4 ms·
Sure, gps geek and time-nut here. The rollover problem is a hard one to fix, because there's no reasonable way for a GPS (at least one using LNAV (legacy) sign
by elfchief 8y ago
Sure, gps geek and time-nut here.
The rollover problem is a hard one to fix, because there's no reasonable way for a GPS (at least one using LNAV (legacy) signals -- pretty much all of them, though I don't know what modern passenger jets use) to know which GPS "epoch" they're in. If you don't know what epoch you're in, you don't know what the date is.
Some GPSs solve this by recording the week number of their firmware build, and if the signals they get indicate the week number is less than that, they assume they're in the next epoch and update accordingly. Still means you run out of time, but you get a full ~19 years of life first (and then fail at some random GPS week). Some use fuses to record when an epoch has passed. Some use out of band information. Some store the current epoch in flash or battery-backed RAM. Some just never address the problem.
Thing is, though -- the only effect this has, is that it makes the GPS return the wrong date. That's it. The week rollover has no effect on navigation unless there's some significant bugs in the unit, or something external to the GPS relies on the date being output and doesn't deal well with the date suddenly going back in time. That's it.
I'd be kinda shocked if airplanes were just jam-syncing the clocks of their nav computers to the output of a GPS, and then had those nav systems be dependent on that time. But then again, I work in the tech industry, and I've seen the kinda code that goes into most products, so maybe I wouldn't be that shocked.
(I also can't imagine that GPS units used in aircraft aren't directly tested for their behavior during the week rollover. It's a well known and understood problem, and even if some random GPS manufacturer drops the ball, stuff that goes into aircraft has to go through certification for a reason...)
- sho 8y ago> I'd be kinda shocked if airplanes were just jam-syncing the clocks of their nav computers to the output of a GPS, and then had those nav systems be dependent on that time Obligatory not an aircraft software engineer, but I can say with some certainty that this is not the case. Aviation software, despite the recent issues with the 737 MAX, is generally designed and implemented to very high level of redundancy and SPOF-avoidance. From an airbus manual, to give you some idea: A300-600s, A310s, A320s, A330s and A340s with GPS PRIMARY The navigation system of these aircraft consists of 2 FMS, 3 IRS, 2 GPS and radio navaid sensors. The GPS position is primarily used for FM position updates, however if GPS PRIMARY is lost, FM reverts to radio updates or to IRS ONLY navigation when outside radio navaid coverage. FM(S) = Flight Management (System). IRS = inertial reference system (THREE of them). If GPS is lost or malfunctioning, the FMS falls back to that - which is periodically updated by reference to RNAV ground-based radio beacons. GPS could simply switch off worldwide and aircraft would be fine, just like they were pre-GPS. There is no conceivable reason the navigation system would especially care about the time being received from its GPS units, let alone perform some critical calculation based upon it. This article is pretty uninformed fear-mongering.
- bloak 8y agoThe trouble with fuses is that if a receiver is tricked by a bogus or incorrectly received signal into moving into the next (sub)epoch then you can never get it to move back, unless there's a manual override, which is an option you didn't mention: not possible in some embedded applications, perhaps, but many consumer GPS devices already force the user to answer several questions whenever the device is rebooted. Another option is trying to guess the epoch from the TAI-UTC offset, which, of course, can't be known in advance and to some extent depends on political decisions so that's more of an "interesting" option than a good or practical one.
- nyc111 8y ago> gps geek and time-nut here. This is off topic, but since you describe yourself as "gps geek" I thought I ask. Is it possible to gain access to the actual operational code where GPS operators correct for General Relativity? I'm not disputing GR, I just want to find out if this is a myth or true. Thanks.
- jimktrains2 8y agoNot op and maybe I'm misunderstanding, but the correction is part of the spec in satellite design, not something that can be kept secret[1]. The timing code that is translated into navigational data, however, is encoded and encrypted, which have some well-known keys, and that used to prevent high accuracy. My understanding is that the us government can change those to prevent GPS usage or to limit accuracy. [2] [1] http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.... [2] https://www.trimble.com/gps_tutorial/sub_pseudo.aspx https://www.trimble.com/gps_tutorial/sub_pseudo.aspx
- TickleSteve 8y agoHaving worked on gps chipsets, one of the corrections that need to be applied is for relativistic effects,those calculations are happening in every GPS chipset as it needs to iterate down to a very precise position estimation of the satellites in order to produce an estimation of your position. There is no magic involved, relativity/time-dilation is not a disputed effect. As an example, clone https://github.com/gnss-sdr/gnss-sdr https://github.com/gnss-sdr/gnss-sdr and search for "relativistic correction"