4 ms·

One of the things I love about GPS is: Since you know your exact position, you can pick a good GPS satellite (one of the satellite's you're using to calculate y

One of the things I love about GPS is: Since you know your exact position, you can pick a good GPS satellite (one of the satellite's you're using to calculate your position), look at the timestamp from that satellite, and use it as a highly-accurate time source! Purpose-built GPS time servers (like those from Meinberg) give you an option to enter the length of the coax cable connecting the receiver and the antenna, so that it can correct for the extra time it takes for the signal to travel over the cable (for example, see https://www.meinbergglobal.com/download/docs/manuals/english/m300_gps_ltos5.pdf https://www.meinbergglobal.com/download/docs/manuals/english... page 19).

withinboredom 5y ago
Then you’d have a Stratum 0 source for a stratum 1 NTP server!
- AceJohnny2 5y ago
  Which is what Google did to have a quality time-source for synchronized time for their global database: https://www.wired.com/2012/11/google-spanner-time/ https://www.wired.com/2012/11/google-spanner-time/
  - jlgaddis 5y ago
    > ... quality time-source ... Except they decided to "smear" leap seconds [0], instead of handling them appropriately. For that reason, if you "require correct timestamps for legal purposes" [1], you may want to make sure that you aren't using Google's NTP servers. (N.B.: Amazon (AWS) too, FWIW.) --- [0]: https://developers.google.com/time/smear https://developers.google.com/time/smear [1]: https://docs.ntpsec.org/latest/leapsmear.html https://docs.ntpsec.org/latest/leapsmear.html
floatrock 5y ago
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.
jcrawfordor 5y ago
It's a bit more complex than this, the entire GPS fix is 4D since position depends on time and vice versa. The time reported by a GPS receiver, once fix is attained, is not just the time from one of the satellites but the time resulting from the 4D fix in space and time. This eliminates (to within a certain precision) the latency. A lot of discrete GPS receivers have some nonvolatile storage where they "cache" fixes to reduce fix time. This has the amusing result that when you buy a GPS receiver and monitor its output immediately you usually find out the time and location where QA was performed, as the first fixes emitted without the quality flag.
- CaliforniaKarl 5y ago
  Yup, that's covered by the functions on Pages 20 and 21, which let you either completely wipe all stored state, or update it to account for being moved a long distance (while still retaining satellite data).
- myself248 5y ago
  Or a test location emitted by QA's satellite simulator. I have a small list of funny locations I like to pipe into gps-sdr-sim, including Null Island, the north pole, 500 feet above the Kremlin, the middle of Lake Erie, and a quiet beach in the Bahamas. Not that I expect anyone to look at the first few sentences of output after I hand them hardware, but if they _do_...
- litoE 5y ago
  I have a hand-held GPS receiver that was last used in Chicago in November. I just turned it on again in another part of the world but inside a reinforced concrete building, where is gets no satellite signals. It still thinks it's at the Chicago airport.

withinboredom 5y ago

Then you’d have a Stratum 0 source for a stratum 1 NTP server!

AceJohnny2 5y ago
Which is what Google did to have a quality time-source for synchronized time for their global database: https://www.wired.com/2012/11/google-spanner-time/ https://www.wired.com/2012/11/google-spanner-time/
- jlgaddis 5y ago
  > ... quality time-source ... Except they decided to "smear" leap seconds [0], instead of handling them appropriately. For that reason, if you "require correct timestamps for legal purposes" [1], you may want to make sure that you aren't using Google's NTP servers. (N.B.: Amazon (AWS) too, FWIW.) --- [0]: https://developers.google.com/time/smear https://developers.google.com/time/smear [1]: https://docs.ntpsec.org/latest/leapsmear.html https://docs.ntpsec.org/latest/leapsmear.html

floatrock 5y ago

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.

jcrawfordor 5y ago

It's a bit more complex than this, the entire GPS fix is 4D since position depends on time and vice versa. The time reported by a GPS receiver, once fix is attained, is not just the time from one of the satellites but the time resulting from the 4D fix in space and time. This eliminates (to within a certain precision) the latency. A lot of discrete GPS receivers have some nonvolatile storage where they "cache" fixes to reduce fix time. This has the amusing result that when you buy a GPS receiver and monitor its output immediately you usually find out the time and location where QA was performed, as the first fixes emitted without the quality flag.

CaliforniaKarl 5y ago
Yup, that's covered by the functions on Pages 20 and 21, which let you either completely wipe all stored state, or update it to account for being moved a long distance (while still retaining satellite data).
myself248 5y ago
Or a test location emitted by QA's satellite simulator. I have a small list of funny locations I like to pipe into gps-sdr-sim, including Null Island, the north pole, 500 feet above the Kremlin, the middle of Lake Erie, and a quiet beach in the Bahamas. Not that I expect anyone to look at the first few sentences of output after I hand them hardware, but if they _do_...
litoE 5y ago
I have a hand-held GPS receiver that was last used in Chicago in November. I just turned it on again in another part of the world but inside a reinforced concrete building, where is gets no satellite signals. It still thinks it's at the Chicago airport.