4 ms·

My car does this. Unfortunately, it's a Honda/Acura, and there's a downstream bug in the way the receiver sends the info to the clock display that this year, al

My car does this. Unfortunately, it's a Honda/Acura, and there's a downstream bug in the way the receiver sends the info to the clock display that this year, almost all older Hondas/Acura's are reporting the wrong time: https://didhondafixtheclocks.com/ https://didhondafixtheclocks.com/ > Honda’s head unit receives a GPS signal for date and time including a number representing a week, coded in binary. These digits count from 0-1024 and rollover to 0 after the completion of week 1024. Honda’s head unit supplier did not code their head units to account for the rollover and, on January 1, 2022, reverted to a date and time 1024 weeks in the past [1024/52 = 19.7, so 20 years in the past or 2002] So despite all the almost-magic level of engineering that has gone into the GPS system that has stayed consistent for 40-some-odd years, a classic integer overflow has ruined it all because some subcomponent test engineer didn't think to check the inputs against the expected lifetime duration of the car's equipment. Another fun issue with these is DST databases. The satellites will tell you the time, but it's up to you know how your location translates into a DST zone. And if you have long-running offline equipment (say, a car), and the DST dates change, well, your smarts are only as smart as the update procedure.

myself248 5y ago
It's been said that week number rollover occupies a "sweet spot of awfulness" where it happens infrequently enough that it doesn't get much testing, but often enough to impact equipment deployed in the real world. The designers of GPS either should've made it use like 64 weeks so WNRO would happen constantly and we'd have to get good at handling it, or 32768 weeks so we could ignore it for the entire life of the system and any successors.
- michaelt 5y ago
  > It's been said that week number rollover occupies a "sweet spot of awfulness" It's even worse than that: The traditional way of handling week number rollover is a rollover count in nonvolatile memory, incremented every time a rollover is seen (for a standalone receiver there's no other source for how many rollovers there have been) So a griefer with a software-defined radio can radio out repeated week number rollovers and GPS receivers will increment their rollover counters. In 99% of cases there's no way to decrease them, and now your GPS receiver is convinced it's year 2100.
  - myself248 5y ago
    Perhaps traditional in devices without a UI. My car's stereo (which is where the GPS/nav functions also live) has an epoch dropdown in the settings menu. I presume you're referencing https://users.ece.cmu.edu/~dbrumley/pdf/Nighswander%2520et%2520al._2012_GPS%2520software%2520attacks.pdf https://users.ece.cmu.edu/~dbrumley/pdf/Nighswander%2520et%2...
    - myself248 5y ago
      Ooh it just hit me. Maybe that rollover counter itself can be rolled over, so just transmit 253 more rollover events....
gsich 5y ago
Your car does it somewhat different then those Meinbergs. Yes you can get datetime from GPS, but what you really want is a clock signal that triggers 1 PPS.

myself248 5y ago

It's been said that week number rollover occupies a "sweet spot of awfulness" where it happens infrequently enough that it doesn't get much testing, but often enough to impact equipment deployed in the real world. The designers of GPS either should've made it use like 64 weeks so WNRO would happen constantly and we'd have to get good at handling it, or 32768 weeks so we could ignore it for the entire life of the system and any successors.

michaelt 5y ago
> It's been said that week number rollover occupies a "sweet spot of awfulness" It's even worse than that: The traditional way of handling week number rollover is a rollover count in nonvolatile memory, incremented every time a rollover is seen (for a standalone receiver there's no other source for how many rollovers there have been) So a griefer with a software-defined radio can radio out repeated week number rollovers and GPS receivers will increment their rollover counters. In 99% of cases there's no way to decrease them, and now your GPS receiver is convinced it's year 2100.
- myself248 5y ago
  Perhaps traditional in devices without a UI. My car's stereo (which is where the GPS/nav functions also live) has an epoch dropdown in the settings menu. I presume you're referencing https://users.ece.cmu.edu/~dbrumley/pdf/Nighswander%2520et%2520al._2012_GPS%2520software%2520attacks.pdf https://users.ece.cmu.edu/~dbrumley/pdf/Nighswander%2520et%2...
  - myself248 5y ago
    Ooh it just hit me. Maybe that rollover counter itself can be rolled over, so just transmit 253 more rollover events....

gsich 5y ago

Your car does it somewhat different then those Meinbergs. Yes you can get datetime from GPS, but what you really want is a clock signal that triggers 1 PPS.