Overview
This page configures the receiver's system clock to be disciplined by GPS time, using chrony fed by the GPS daemon (gpsd). This is useful when you need accurate, GPS-referenced timestamps, for example when recording drive-test measurements (see MBMS Modem - Measurements & GPS). It is optional: the modem runs without GPS time synchronization if you do not need GPS-referenced timing.
Precondition: install, configure and enable gpsd by following this guide.
Step 1: Install chrony
Install the chrony time daemon:
sudo apt install chrony
Step 2: Add a GPS reference clock to the chrony configuration
Edit /etc/chrony/chrony.conf and add the following line at the end. It tells chrony to use the shared-memory (SHM) reference clock that gpsd publishes, labelled NMEA, with a small delay correction:
refclock SHM 0 delay 0.5 refid NMEA
Step 3: Enable the gpsd options for chrony
Edit /etc/default/gpsd and add the options -n -b (which make gpsd poll the receiver without waiting for a client, and enable broken-device safety):
# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group dialout.
DEVICES="/dev/ttyACM0"
# Other options you want to pass to gpsd
GPSD_OPTIONS="-n -b"
Step 4: Restart gpsd and chrony
Apply the changes by restarting both services:
sudo systemctl restart gpsd
sudo systemctl restart chrony
Step 5: Verify
Check that chrony is receiving time data from your GPS device with chronyc sources.
After around 30 seconds, you should see recent sample data in the NMEA line, for example:
210 Number of sources = 9
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
#- NMEA 0 4 377 11 +73ms[ +73ms] +/- 251ms
^- pugot.canonical.com 2 6 377 53 +394us[ +394us] +/- 54ms
^+ chilipepper.canonical.com 2 6 377 54 +156us[ +185us] +/- 44ms
^- alphyn.canonical.com 2 6 377 52 +624us[ +624us] +/- 127ms
^+ golem.canonical.com 2 6 377 55 +324us[ +352us] +/- 53ms
^+ extern4.nemox.net 2 6 377 54 -177us[ -149us] +/- 48ms
^+ 194.112.182.172 2 6 377 55 -151us[ -123us] +/- 29ms
^* ntp.candystore.at 2 6 377 53 +132us[ +160us] +/- 23ms
^+ svn.mediainvent.at 2 6 377 55 -94us[ -66us] +/- 36ms
Success looks like an NMEA line that shows a recent LastRx value and a Last sample figure (as above), meaning chrony is reading time from the GPS device.
If the NMEA line stays empty or shows no recent sample, confirm that gpsd sees your receiver (for example with cgps, as described in MBMS Modem - Measurements & GPS), that the correct device is set in /etc/default/gpsd, and that the gpsd precondition steps above were completed.