At Dulles-based Orbital Sciences Corp., officials counted down the weeks to their own miniature version of the year 2000 moment of truth.

Orbital Sciences' Magellan division is one of the primary manufacturers of hand-held global positioning system (GPS) receivers that use data from satellites to pinpoint an exact geographic location for mariners, pilots, surveyors and the military.

On Saturday, the 19-year-old GPS internal calendar, which was designed to last 1,024 weeks, completed its cycle and returned to its original starting point, week 0000. But at Orbital and other companies where GPS systems are made, the impending event had already become a non-event.

The popularity of GPS units is a recent phenomenon, says Mike Griffin, Orbital's chief technical officer, and devices purchased since 1995 include software that should handle the rollover seamlessly. The Magellan folks were taking the weekend off, Griffin said, confident that months of alerts had prompted GPS users to check out their systems or replace them.

Even with older devices, while the date and time information may be garbled, the geographic positions relayed by satellites should not be off-target, he said.

Donna Bailey, vice president of TowBOAT/U.S. Solomons in Maryland, uses Garmin equipment on the company's towboats and has taken special precautions to make sure GPS units are working. "We needed to be on top of this, because sometimes our towboat operators get called out during the night and we needed to make sure that we were prepared," Bailey said.

GPS owners should check the Web sites of the manufacturers, including Magellan, Trimble and Garmin. These sites list compliant, upgradeable or noncompliant units. There is also an online directory of other GPS product manufacturers. The U.S. Coast Guard also provides a directory of phone numbers and Web sites for manufacturers.

At Motorola Inc., which manufactures GPS components, the GPS marketing unit scheduled a barbecue to watch the date changeover, the Chicago Tribune reported. "We expect it to be absolutely uneventful, but we thought it would be a nice time for a party," said GPS product manager Paul Dowell.

For more information, contact www.navcen.uscg.mil/gps/default.htm, which lists GPS receiver manufacturers and contact information. The President's Council on Y2K Conversion maintains a hot line, 1-888-USA4Y2K.