[ntp:questions] Linux NTPd using a older Trimble Thunderbolt GPS Receiver

William Unruh unruh at invalid.ca
Tue Jan 20 18:45:03 UTC 2015

On 2015-01-20, Rob <nomail at example.com> wrote:
> William Unruh <unruh at invalid.ca> wrote:
>> On 2015-01-20, George Ross <gdmr at inf.ed.ac.uk> wrote:
>>> --===============2288611982837908707==
>>> Content-Type: multipart/signed; boundary="==_Exmh_1421754685_7720P";
>>> 	 micalg=pgp-sha1; protocol="application/pgp-signature"
>>> Content-Transfer-Encoding: 7bit
>>> --==_Exmh_1421754685_7720P
>>> Content-Type: text/plain; charset=us-ascii
>>>> ... Presumably PPS
>>>> was ignored because the event based timing packets yield reliable
>>>> sub-millisecond offsets.  The driver and document should be brought into
>>>> the PPS era and be renamed the TSIP refclock rather than Palisades.
>>>> Palisades/NMEA + ATOM is the way to use these receivers.
>>> From the Acutime 2000 user guide: "The time tag provides a resolution of
>>> 320ns ...".  Is PPS going to be sufficiently better that it would outweigh
>>> the additional setup complexity?
>> ??? The question is not what the resolution of the time tag is. The
>> question is how accurately you can get that time into your computer.
> It turns out that the device has a mode where you can SEND a pulse at
> a moment you decide and then the device RETURNS the timestamp of that
> pulse you sent in a serial message.
> Presumably you can take a nanosecond timestamp and change the output line
> as closest together as possible, then read the returned timestamp and
> compare the two.
> That is equivalent in precision to receiving a PPS pulse, maybe even better.
> (because you don't have interrupt latency issues, the only issue is how
> close the pulse moment can be to the reading of the system time)

OK, that would certainly be a different situation. You could presumably
read the clock and toggle the pin to sub microsecond precision. You
would presumeably want a daemon to read the clock and toggle the pin,
perhaps with interrupts turned off for the microsecond required so that they could not get between
the two. Not sure how long a pulse it requires.

> The Datum we use has that option too, but I never tried it.  Maybe soon.

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