[ntp:questions] fudge time1 for gps-18x-LVC?

Dave Hart davehart at gmail.com
Sun Feb 7 17:22:18 UTC 2010

For some reason the copy of this message I received via
questions at lists.ntp.org was truncated after the Terje quote,
conveniently omitting everything I said.  In Google's Group's usenet
web interface, the complete message is displayed.

So, once more with feeling, and without any correction like removing
the duplicate signature...

---------- Forwarded message ----------
From: Dave Hart <daveh... at gmail.com>
Date: Feb 7, 16:40 UTC
Subject: fudge time1 for gps-18x-LVC?
To: comp.protocols.time.ntp

On Feb 7, 14:59 UTC, Terje Mathisen wrote:

> David J Taylor wrote:
> > Indeed, but I think we've talked about this before with David and
> > "default" may mean "default for use with NTP". We'll see. Certainly the
> > single sentence is the best - I happen to use: $GPRMC.

> I'm trying to make $GPGGA the default, it is a much more informative
> sentence (using 'mode 2' in the server statement specifies this):

> It contains the current time (hms), position (ddmm.mmmm), nr of visible
> sats, DOP etc, which makes it easy to write a little perl script to
> process the clockstats file to gather all sorts of interesting statistics.

Historically, the NMEA driver claimed to use any/all sentences in the
default (mode 0) configuration, but it lied and in fact treated mode 0
the same as mode 1, which is to say it ignored all but $GPRMC.  This
was fixed during the period 4.2.5 was ntp-dev, and with the current
code, you really can use any/all of the supported sentences by
configuring mode 0.  That allows you to choose which sentence to use
without touching ntp.conf, by reconfiguring the NMEA device to send a
single sentence of your choosing.  I don't know about you, but looking
up modes on the NMEA driver's driver22.html and configuring them is
tedious to me -- I'd much rather fire up the Garmin configuration
software and switch sentences that way.  Moreover, using mode 0, there
is need to match changes to the device configuration with changes to
the mode number.  You can see which is being used easily enough:

ntpq -ccv

If you have more than one reference clock driver configured, you can
select which using the longer form:

ntpq -p -c "cv &1"

&1 is equivalent to the first association ID and must come after a
command which lists the associations such as -p / peer or assoc.

The cv (clockvar) output looks like:

associd=35286 status=0011 , 1 event, clk_no_reply,
device="NMEA GPS Clock",
timecode="$GPGLL,____.____,N,_____.____,W,163332,A*__", poll=27796,
noreply=1, badformat=0, baddata=0, fudgetime1=-6.729, stratum=0,
refid=uPPS, flags=8

(that's with fudge flag4 1 set to obscure the location.)

So to be clear, the NMEA driver no longer has any preference for
$GPRMC in 4.2.6 and later.  With mode 0, every recognized sentence is
processed, with the effect that the last recognized one seen is the
one used.

Dave Hart

Dave Hart

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