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Re: [gpsd-users] Garmin 18X-5Hz

From: Gary E. Miller
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Mon, 15 Aug 2016 15:21:31 -0700

Yo Robin!

On Mon, 15 Aug 2016 23:55:29 +0200
Robin Schwab <address@hidden> wrote:

> Dear Gary
> Am 15.08.2016 um 19:57 schrieb Gary E. Miller:
> > gpsd does discard bad time fixes.  It does so using the
> > manufactureres recommened method, which is to only use time stamps
> > when the GPS has had 3 valid fixes in a row.  
> How up to date is this? Which manufacturer?

Every one is different.  I just looked at a few:

Garmin 18: for all conditions where a valid position and accurate fix has 
           been reported for at least the last 4 seconds.

Garmin 18x: "a valid position fix"

TSIP: time only mode with one satellite in use, otherwise needs a full position

Venus 634: "active after a position fix"

ublox 6: the 1 Hz output is maintained whether there the receiver is locked
         to GPS time or not.

Sirf IV: reports PPS validity is message ID 52.

Some GPS do not say, but I have seen bad behavior of all sorts.
Requiring 4 valid fixes in a row seems good enought, I have seen that
1 alone is not.  If anyone can identify a way to identify bad PPS
that can be shown to catch new problems I'd loe to hear about it.

If it matter to you, best to check the GPS you have, instead of mine.

ntpd driver 22 does not check for a valid fix, thus will pass bad

> > If you can come up with something better, and can prove it with some
> > graphs, we'd ike to hear about it.  
> Well I do not have a running timeserver right now and yours has a 
> problem. Switch on NMEA TDOP, log it and see if it correlates with
> your spike.

Yeah, I have a lot of stuff to log, for now I'm testing the new ntpviz
and hoping it improves my LF problems.  It would be interesting to see if
the TDOP is bad when the GPS reports a valid fix.  My guess right now is
more a kernel stall, leading to delayed NMEA reading.  But only a guess.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

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