[ntp:questions] Timing GPS recommendations

unruh unruh at invalid.ca
Mon Aug 20 16:13:17 UTC 2012

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

> --< end >--
> I have been using FreeBSD 8.3 with the Sure GPS board for the last while. I get accuracy in the single digit microsecond region most of the time.
> To move to the next level, I believe I need one of these "Timing GPS" units. Any recommendations on specific brands and models for the person not familiar with these devices?  I am familiar with the Thunderbolt unit and the HP z2801 series.  I do not know what a "UT+" is. Are there others? Maybe less expensive or better performing than the Thunderbolt or HP?

You cannot get better than that with a PC. The limitation is NOT the
gps. It is the PC. (The Sure delivers sub 100nsec precision already) It
does not have the sawtooth corrections, but they would be useless

> Ron

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