Yes: a Raspberry Pi can serve as a Stratum 1 NTP server when chrony is synchronized to a GPS receiver’s one-pulse-per-second (PPS) signal and the receiver’s serial data supplies the matching UTC time. The PPS wire must reach a Linux-supported GPIO, appear as a kernel PPS device such as /dev/pps0, and be paired with a usable GPS time source. A running GPS daemon alone does not prove that the Pi is synchronized.
What the server does
The NEO-6M provides two different kinds of timing information. Its NMEA serial messages convey the time and date needed to identify the PPS pulses; its PPS output supplies a precise edge once per second. GPSD reads the receiver’s serial stream, while Linux exposes the pulse through its PPS driver. Chrony combines the time context and pulse to discipline the Pi’s clock, then serves NTP to permitted devices on the local network.
Stratum 1 means the server is synchronized directly to a reference clock. In this build, that reference is GPS, not another network time server. The chrony project documents a GPSD arrangement using a SHM or SOCK reference together with a u-blox NEO-6M PPS source. GPSD’s Time Service HOWTO likewise describes using a GPS receiver with 1PPS to set up a high-quality NTP server.
What you need
- Raspberry Pi: A Pi 4 Model B is a straightforward baseline; the implementation guidance highlights its 1 Gbit network interface. A Pi 5 can also be used, but its UART arrangement differs, so follow serial-port instructions for that model rather than copying Pi 4 wiring or settings.
- u-blox NEO-6M receiver: Choose a breakout that exposes UART and a genuine PPS output. Board pin order and logic voltage vary, so check the markings and specifications for the exact board.
- Active GPS antenna: Use an antenna compatible with the receiver and place it where it has a clear view of the sky. A receiver without a valid fix may not provide the time context needed to lock PPS.
- Connections and network: Have jumper wires, a shared ground, stable Pi power, and Ethernet or another reliable network connection.
Wire the receiver and prepare the Pi
Connect UART, ground, and PPS
Connect the receiver’s TX to the Pi’s RX and connect grounds. Connect receiver RX to Pi TX only if you need bidirectional communication, such as configuring the receiver. Connect the receiver’s PPS output to a GPIO supported by the Linux PPS GPIO driver. Verify the breakout’s pin labels and signal voltage before connecting it: NEO-6M board layouts and electrical details are not interchangeable.
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Enable serial and kernel PPS
Enable the Pi’s serial interface, but do not assign that UART to a login console. Configure the PPS GPIO overlay or driver for the chosen pin, then check whether Linux has created /dev/pps0. GPSD’s documentation identifies kernel PPS support as a prerequisite for using this timing path. If the device does not appear, chrony cannot use that PPS device.
Install and check the time-source software
Install GPSD, its client utilities, PPS tools, and chrony using the package manager for your Pi operating system. Configure GPSD to read the receiver’s actual serial device. Device names and configuration details depend on the Pi model and operating-system release, so confirm the UART path rather than assuming it is the same on every installation.
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Use cgps or gpsmon to check for a valid GPS fix. GPSD’s HOWTO specifically recommends gpsmon for checking PPS detection. For example, use gpsmon /dev/ttyS* only if that pattern matches the active UART on your system; substitute the actual device path otherwise. Confirm that PPS events are visible, not just that GPSD is running.
Configure chrony to combine GPS time and PPS
Use serial-derived GPS time for coarse UTC context and PPS for precision. A representative chrony configuration pattern is:
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refclock PPS /dev/pps0 lock GPS
refclock SHM 0 refid GPS precision 1e-1 offset 0.0 delay 0.2 noselect
allow 192.168.0.0/16
rtcsync
driftfile /var/lib/chrony/driftThis is a pattern, not a drop-in configuration for every distribution: the GPSD transport may be SHM or SOCK, and the corresponding source definition must match how GPSD and chrony are configured on that system. Chrony’s official example documents both the socket arrangement and the PPS lock relationship. In the sample, lock GPS associates PPS with the GPS time source, while noselect keeps the SHM source from being selected as the system clock source on its own.
Restrict allow to the LAN subnet or subnets that should receive time. The sample 192.168.0.0/16 is broad for many home networks; replace it with the range your clients actually use. Keep ordinary upstream NTP servers configured as a fallback and independent sanity check, especially if GPS reception can be interrupted.
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Verify lock before relying on the server
- Confirm the kernel PPS device: Run
ls -l /dev/pps0. The device should exist if the GPIO driver and chosen pin are configured correctly. - Confirm GPS and PPS activity: In
gpsmon, check for a valid fix and visible PPS events. A valid serial fix without PPS events is not enough for this configuration. - Inspect chrony’s sources: Run
chronyc sources -v. Check whether the GPS and PPS sources are reachable and whether chrony is selecting the intended reference. - Check the server’s status: Run
chronyc tracking. Once the system is locked to its directly connected GPS reference, the output should identify the selected reference and reportStratum : 1. - Observe it over time: Watch several minutes of samples before treating the setup as stable. A single successful status check does not establish that reception and synchronization will remain reliable.
Troubleshoot a server that will not reach Stratum 1
- No
/dev/pps0: Recheck the GPIO selection, PPS overlay or driver, wiring, and common ground. The chosen GPIO must be supported by the Linux PPS GPIO driver. - GPS fix but no PPS events: Confirm that the receiver board actually exposes PPS, that the correct pin is connected, and that the antenna has a clear sky view. Do not assume a board’s unlabeled or differently ordered pin is PPS.
- No GPS fix or usable serial time: Check antenna placement, the UART device path, and serial configuration, including baud rate. A baud mismatch can prevent GPSD from interpreting the receiver’s messages.
- PPS exists but chrony does not use it: Check that the chrony PPS source points to the actual device and that the GPS source identifier used by the PPS lock matches the configured time source. Also check whether chronyd has permission to access
/dev/pps0. - It synchronizes intermittently: Consider blocked or variable antenna visibility, Pi load, UART scheduling, GPIO interrupt latency, and network hardware. Retain upstream NTP sources for fallback and comparison when GPS reception is unavailable.
What accuracy to expect
PPS is the precision signal; NMEA alone is noisier because its time information travels over a serial path. GPSD describes a suitable 1PPS receiver as a way to condition a high-quality local NTP server, and chrony’s example demonstrates Stratum 1 operation. Neither establishes a universal accuracy figure for every Raspberry Pi and NEO-6M installation. Antenna placement, receiver firmware, UART scheduling, GPIO interrupt latency, Pi load, and network hardware all influence the result seen by clients. Treat Stratum 1 as a statement about the source hierarchy, not a guarantee of a particular microsecond or nanosecond offset.
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