GPS Q&A
By Don Meyer

Q.What is the GPS Week Rollover?
A.B. Portland Ore.

A.Rather than using a second/hour/day/month/ year approach to reporting time, as we are accustomed to doing, the Global Positioning System (GPS) represents time as a numbered week and the number of seconds into that week. The GPS weeks are numbered zero through 1,023. Week zero of the GPS clock began in January 1980. At the completion of week 1,023 on August 21/22, 1999, the GPS week will "rollover" and begin again with week zero.

Q.What impact will the rollover have on a GPS receiver?
T.M. New Orleans, La.

A.The message broadcast by each GPS satellite includes the precise time (numbered week and second of the week), the position of the satellite, the health of the satellite, and orbital information about the other satellites in the system. GPS receivers use clock synchronization and time codes to determine how long it takes the signal from each satellite to reach the receiver. The time, divided by the speed of light (186,000 miles per second) equals distance. The distance of the receiver from three or four satellites is triangulated resulting in an intersection representing the position of the GPS receiver on or above the Earth.
     The rollover of the GPS week to zero in 1999 could have the following effects:
     A GPS receiver which had been in use prior to the rollover may, on its first attempt to lock onto satellite signals after the week rollover, begin looking for satellites where they aren't, thinking that it is 1980.
     A GPS receiver may reject satellites as "unhealthy" because it believes those satellites are broadcasting incorrect time. In either instance, it is possible that the GPS receiver may not be able to calculate an accurate position or report accurate time.
     Anticipating the GPS week rollover issue, Magellan has been inserting its own rollover correction into the software of all of its GPS receivers. This correction will allow the unit to function unaffected by week rollover for 19 years from the software production date. This is accomplished as follows:
     Every version of software for Magellan's GPS receivers has the then current GPS almanac (the complete data on the orbits and location of the GPS satellites) installed in non-volatile Read Only Memory (ROM). This almanac is anywhere from one to six weeks old, and has a week number associated with it.
     Magellan's software assumes that the current date is always after the week in the ROM almanac.
     When the week number broadcast by the satellite is less than the week in the Magellan ROM almanac, the software automatically increases the week number by 1,024 to account for the rollover. Therefore in August 1999, when the GPS week resets to zero, any Magellan receiver in service at that time will increase the week broadcast by the satellites by 1,024, thus synchronizing the GPS week with that recognized by the receiver.
     In this manner, a Magellan GPS receiver will function through the GPS week rollover period unaffected. It will have its own rollover in 1,024 weeks from the time the software version was created, however, if the receiver is initialized at that time with the then current day, date and time, the receiver will again add 1,024 weeks and begin a new 19-year cycle.
     Example: A Magellan GPS receiver with software released on 11/17/95 contained a ROM almanac for GPS week 824 (week starting 10/22/95). On August 22, 1999, the Magellan receiver enters week 1,024 and the GPS week rolls over to week zero. By adding 1,024 to the GPS week, the Magellan receiver brings both time periods into synch at 1,024. The Magellan GPS receiver continues to function until week 1,848 (824 plus 1,024) which is the calendar date of May 31, 2015. The software has been designed such that if the user merely enters in the current date and time, the receiver will recycle and begin another 1,024 weeks.

QUESTIONS ABOUT GPS? Send your GPS questions to us at: EOM, 4901 E. Dry Creek Rd., Suite 170; Littleton, CO 80112, fax to 303-713-9944, or send via e-mail: [email protected]

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