SW Aviator Magazine - June/July 1999 issue
Week Number Rollover and Your GPS
by Alisa Christensen, Santa Fe Avionics
Last issue, I discussed Y2K and your avionics. Hopefully, everyone called their local avionics shop or manufacturers and got all the Y2K information they require. Well, I hate to tell you this, but you may need to call your local avionics shops and manufacturers again.

You might experience problems this August with your GPS systems (including handhelds). At 23:59:59 on August 21st, the satellite's week counters will reach the 1024th week since they were initialized on January 6, 1980. At this precise time, the satellites will reset back to zero.

The Global Positioning System (GPS)—established in1980 and later made famous by Desert Storm—is now used by both military and civilians alike. The system consists of 21 satellites. The basis of GPS is a measurement of time spent by radio signals travelling between the satellite and the receiver.

Each satellite is equipped with an atomic clock, which broadcasts time a little differently than we are used to. It reads time in the form of a GPS week number; no year or century is included in this time code. The clock was set with a base date of Sunday, January 6, 1980. This time keeping system was designed to operate in cycles of 1024 weeks—meaning that about every 19.6 years, the atomic clocks will reset to week zero.

As with the Y2K problem, if your GPS has not been programmed to take a date rollover into account, several problems may occur. The GPS may not be able to calculate your position properly—thus giving you a false location. You may have a hard time getting your GPS to "lock on." Or it simply may not be able to calculate the proper date.

Like I said earlier, the best thing to do is call your local avionics shop or GPS manufacturer and find out if your unit is WNRO safe. This applies to your marine GPS receivers as well. Most GPS receivers made in the past two to four years should not have any problems with the upcoming first rollover.

I suggest—whether your GPS is new or not—you find out if it is ready. Those units that are not ready for the rollover can generally be updated by the manufacturer or your local avionics shop.

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The material in this publication is for advisory information only and should not be relied upon for navigation, maintenance or flight techniques. SW Regional Publications and the staff neither assume any responsibilty for the accuracy of this publication's content nor any liability arising out of it. Fly safe.