[ntp:questions] Timing GPS recommendations

Chris Albertson albertson.chris at gmail.com
Mon Aug 20 17:06:06 UTC 2012

On Mon, Aug 20, 2012 at 9:13 AM, unruh <unruh at invalid.ca> wrote:

> On 2012-08-20, Hahn, Ron <ron.hahn at fmr.com> wrote:
> > Colleagues,
> >
> > Chris Albertson, made the following statement:
> >
> > --< begin >--
> > I'm using the t-bolt.  It seems to work.   I guess there might be more
> > functions the driver could implement.  Perhaps better support for
> > self-survey or logging or whatever.  I'd like to see internal temperature
> > logged.  But for normal NTP use it works fine.
> >
> > If you only have one GPS the t-bolt is the one to have.  But if you can
> > afford two there are lower priced GPSes that use less power that work
> well
> > with NTP.   I have the t-bolt and two UT+ units.
> >
> > Whatever you get make sure it is a TIMING gps.  The Garmin units and the
> > Sure Electronic board at not timing GPS although they do have PPS
> outputs.
> >  They work but they are "uSec" level devices.  I'd look for 100ns or
> better.
> No The sure is 10s of ns device. Unfortunately this is useless as you
> cannot get the time into your computer to better than usec. The
> interrupts are not serviced fast enough on any PC to give better than a
> few us and the interrupt routines get delayed on a working system by
> more than that at times (eg due to disk priority, or things switching
> off interrupt processing).
> You would have to build a special board for your computer to better than
> usec.
The errors add together.  If the GPS and the computer both had 1 uSec of
uncertainty the total is 2uSec.  So there is an advantage to having a GPS
that is in effect "zero".  I think this is the same ia when you build a
circut with a bunch of 5% tolerance resisters.  You can get much larger
than 5% error because of the ways resisters add.

I think I would some day like to try and build an external PPS interface
that works at nanosecond resolution.  It would not be hard.  A fast counter
is PPL'd to a 10Mhz lab reference and then the PPS captures the counter and
then interrupts the computer.  the computer then reads my external box
rather then the CPU counter.  Only a few lines of C code need to change
inside the Linux driver.

Chris Albertson
Redondo Beach, California

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