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Most GPS devices made at least after 1999 implement an unwrapping algorithm that has an epoch at some random date set by the maker in the firmware. Essentially
by nullc 8y ago
Most GPS devices made at least after 1999 implement an unwrapping algorithm that has an epoch at some random date set by the maker in the firmware.
Essentially the firmware knows that the day is later than say, July 4th 2004 because that's when it was built. If the GPS date is less than that date, it adds 1024 weeks repeated until its greater than the hardcoded date.
This means that the exact wrap date will depend on the receiver, though some will wrap on April 6th
Fortunately GPS rollover doesn't normally get in the way of positioning. The only way it would is if the 'wrong' dates triggered some other bug.
- makomk 8y agoApparently it did cause positioning issues last time around. One major problem was that GPS receivers calculate which satellites they expect to see in the sky based on the current time and the almanac+ephemeris data, and at the wraparound some receivers started expecting all the satellites to be where they would've been 1024 weeks ago and completely lost lock. I think there were some problems caused by individual satellites wrapping their counts slightly before the actual GPS time too. Receivers which are started up after the 6th should be fine though.
- gsich 8y agoWrong almanac means a somewhat longer time for first fix.