. But with no way to send a signal, he ended up marooned for 4 years. Wouldn't the FedEx plane have had a beacon?

"Probably not," Mehta said. But he added that a lot of commercial planes have begun to install them in their life rafts. And SARSAT claims total global coverage. In most cases, a distress signal is processed within a couple of minutes.

"If Hanks' plane had had a beacon," Mehta speculated, "it would have been detected, and he certainly would have been rescued a lot sooner."

That would have ruined the final 123 minutes of the movie (but spared us from gawking at the actor's aging body).