Blog entries from July, 2019

GPS Week Number Rollover

Saturday, July 27th, 2019 at 7:42 pm

A quirk of the GPS system is that it uses a week counter to represent the date, this can only represent a maximum of 1024 weeks, so every ~19.7 years it resets to zero, known at the GPS Week Number Rollover.

The second rollover was earlier this year and there was plenty of media coverage as well as government statement at the time. I also saw a number of YouTube videos about it, such as this one from Matt Parker:

The handheld GPS unit I use for geotagging photos is from Garmin, so I was particularly interested in what they had to say about it. Well that is non-specific… is my device (obtained second hand twelve years ago and last firmware update was ten years ago) part of the “vast majority” that “will handle the WNRO without issues”?

I wasn’t that concerned so opted for a wait and see approach, so when I used it in late April I was pleased to see that it was logging the correct date. However when I used it in May and June I noticed that there would be spurious log files, as if it was starting up with the wrong date and then adjusting to be correct.

Today was my first time heading out to take photos since June, so I was disappointed to find that my GPS device is now stuck in 1999. The location and time are correct, just the date is wrong.

When geotagging my photos from the GPS track I would specify the time offset between the camera (local time of where I was) and UTC, so for Melbourne at UTC+10 that is an offset of -36000 seconds. Since the date it is logging is now 1024 weeks out, I could specify an additional offset of -619315200 seconds…

The more reasonable option is to look for a new GPS unit, but with my key requirements of AA (or AAA batteries) and a GPX file written to an SD card I suspect that I might just end up with a newer version of what I have. I have seen some nice small dedicated GPS logging devices, but I want to be able to easily swap in fresh batteries or don’t want to be forced to use their software to access the tracks.

I had a quick look at eBay to get an idea of what might be available, saw plenty of devices like what I already have, but don’t see any mention of GPS Week Number Rollover in the item descriptions…

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