[ntp:questions] Garmin GPS 18LVC Setup but questions on best way

Unruh unruh-spam at physics.ubc.ca
Sat Dec 27 18:10:51 UTC 2008


"Maarten Wiltink" <maarten at kittensandcats.net> writes:

>"George R. Kasica" <georgek at netwrx1.com> wrote in message
>news:refbl4deg778k7rtgdnqcbjhajpuf3250v at 4ax.com...
>[...]
>>>> 3) I'd like to share the GPS/PPS signals via gpsd with another Linux
>>>> system if possible would that be usable or accurate enough or should
>>>> I just synch off this one at stratum 2?

>>> The gps/pps signals are hardware signals. What do you mean "share
>>> them?" If you mean installing a splitter so that the same PPS signal
>>> is delivered to the two machines, yes you can do that. If you mean
>>> something else you need to say what.

>> I was hoping that gpsd would be able to simulate or transmit the pps
>> similar to how it does with NEMA data but as you day they are hardware
>> signals so apparently it cannot do that. Again, I don't really want to
>> make the hardware any more convoluted than necessary here.

>The PPS signal is a simple wire driven by one end and only read in the
>most rudimentary way by the other. You can easily wire it to another
>detector. Configure the other NTP to accept a PPS signal just like the
>first and it'll work.

Well, not really. Each of the computers loads the signal line with about a 2K
impendance, and the driver on the gps will run out of juice to deliver a
sharp signal. Ie, the front edge will start to smear out. Now I have not
done the tests to see how many could be run off the one PPS signal without
distortion, so you might be right that it could handle one more. 


>As for the NMEA data, I'd be inclined to distribute it in standard
>NTP format - just configure the stratum 1 server as a server in that
>other NTP server. It needs to be certain about its second boundaries
>to use its PPS source, but that doesn't need to come (directly) from
>a hardware reference clock.

>Groetjes,
>Maarten Wiltink





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