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Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous

From: tz
Subject: Re: [gpsd-dev] Direct GPS support in ntpd makes me nervous
Date: Tue, 22 Oct 2013 14:51:52 -0400

It depends on the vote.  Some GPS will have problems like the date NOT rolling over at midnight exactly but a few samples, so if there is no network or something else, bad things can happen.  If there is a sanity check (and assumes thorazine, straitjackets and padded cells), maybe, but the only thing you should use a GPS for is the PPS signal itself.  The "fix" will usually occur well after (sometimes a half second).

There is the accuracy v.s. precision dichotomy.  The PPS can make the time value very precise (assuming it doesn't emit it when the fix isn't good).  But the NMEA nonsense can result in inaccuracies - of a day as in the case of the above (a Garmin 5hz unit I used).

A Dallas Semiconductor I2C RTC chip can be tuned and synced so its "PPS" syncs with the GPS and it would be far better a time source (2PPM!) than any raw GPS not designed for time (SkyTraq has a 10nS GPS chip designed for just this)

On Tue, Oct 22, 2013 at 1:46 PM, Gary E. Miller <address@hidden> wrote:
Yo Eric!

On Tue, 22 Oct 2013 11:14:21 -0400
"Eric S. Raymond" <address@hidden> wrote:

> > Type 20 is Generic NMEA, but there are others (Jupiter, Oncore,
> > Palisade, etc.).
> Heads up, Harlan. You need to hear this.
> Learning that there is direct GPS support in ntpd makes me nervous.
> I recommend you take a good hard look at *removing* it.

I think this is a tad harsh. ntpd assumes the daemon is network
connected and so has a number of other sources to get data like the
current century and the leap seconds.  It also has no need for position,
velocity, error estimates, sky view, etc.  All ntpd really needs is the
top of the second from the PPS.  Then it takes that data from the
multiple sources, applies voting rules and comes up with a best guess.

If it misses some GPS time thing then the voting will ignore the GPS
data until later it seems valid.

So if time is all you want, ntpd is the way to go.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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