Ntpd won't save you from one particular rogue bit

Re:Ntpd won't save you from one particular rogue bit by Lx1oG-AWb6h_ZG0 on Thursday September 28, 2017 @09:26AM (#15356920)

The precise clock is located on the satellites, not receivers. The receivers figure out the correct time by listening for the signals from multiple satellites and matching them against each other and it’s own internal tables. This is the expensive part.

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Re:Ntpd won't save you from one particular rogue bit by Piskvorrr on Thursday September 28, 2017 @10:15AM (#15357236)

Indeed, but the GPS fix you're getting is 4-dimensional: if you have position, you also have precise time. Therefore - listening for the GPS time is not necessary all the time, just update when a precise-enough fix is achieved.

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Re:Ntpd won't save you from one particular rogue bit by monochromatic on Thursday September 28, 2017 @12:19PM (#15358335)

Probably cheaper in terms of battery life to just use NTP than to periodically get a GPS fix.

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Re:Ntpd won't save you from one particular rogue bit by Dylan16807 on Thursday September 28, 2017 @02:48PM (#15359751)

It's worth noting that to get time with .1s accuracy you don't actually need a GPS fix, just to pick up the 50-bit-per-second navigation signal for a couple seconds, from one satellite.

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Re:Ntpd won't save you from one particular rogue bit by monochromatic on Thursday September 28, 2017 @05:26PM (#15360875)

How would that work unless you know the distance to the satellite?

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Re:Ntpd won't save you from one particular rogue bit by nucleardog on Thursday September 28, 2017 @06:26PM (#15361301)

Based off of some quick and dirty calculations for a GPS satellite at approximately 20,000km above sea level, the difference in distance between a satellite directly overhead and one straight out over the horizon is around 5500km.

That's actually only about 18ms difference in delay. Unless there's something I'm failing to account for here, you should be able to get more than 0.1s precision based off of a single satellite's signal.

Subtract a flat ~75ms to account for the minimum delay (~66ms @ 20,000km) and half of the variable delay, and you shouldn't really be more than about 9ms out.

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Re:Ntpd won't save you from one particular rogue bit by Dylan16807 on Thursday September 28, 2017 @06:18PM (#15361233)

The distance to an overhead GPS satellite is somewhere in the range of .07-.09 light-seconds. The planet's width is basically irrelevant when your goal is .1 second accuracy.

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Re:Ntpd won't save you from one particular rogue bit by p1mrx on Thursday September 28, 2017 @06:16PM (#15361219)

If you're on Earth, you can assume that the satellite is closer than 30000 km or so. Light takes 0.1s to cross that distance.

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Re:Ntpd won't save you from one particular rogue bit by Piskvorrr on Thursday September 28, 2017 @12:37PM (#15358551)

Of course. I was thinking "when another app is requesting a GPS fix anyway, might as well use the temporal part, as you're getting it for free."

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