In five months, the Global Positioning System (GPS) will get a foretaste of the Y2K computer bug.

GPS is a satellite-based navigation system administered by the U.S. military, primarily for naval and airborne navigation. In recent years, small, inexpensive GPS receivers have been produced for consumer use as well, finding their way into the hands of hikers, private and commercial pilots, sailors and fishermen - even luxury-car owners.

Due to the way the system keeps time, the GPS system will have a date "rollover" on Aug. 22, 1999, much like the so-called millennium bug. Most modern GPS receivers have accounted for the rollover. But not all.

"Certainly, there are old receivers out there that are still in use," says Richard Langley, professor of geodesy and precision navigation at the University of New Brunswick. "Fixing the problem is easy enough for receiver manufacturers to do, but the trick is finding all the receivers in use."

Finding Your Place in the World

Developed by the military as a precise navigation system for ships, planes and ground troops, GPS navigation is used in everything from Tomahawk cruise missiles and smart bombs to Coast Guard cutters and tankers.

There are currently 27 GPS satellites in orbit 12,000 miles above the Earth. Each satellite continuously broadcasts its position, and a navigation receiver uses three such broadcasts to mathematically deduce its latitude and longitude position on Earth. The system is accurate to about 80 feet.

A time stamp on the satellite transmission is used to make sure that the correct data from each satellite is used. Some GPS-based navigation devices calculate the positions of GPS satellites using the last known time stamp. Extrapolating from the last time stamp and the satellite's speed in orbit, such systems find the satellite again for another reading.

The current GPS timekeeping system was inaugurated on Jan. 6, 1980. To keep time, the system uses a 10-digit binary number, which means that the system runs out of 10-digit numbers once every 1,024 weeks. That will occur on 12:01 a.m. GMT on Aug. 22.

Looking for the Satellite

According to Pentagon spokeswoman Susan Hanson, the Defense Department's Y2K remediation efforts will fix the GPS rollover problem prior to Aug. 22. U.S. Coast Guard spokesman Jack O'Dell says Coast Guard ships are already compliant, and the Coast Guard is now working to educate boaters about possible problems.

"We're ready to go, and most manufacturers have already taken care of the problem," O'Dell says. "But that doesn't take away from GPS users' responsibility to contact the manufacturers and make sure they check everything they've got."

GPS experts say that most receivers will be upgraded or replaced by Aug. 22, and should operate normally. But as with the Y2K issue, 100 percent compliance is unlikely. What will happen with the remaining non-compliant receivers?

In many cases, the GPS receiver will function normally, but will have a 19-year-old time stamp. If the time stamp is incorrect, the device could be looking for the satellite at its location back on Jan. 6, 1980.

A GPS device with a faulty time stamp, in other words, could be rendered useless; and a ship or a hiker with a faulty GPS device could be lost.

Beyond Navigation

The time stamp on GPS transmissions has other uses aside from navigation. The system uses composite readings from 230 atomic clocks around the world, which allow it to keep time down to the millisecond. Because of their accuracy, GPS receivers have been placed in computer networks, ATM networks and cellular phone towers to help keep track of time.

Datum, Inc., a San Jose, Calif.-based company that integrates GPS timing devices into computer and banking networks, discovered the problem early last year.

"If this had been a perfect world, everybody would've read the documentation and prepared for this," says David Robinson, technology specialist at Datum, Inc. "Some people knew about it, some didn't."

Since then, Datum has been poring over its customer database, informing as many people as it can about the potential problem.