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title: Yet another Raspberry Pi Stratum 1 NTP server
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<title>Yet another Raspberry Pi Stratum 1 NTP server</title>
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<body>
There are lots of instructions for setting up a Stratum 1 NTP server
on your Raspberry Pi. A lot. After much research, I found a simple
configuration that uses a single ntp reference clock and does not involve
gpsd, but uses both NMEA and PPS for the most accurate timekeeping possible.
NMEA+PPS is a mode of NTP's &quot;<a href="http://doc.ntp.org/4.2.8p10/drivers/driver20.html">Generic NMEA GPS Receiver</a>&quot;.
<p>Here's what worked for me:
<p>Start with a pi3 and raspbian stretch. Add a GPS with TTL-level PPS and 9600
baud NMEA outputs.
<p>Hook up you GPS: GND and +3.3V or +5V (according to the specifiics of your GPS)
to any matching pin, the GPS's TX output to the Pi's RX input on pin 10
of the header, and the GPS's PPS output to the Pi's pin 12. (You can hook
up your GPS's RX input to the Pi's TX output on pin 8 if you like, but I don't
think this is necessary)
<p>Using raspi-config, disable the serial console and enable the serial port
hardware.
<p>Manually edit /boot/config.txt to add: <tt>dtoverlay=pps-gpio</tt> —
If you have to (or prefer to) use a different pin for pps, you can apparently
specify it as <tt>dtoverlay=pps-gpio,gpiopin=##</tt>, where ## is the
internal numbering of the GPIO pins, not the number on the 40-pin header.
<p>Manually edit /etc/modules and add a line that reads <tt>pps-gpio</tt> — According to some sources, this step is not necessary.
<p>Install ntpd with apt-get.
<p>Delete all the content in /etc/dhcp/dhclient-exit-hooks.d/ntp, leaving an
empty file.
<p>Remove the file /run/ntp.conf.dhcp if it exists.
<p>Edit /etc/udev/rules.d/99-com.rules. On each of the two lines that ends
<tt>, SYMLINK+="serial%c"</tt>, append <tt>, SYMLINK+="gps%c"</tt>
<p>Create /etc/udev/rules.d/99-ppsd.rules with the content <tt>SUBSYSTEM=="pps", GROUP="dialout", MODE="0660", SYMLINK+="gpspps%n"</tt>
<p>In /etc/ntp.conf, add a stanza to access the GPS:
<pre>
# gps clock via serial /dev/gps0 and /dev/gpspps0
server 127.127.20.0 minpoll 3 maxpoll 3 mode 16 burst iburst prefer
fudge 127.127.20.0 refid GPS time2 +.250 flag1 1
</pre>
(leave the &quot;pool&quot; line(s) or other ntp server lines; if your pi doesn't
have a battery-backed RTC, you need a way to get the correct time initially,
before a GPS fix may be available!)
<p>&quot;time2&quot; doesn't seem critical with this setup, because the PPS time is
preferred over the serial reception time. &quot;minpoll&quot;, &quot;maxpoll&quot;, &quot;burst&quot;
and &quot;iburst&quot; may be superstitious and unnecessary.
<p>Reboot now and have a look at `ntpq -c peers`. You should see something
like this:
<pre>
remote refid st t when poll reach delay offset jitter
==============================================================================
oGPS_NMEA(0) .GPS. 0 l 1 8 377 0.000 -0.001 0.003
+ntp.u .GPS. 1 u 2 8 377 1.104 0.013 0.064
</pre>
<p>&quot;GPS_NMEA&quot; is selected as peer and is using PPS (this is what &quot;o&quot; in the
first column means). delay, offset, and jitter should all be extremely small
(Here, .001 is 1 microsecond). &quot;ntp.u&quot; is my other local stratum-1 NTP server.
The &quot;+&quot; indicates it's in good agreement with the local GPS. If you use pool,
you will see multiple lines here; some may have &quot;+&quot; and some may have &quot;-&quot;.
<p>If something's not working, you will get, &quot; &quot; (blank), &quot;-&quot; or &quot;x&quot; next to
GPS_NMEA. If it's got &quot;*&quot; then NMEA is working but PPS isn't. Now you get to do things like debug whether the PPS signal is working properly according to ppstest, whether NMEA messages are actually coming in
at 9600 baud, etc. Or you can follow one of those other guides. :wink:
<p>Here's how the local time to GPS offset has looked over the last
10 hours or so — I find it awesome that my computer appears to be
synchronized to GPS to within ±5 microseconds almost all of the time:
<p><img src="https://media.unpythonic.net/emergent-files/01559823543/ntp-offset.png">
<p><br><br><font size=-2>Entry first conceived on 6 June 2019, 12:19 UTC, last modified on 6 July 2019, 21:49 UTC</font>
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