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

Rob nomail at example.com
Tue Jan 20 18:34:31 UTC 2015

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)

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

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