[ntp:questions] Tracking the drift of a GPS clock relative to a HW clock

ryad.bek at gmail.com ryad.bek at gmail.com
Wed Jan 21 17:05:22 UTC 2009


On Jan 21, 9:24 am, "Maarten Wiltink" <maar... at kittensandcats.net>
wrote:
> <ryad.... at gmail.com> wrote in message
>
> news:896fd879-86c6-4c12-9528-139e59d24093 at w1g2000prk.googlegroups.com...
> [...]
>
> > I'm trying to track the drift of my GPS clock RELATIVE to
> > the clock that I would have obtained without GPS (and vice versa).
>
> If I understand you correctly, you want to generate a log with
> two timestamps on each entry: one free-running, one GPS-synced.
>
> You could _not_ run NTP on your test machine, and whenever a message
> is logged, fetch a timestamp from another machine that does run NTP.
> Microsecond accuracy on that machine is not very hard with a GPS
> clock, but getting it into the test machine with the same accuracy
> may be a bit of a problem. It's mostly a matter of having the transfer
> take about the same time each time; if a good timestamp always arrives
> in exactly thirty milliseconds, that's good, you can correct for that.
>
> Another avenue is to run NTP on the test machine but not let it touch
> the clock (or not let it believe the other NTP server or for that
> matter any other server; you want a free-running clock). You still
> need the GPS-synchronised other NTP server, but running NTP on the
> test machine gives you easy, continuous, and fast access to an estimate
> of the current error in the local, free-running clock, and thus an idea
> of what the 'real' time is.
>
> > My final goal is to convert a series of gps timestamps to the
> > equivalent unsychronized timestamps.
>
> Whereas I propose doing it the other way around. It produces the same
> information and it's easier to do. I think.
>
> Groetjes,
> Maarten Wiltink

Hi Marteen,

I've done it (a log with two timestamps on each entry: one free-
running, one GPS-synced) with a single machine and I think the results
sound good.
I've posted a new message which expains how I does it:
"A practical implementation for tracking the drift of a GPS clock
relative to a non steered clock (& the associated results)"

see also http://www-phare.lip6.fr/~kezadri/temp/tstamp_conv.html

Note that the non steered clock is the clock that the PC would have
used if no GPS was installed.

Regards
Ryad




More information about the questions mailing list