Netgate SG-1000 microFirewall

Author Topic: NTP GPS-Problem II  (Read 2569 times)

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Offline charliem

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Re: NTP GPS-Problem II
« Reply #15 on: March 16, 2015, 11:01:14 am »
i use a Trimble Acutime 2000

This is a great system, but will take some work to get PPS delivered to the pfSense box.  It's not a simple plug & play like some other GPS units with PPS, it has special firmware for accurate timing.  There are also a large number of configuration settings that you need to check, like using NMEA output rather than TSIP (a binary protocol from Trimble).  If you don't already have it, grab the trimble "GPS Studio" windows program; it will allow you to check out and set up the unit properly (near the bottom of this page: http://www.trimble.com/timing/resolution-t.aspx)

I have not used one off these, but I did find a full manual here: www.cubinlab.ee.unimelb.edu.au/probing/Acutime2000.pdf

Assuming you have a 'synchronization interface module' box, as shown on p 33 of that guide.  Inputs are power and 12-pin connector to the external dome antenna/GPS unit.  Outputs are RS-232 port A, RS-232 port B, and PPS (looks like a BNC output for PPS).  So, it looks like you do not have PPS going to your pfSense box unfortunately.

You may be able to hack the PPS from the BNC into the 9-pin connector for the serial port (Pin-1, I think).  Be careful though as PPS is a differential signal driven by the RS-422 from the GPS unit.  Maybe get a hardware hacker buddy to help with this part ....

Finally, be sure you go to a real serial port on your pfSense machine, the PPS signal will not work with a USB serial port adapter.

[Edit] You may be able to get quite good time accuracy (20 uS) even without using PPS.  NTPD can use the native TSIP protocol rather than NMEA.  Not as good as NMEA+PPS, but much much improved over NMEA w/o PPS, and no soldering involved! 

To try this, you would need to modify /var/etc/ntpd.conf to use the 'Trimble Palisades' refclock driver, number 29, rather than the NMEA GPS number 20.  See https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver29.html for details.  If it works you would need to modify /etc/inc/system.inc to make the changes permanent (until the next upgrade).
« Last Edit: March 16, 2015, 04:05:58 pm by charliem »

Offline stephenw10

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Re: NTP GPS-Problem II
« Reply #16 on: March 16, 2015, 11:03:28 am »
Yes. Work out the difference in offset between the active peer and your GPS source and add it to Fudge time 2. So ~0.3 in your case.
The jitter is still too high though, PPS isn't working.

Steve

Offline kevinsystorm89

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Re: NTP GPS-Problem II
« Reply #17 on: February 12, 2018, 03:49:14 am »
Can't you enable both GPS and PPS on pfSense? That's how my RaspberryPi is set up and it always picks the PPS as used source over GPS. GPS has multitudes higher jitter than PPS.

Offline stephenw10

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Re: NTP GPS-Problem II
« Reply #18 on: February 15, 2018, 07:47:38 pm »
Yes you can.

The issue with most USB connected GPS devices though is that they don't provide the PPS signal. And yes USB connected GPS as a time source is bad....very bad! But also fun.  ;)

Steve