GPS End-Of-Week Rollover

On August 22, 1999, some GPS units will think that the date is actually January 6, 1980, causing all sorts of navigational problems, due to the GPS "End-of-Week" Rollover (EOWR) problem.

While the twenty-four Global Positioning System (GPS) satellites and the ground control center will breeze through the night of August 21 without a hitch, the EOWR problem is limited to GPS receiving units (especially those constructed prior to 1994). Exact time is essential to GPS to calculate precise positioning with respect to the satellites.

The internal calendar of GPS units is based on a 1024-week (approximately 19 years, 8 months, and 15 day) cycle which began on January 6, 1980. The last week of the current GPS cycle begins on August 15, 1999. After the last day of that week, August 21, the GPS unit needs to switch to a new cycle of 1024 weeks.

Most newer GPS units will switch to a new cycle. The best way to determine whether a GPS unit will be ready for August 22 is by contacting the manufacturer. Most companies, such as Magellan, Trimble, and Garmin, provide online lists of compliant, upgradable, or noncompliant units online. There is also an online directory of other GPS product manufacturers with links to their EOWR information.

Problems that are like to occur for incompatible GPS units include:

• The GPS unit would be unable to locate any of the satellites.
• It might take the GPS unit much longer to locate the satellites.
• The unit may appear to function although it may provide incorrect location, time, or date (especially 1024 weeks off).

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