GPS / PPS time sources
GPSd time source
Instead of using another NTP server as a time source, it is also possible to use time data from GPSd as a time source for ntpd-rs. GPSd is able to interpret GPS signals from a GPS receiver, from which it can derive the current time.
To provide this information, gpsd tries to open a socket at /run/chrony.XXXX.sock, where XXXX is replaced with the device name of the GPS device. When running ntpd-rs with reduced permissions, this socket cannot be created by ntpd-rs. We therefore have to configure systemd (or an equivalent startup daemon) to create a symlink to a folder ntpd-rs can create the socket in. We recommend linking to /run/ntpd-rs/chrony.XXXX.sock, which can be done with the following systemd unit:
[Unit]
Description=GPSD Socket Shim
Documentation=https://github.com/pendulum-project/ntpd-rs
[Service]
Type=oneshot
ExecStart=/bin/bash -c "ln -s /run/ntpd-rs/chrony.XXXX.sock /run/chrony.XXXX.sock"
GPSd can then be added as a time source for ntpd-rs by adding the following to the configuration:
For socket sources such as a GPS device from gpsd, ntpd-rs is unable to estimate the uncertainty of the timing data. Therefore, you should provide an estimate (corresponding to 1 standard deviation) of this noise yourself through the precision field. For typical GPS receivers, an uncertainty somewhere in the range of 1 to 100 ms is reasonable.
Setting up GPSd
In order for GPSd to connect to ntpd-rs, GPSd must start after ntpd-rs and after the socket shim has been created. This is because ntpd-rs needs to create the socket for GPSd, which can only find the socket if it exists at the moment GPSd starts.
GPSd can be manually restarted using:
Systemd can be told to enforce such a starting order using the Wants, Before and After keys in the Unit section. It is recommended to modify the unit files for ntpd-rs and gpsd through systemctl edit when making these changes.
For help with setting up GPSd on a Raspberry Pi, see for example this guide.
Pulse Per Second (PPS)
Ntpd-rs also supports using PPS timing data via Kernel PPS, based on RFC 2783.
Here is an example configuration of a 1 PPS device in ntpd-rs:
By default, PPS sources are treated as 1 PPS sources, which send a pulse every rounded second. For e.g. a 10 PPS device, the source can be configured with a period of 0.1 seconds: