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

unruh unruh at wormhole.physics.ubc.ca
Fri Feb 5 17:58:05 UTC 2010

On 2010-02-05, David Lord <snews at lordynet.org> wrote:
> Hi
> Now I have gps-18x-LVC nmea along with
> parallel port PPS using pps0 at ppbus0.

What sentences is the nmea sending out?
Why not use something like gpsd and the shm driver to combine the nmea
and the PPS into one very accurate time stamp?
> This seemed to work ok until pps timestamps suddenly jumped to
> -350ms then PPS became false then deselected.
> To correct for this I've set fudge " time1 0.65"
> also "tos mindist 0.4"

That sounds far too big. Even at the slowest serial spped it takes a lot
less than that for a single nmea sentence to come it. 

> Somehow I think I might be in sync with wrong edge of PPS.


> Eventually I've got an ethernet cable running upstairs
> (3 x long cables + 2 couplers).

Depends on what you call long. 10m should easily  "get it upstairs" but
10 m is not that long. 

> I've added 3 x local servers sync to MSF + public servers
> and marked noselect.
> About 30 min later "ntpq -p" has:
> source reach offset(ms)
> + GPS   377  23.247

Not surprizing. NMEA takes a long time delivering. 

> o PPS   377  -0.298
>    serv1 377  -1.295
>    serv2 INIT  0.000
>    serv3 377  -0.091
> I suspect reason for INIT is both serv1 and serv3 have serv2
> selected. The GPS offset has been varying +/- a lot but pps
> still seems to be converging.

??? What serv1 and 2 do should not matter. What are their stratums?

> That looks ok to me except only Google hit for gps18x-lvc +
> "fudge time1" gives setting of 0.0001
> What does anyone else use for this fudge time1?

It depends on how many nmea sentences are being collected.

> I'm using default nmea strings as garmin loses its settings
> otherwise whilst not powered up.

That is a mistake. YOu only need the one. And the garmin does not loose
its settings -- it has an onboard battery. Besides the software should
reset the system when you power up.

> David

More information about the questions mailing list