PPS

SynchNTPtoGPS.md

gpsd howto

A github repo with a guide for different receiver

ntpsec quick guide

Importantly:

• Connect a PPS-capable GPS receiver to one of your serial or USB ports. A random cheap consumer-grade GPS receiver won’t do; you may have to do some hunting to find a usable one. Test connection:

stty -F /dev/ttyF0 ispeed 115200 && cat </dev/ttyF0
ldattach pps /dev/ttyF0

Create symbolic links:

sudo ln -s /dev/ttyF0 /dev/gps0  
sudo ln -s /dev/pps0 /dev/gpspps0  

Set AppArmor policy to /usr/sbin/ntpd

sudo aa-complain /usr/sbin/ntpd

Add KERNEL=="ttyF0", SYMLINK+="gps0" to udev rules

sudo vim /etc/udev/rules.d/80-gps-pps-custom.rules

Reboot, if this works, you should see /dev/gps0 after reboot.

# Make sure there aren't any instances of gpsd running already
sudo service gpsd stop
# stop processes that are using your device:  
lsof -n | grep /dev/ttyF0
# run gpsd  
sudo gpsd -n -N -D3 /dev/ttyF0

Add to /etc/ntp.conf:

server 127.127.28.0 minpoll 4  
fudge 127.127.28.0 refid GPS  
server 127.127.28.1 minpoll 4 prefer  
fudge 127.127.28.1 refid PPS0

TODO: run gpsd automatically at startup /etc/default/gpsd ? NOTE: It takes a few minutes for the clock to synchronize nicely with the PPS, check synch with ntpq:

ntpq -p

TODO: adjust GPS offset time 1 parameter so that the PPS synch gets better... (guess: 0.910, -0.910)

More details:

Use the gpsd daemon, edit to the following: /etc/default/gpsd

• Start the gpsd daemon automatically at boot time: START_DAEMON="true"
• Use USB hotplugging to add new USB devices automatically to the daemon: USBAUTO="false"
• Devices gpsd should collect to at boot time. They need to be read/writeable, either by user gpsd or the group dialout. DEVICES="/dev/ttyF0"
• Other options you want to pass to gpsd: GPSD_OPTIONS="-n -D3"
• Add the following to run at startup at : /etc/rc.local:

stty -F /dev/ttyF0 ispeed 115200
service gpsd restart

• if the gpsd daemon is working properly, run: sudo service gpsd status. You should see something like:

Jul 22 17:47:43 boreas gpsd[1450]: gpsd:INFO: PPS:/dev/ttyF0 Assert hooks called clock:  1626990463.000070278 real:  1626990463.000000000: accepted

• final step, run: ntpq -p
• And check that your offset wrt PPS is less than 1 ms, then you're good to go.

Connecting GPS receiver to a computer via DB9 RS-232 port

Typical DB9 pinout for RS-232: Reach RS/RS+ bottom connector pinout Cable needed to connect access the PPS signal from the receivers