<!-- Do **not** add `local stratum 1` for the GNSS/PPS setup. A selected local refclock already makes chronyd serve the correct NTP stratum; `local` is a separate fallback/local-reference mode. -->
Restart chrony and verify that PPS becomes the selected source:
```bash
sudo systemctl restart chrony
chronyc sources -v
chronyc sourcestats
chronyc tracking
```
A healthy setup should show PPS selected (`#* PPS` on common chrony versions).
The NMEA `offset 0.0` in the example is only a starting point. If the serial NMEA sentence latency is large, measure it and adjust the NMEA offset later.
## 5. Verify hardware timestamping on `eth0`
Before enabling PTP, verify the Pi 5 NIC exposes a PHC and hardware TX/RX timestamping:
```bash
ethtool -T eth0
```
You want hardware TX/RX/raw timestamp support and a non-negative `PTP Hardware Clock` number.
You can also identify the PHC with:
```bash
readlink -f /sys/class/net/eth0/device/ptp/ptp*
```
## 6. Install the PTP configuration and services
From this directory, install the files as follows:
`clockAccuracy 0xFE` is intentionally conservative until actual accuracy has been measured. The `utc_offset` value is the PTP/TAI-to-UTC offset advertised to clients; review it if a future leap second changes the TAI-UTC difference.
Reload systemd and enable the services:
```bash
sudo systemctl daemon-reload
sudo systemctl enable --now ptp4l-gm.service
sudo systemctl enable --now phc2sys-gm.service
```
The direction is important:
```text
chrony: GNSS/PPS -> CLOCK_REALTIME
phc2sys-gm: CLOCK_REALTIME -> eth0 PHC
ptp4l-gm: eth0 PHC -> PTP network
```
## 7. Optional NIC coalescing tuning
`ptp_nic_coalesce.service` applies:
```bash
ethtool -C eth0 tx-usecs 4 rx-usecs 4
```
Treat this as an optional latency/jitter tuning step, not a requirement for PTP to work. First check what the driver supports:
```bash
ethtool -c eth0
```
If the settings are accepted and you want to keep them:
The `grandmasterIdentity` reported here is the identity that should remain visible through WOLF-HUNTER and NYATER.
## 9. Troubleshooting checklist
```bash
# PPS exists and pulses
sudo ppstest /dev/pps0
# GPS data is arriving
cgps -s
# chrony actually selected PPS
chronyc sources -v
chronyc tracking
# eth0 supports hardware PTP
ethtool -T eth0
# PTP services are healthy
journalctl -u ptp4l-gm -n 100 --no-pager
journalctl -u phc2sys-gm -n 100 --no-pager
```
Do not run another service that also tries to discipline the `eth0` PHC. Chrony may continue to discipline `CLOCK_REALTIME`; that is intentional on the grandmaster.
