GPS End of Week Rollover Bug
By Jeff Porcaro

Most of us have heard of the Y2k bug and how on January 1, 2000 some non-compliant software and hardware will fail. Most people don't realize that a similar problem can cause a GPS receiver to fail. This is commonly referred to as the GPS end of week rollover bug.

The end of week Rollover can be explained like this. The atomic clocks in the satellites were set up with a base date of January 6, 1980. The clock keeps track of each week by assigning it a number, the limit on the number used in a satellite's clock is 1024. So when August 22, 1999 rolls around the clock will hit that limit and the number will rollover to 0 and start counting up again.

You are probably sitting there asking yourself what does this have to do with my receiver and me? To answer this we must give you a brief GPS primer.

GPS Primer
A GPS receiver determines it's precise position by tracking signals from each satellite in range. The signals are timed by an atomic clock in the satellite. The time it takes for the signals to reach the receiver indicates how far away the satellite is from the receiver.

To make position calculations, the receiver needs signals from four or more satellites. The first three satellites triangulate the position. The fourth improves the position's accuracy by computing time offset between the satellite system's clock and the GPS receiver's clock. The receiver then displays the current information.

GPS End-of-Week Rollover Impact
If the receiver manufacturer did not program the hardware and software to handle the End of Week Rollover the End of Week bug could cause the receiver to cease functioning because it doesn't know where to find the satellites. Other potential problems include; longer time to lock in on the satellite and the dates returned could be incorrect or even worse the position returned could be incorrect causing you to rely on erroneous information. Affected receivers may need to have the firmware updated or the receiver may become obsolete.

To know if your receiver is going to be affected you need to go to your receiver manufacturer's web site for information on the End of Week (EOW) bug or contact your manufacturer's support to know if you need to worry.

Jeff Porcaro is a Contributing Editor and co-founder at GearReview.com. He specializes in stoves, GPS systems, navigation, and mountaineering.

For manufacturer compliance information:
Trimble
Magellan
Lowrance
Garmin