[ntp:questions] Synchronize distributed PCs with GPS 1 PPS and NTPd for OWD measurements

Sander Smeenk ssmeenk at freshdot.net
Mon Sep 14 07:13:22 UTC 2015

Quoting sandip gangakhedkar (sandipfloyd at gmail.com):

> Any suggestions/comments on my approach are much appreciated.

Try using real serial ports, avoid USB-serial.

I'm using the Garmin GPS18x LVC puck, it has a PPS signal and talks
RS232 serial. Sells on eBay for $50/$75. You can either try and use NTPs
own NMEA/PPS clocks or use GPSd to provide the SHM-segments for NTPs

You'll probably need to tweak (fudge) some parameters to get a stable
clock but when it does, it's rock solid. It might also help to change
the baud-rate of the puck's output: higher is not always better, but
i've seen improvements in stability with baud rates >4800bps.

Relevant config parts for my GPS18x LVC:

| # NMEA  http://doc.ntp.org/4.2.6/drivers/driver20.html
| server mode 2 minpoll 4 maxpoll 4 prefer
| fudge time1 0.000 time2 0.684076
| fudge flag1 1 flag2 0 flag3 0 flag4 0
| # PPS   http://doc.ntp.org/4.2.6/drivers/driver22.html
| server minpoll 4 maxpoll 4
| fudge time1 0.000
| fudge flag1 0 flag2 0 flag3 1 flag40

Also take a look at 'pps-tools' from http://linuxpps.org to get better
insight in your kernel PPS-setup

(FYI, i've also used this GPS18x LVC connected to a PL2303 USB-serial
dongle, this worked fine, even the PPS, but the accuracy is so much
better with real serial, because of, well, what has been said already on
the list)

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