[ntp:questions] SIRF time output wobble, GPS or NTP fault?

David J Taylor david-taylor at blueyonder.co.uk.invalid
Thu Mar 8 15:50:16 UTC 2012


"Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message 
news:4F58B4FC.4060500 at c3energy.com...
[]
> On the Status tab, that's where it displays all the offsets, similar to 
> the ntpq -p command.  I don't know how it gets this data.  On the 
> Statistics screen, if you select the directory where your statistics 
> files are, you can graph either loopstats or peerstats files.  I think 
> clockstats is the third type.  I don't know if there is anything to 
> graph in that or not.

No, I've not played with those fields in that program.  I wrote my own 
loopstats analyser.

>> The Fix Quality field.  0 = Invalid, 1 = GPS fix, 2 = DGPS fix.  I 
>> would hope that NTP rejects any data with 0 in this field, but I don't 
>> read the sources at all well.
>>
>
> OK. I see your point.  They call that FIX_MODE on this page which you 
> mentioned before.
>
> http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html
>
> I didn't see that field before.

There are lots of things I can't even find, let alone "see" them!  <G>

[]
> Based on the aforementioned web page, it looks like the possible 
> validity fields are as follows:
>
> GPRMC sentence - POS_STAT
> GPGLL sentence - POS_STAT
> GPGGA sentence - FIX_MODE
> GPZDA sentence - none, as you observed
> GPZDG sentence - V or maybe signal strength
>
> I don't know if my GPS will do GPZDG.  Otherwise I might try it.  It may 
> be proprietary to some GPS's.
>
> We still don't know if NTPD recognizes any of these fields, or if the 
> behavior is different in Windows and Linux.  If it does, I might switch 
> back to another sentence.  However, I already know it will increase my 
> NMEA jitter.  If I can get PPS working, that won't matter as much. 
> That's not an option with the BU-353 though.

Take a look at the source, refclock_nmea.c, around line 868 in the copy I 
have.  It does checks on $GPZDG, $GPMRC, $GPGGA and $GPGLL, but what it 
does with the checks I can't easily follow.

[]
> Peerstats is now on.  Not totally sure what to do with it, but it's 
> there.
>
> Sincerely,
>
> Ron

David Lord answered this, I wrote a small Windows program for my own 
purposes.

Cheers,
David 



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