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Re: [gpsd-dev] GPS week wraparound
From: |
Eric S. Raymond |
Subject: |
Re: [gpsd-dev] GPS week wraparound |
Date: |
Fri, 27 Sep 2013 13:31:50 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Andy Walls <address@hidden>:
> It is obvious the GPS unit has an initial, hard-coded leap second count
> that is out of date.
Good point. I've inserted a sentence about this. Since I had added a
'graph about PPS that will be of interest to NTP people shortly after that,
here's how the whole last section now reads:
5) Another source *may* be available - the GPS leap second count, if we can
get the device to report it. The latter is not a given; SiRFs before
firmware rev 2.3.2 don't report it unless special subframe data reporting
is enabled, which requires 38400bps. Evermore GPSes can't be made to
report it at all. Furthermore, before the almanac load the GPS may report
a fixed (and possibly out of date) offset.
Conclusion: if the system clock isn't accurate enough that we can deduce
what rollover period we're in, we're utterly hosed. Furthermore, if it's
not accurate to within a second and only NMEA devices are reporting,
we don't know what century it is!
Therefore, we must assume the system clock is reliable to within a second.
However, none of these caveats affect the usefulness of PPS, which
tells us top of second to 50ns accuracy and can be made to condition a
local NTP instance that does *not* rely on the system clock. The
combination of PPS with NTP time should be reliable regardless of
what the local system clock gets up to.
I left out my worries about bufferbloat-induced NTP clock skew...
--
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
- Re: [gpsd-dev] GPS week wraparound, (continued)
- Re: [gpsd-dev] GPS week wraparound, Eric S. Raymond, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Harlan Stenn, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Andy Walls, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Hal Murray, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Andy Walls, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Magnus Danielson, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Hal Murray, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Magnus Danielson, 2013/09/28
- Re: [gpsd-dev] GPS week wraparound, Eric S. Raymond, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Harlan Stenn, 2013/09/28
- Re: [gpsd-dev] GPS week wraparound,
Eric S. Raymond <=
- Re: [gpsd-dev] GPS week wraparound, Hal Murray, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Eric S. Raymond, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Gary E. Miller, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Eric S. Raymond, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Magnus Danielson, 2013/09/27
- Re: [gpsd-dev] GPS week wraparound, Magnus Danielson, 2013/09/27