The nation's popular global navigation system resets its clock on Saturday, a long-planned time change that could confuse older receivers used by boaters, hikers, small-plane pilots and cellular networks.

The Global Positioning System, known as GPS, is a constellation of 27 satellites in an orbit about 11,000 miles above the Earth. Set up by the Defense Department and used during the Persian Gulf War to guide missiles and bombs, GPS increasingly is relied upon by commercial and recreational users who need precise determinations of locations and time.

Like many early computer systems, GPS was designed to accommodate technology common two decades ago. By most accounts, engineers then did not want to burden satellites and receivers by requiring lengthy transmission times for data that is the equivalent of a couple of pages of text.

Because of those early engineering concerns, the GPS clock started counting time on Jan. 6, 1980, with week "0000" and continues until 23:59:47 universal time, or 8 p.m. EDT, Aug. 21.

At that moment--the GPS's first rollover since its start-up--the clock will reset to zero, "sort of like your car when the odometer hits all nines," said Col. James B. Armor, director of the GPS Joint Program Office at Los Angeles Air Force Base.

Government officials--from the Air Force to the Federal Aviation Administration to the Coast Guard--said they are confident that GPS satellites and government receivers of the navigational signals will operate without any hitches when the GPS clock resets. The Air Force, for example, spent about $15 million on tests to verify that the satellites and GPS control systems will work, the officials said.

But they urged users of commercial receivers to contact their manufacturers to ensure that the receivers have been programmed to compensate for the clock rollover. Receivers made within the last five years should continue to work properly, but such assurances can be given only by the manufacturer, Armor said.

Receivers that do not correctly recognize the clock rollover could stop working, appear to work but display inaccurate positions or times, or take more time than usual to locate the satellites, Armor said.

Older receivers can be fixed by installing a new computer chip or through changes to their software, officials said.

GPS has grown in popularity with the expansion of digital networks across the world. Some banks use GPS signals to track the time of financial transactions, some power companies use the system to time transfers of electrical power, and some telephone companies use GPS to synchronize cellular networks. In the last few years, boaters and pilots have increasingly relied on GPS as a navigational aid, although most use more than one source of navigation information.

The GPS system also has undergone fixes for the year 2000 date conversion, when many computers using two-digit date fields will be at risk of misinterpreting "00" as 1900, not 2000, and possibly malfunction.

After Saturday's rollover, the GPS clock will tick off time for another 1,024 weeks.