Written on 15/02/19 by Paul Oldham

Got an old GPS? It might stop working on 6th April

We discovered this week via a posting on The Register that older GPSs, satnavs, and other such devices won't be able to use America's Global Positioning System properly after 6th April unless they've been suitably updated or designed to handle a rollover similar to the Y2K problem.

GPS constellation

For the technical minded the issue is that the GPS signal coming down from the satellites includes a timestamp, which your GPS needs in order to work out its location. The problem is that the week number in that timestamp is stored in 10 bits, which means the week number can be in the range 0 to 1023 and 1023 weeks is about twenty years. This means that every twenty years or so the week number rolls over back to 0 and starts counting up again.

The last time the week number overflowed was in 1999, nearly two decades on from the first original week zero in January 1980. Back in 1999 it wasn't a great problem as, outside of the military, not many people had devices which relied on the GPS signal. This time around it's going to be very different.

The problem is that if your device is not designed or patched to handle this latest rollover, it will revert to an earlier year after 6th April, causing attempts to calculate position to fail.

GPS devices with a poorly implemented GPS Time-to-UTC conversion algorithm may provide incorrect UTC following a [week number] rollover

Garmin Foretrex 101

Explained US Homeland Security in its write-up of the issue.

The good news is that more recently built (or patched) GPSs should be fine. Edward Powers of the US Naval Observatory notes:

Most newer receivers are likely going to be OK if ICD-200 was followed

What this means in practice is that newer receivers built after around 2010 should be fine.

We've been looking through our GPS devices. Our Garmin eTrex 10 and eTrex 20x GPSs should be fine as they're recent purchases, our Garmin Foretrex 301, which we bought in 2010, may be OK but we'll be treating it with caution after 6th April. As for our elderly Garmin Foretrex 101 all bets are off with that one. Our TomTom One satnav is also pretty elderly too, although it has had updates so again we might have to stop using that.

Anyway the bottom line is that if you have a GPS and you rely on it when out walking then do check it's still working correctly after 6th April. Similarly with any satnav in your car.

Tagged: GPS, news

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