GPS Grandmaster
Summary
A Raspberry Pi 5-based GPS-disciplined PTP grandmaster is a planned upgrade (Phase B, ~$130-505) to replace the software PTP grandmaster on SERVER-PC. The Pi 5 is the only viable low-cost board for this role. Cheaper alternatives (Pi 3B+, Pi Zero 2 W, Orange Pi Zero 3) all fail the core requirement: hardware PTP timestamping on the ethernet NIC.
Without hardware timestamping, a board can discipline its system clock via GPS but cannot distribute that accuracy over PTP. Software timestamping drops precision from nanoseconds to tens or hundreds of microseconds, which is not representative of how enterprise PTP grandmasters operate. The Pi 5’s native GbE NIC (not USB-attached) supports PTP hardware timestamping, confirmed by Jeff Geerling’s testing.
Three tiers are documented: free-running NTP-synced (~130, 5 microsecond accuracy), and full OCP-TAP TimeHAT with M.2 GNSS module ($505, ~28 nanosecond accuracy). The GPS grandmaster connects to a CRS326 RJ45 port and runs the full enterprise software stack: ptp4l (PTP grandmaster) + chrony (GPS clock discipline) + phc2sys (system clock to PHC sync).
This upgrade is significant for interviews. It demonstrates the complete GPS-to-PTP signal chain that professional broadcast facilities use: GPS receiver with PPS output disciplines the system clock, phc2sys syncs that to the NIC’s PTP hardware clock, and ptp4l distributes GPS-traceable time as grandmaster with hardware timestamps through the CRS326 boundary clock to all slaves.
Timeline
- 2026-04-03: GPS grandmaster options researched and documented with three pricing tiers.
- 2026-04-03: Jeff Geerling and Austin’s Nerdy Things tutorials identified as build guides.
Current State
Not started. Planned for Phase B, after core lab infrastructure is operational. SERVER-PC will serve as software grandmaster in the interim.
Budget option BOM (~$130):
| Component | Price |
|---|---|
| Raspberry Pi 5 | ~$80 |
| USB GPS module with PPS (u-blox) | ~$25-50 |
| GPS antenna (active, SMA) | ~$15-20 |
Full precision option BOM (~$505):
| Component | Price |
|---|---|
| Raspberry Pi 5 | ~$80 |
| OCP-TAP TimeHAT | $200 |
| OCP M.2 Neo-M9N GNSS module | $195 |
| GPS antenna | ~$20 |
Key Decisions
- 2026-04-13: Pi 5 is the only viable low-cost board. Alternatives evaluated and rejected:
- Pi 3B+/3A+: Ethernet is USB-attached (LAN7515). No hardware PTP timestamping. GPS would discipline the system clock but PTP distribution would be software-timestamped (millisecond-range accuracy). Not representative of enterprise PTP.
- Pi Zero 2 W ($15): No ethernet at all. WiFi only. USB ethernet adapter would have same USB-attached timestamping problem.
- Orange Pi Zero 3 ($22): Native GbE ethernet (Allwinner H618), but no confirmed hardware PTP timestamping support on the NIC. Would give GPS-disciplined NTP, not GPS-disciplined PTP with hardware timestamps. Different (and less impressive) thing.
- Pi 4: USB-attached ethernet (VL805 USB 3.0 hub). Same problem as Pi 3B+.
- Pi 5 ($60): Native GbE (not USB), confirmed hardware PTP timestamping, GPIO for PPS input. The cheapest board that checks every box for a real PTP grandmaster.
- 2026-04-03: Phase B priority (after core lab) — GPS GM is impressive but not required for learning 2110.
Experiments & Results
| Experiment | Status | Finding | Source |
|---|---|---|---|
| Budget GPS GM (~5us accuracy) | Future | USB GPS + PPS to GPIO | Future expansion |
| TimeHAT precision GM (~28ns) | Future | Jeff Geerling measured 28ns offset | Switch research |
Gotchas & Known Issues
- GPS antenna needs sky view — indoor placement may limit accuracy. Consider window-mounted antenna with SMA cable.
- Pi 5 availability — check stock before planning build timeline.
- TimeHAT availability — sold via Tindie, may have limited stock.
Why Pi 5 Specifically
The GPS-to-PTP signal chain requires four components working together:
- GPS receiver with PPS output — 1 pulse per second, UTC-traceable (u-blox module, ~$25)
chrony/gpsddisciplines the system clock from GPS+PPS (any Linux board can do this)phc2syssyncs system clock → NIC’s PTP hardware clock (PHC) — REQUIRES hardware PTP timestamping on the NICptp4ldistributes GPS-locked time as PTP grandmaster with hardware timestamps — REQUIRES hardware PTP timestamping on the NIC
Steps 3 and 4 are what separates a real PTP grandmaster from a GPS-disciplined NTP server. Any cheap board can do steps 1-2. Only boards with native ethernet and hardware PTP timestamping can do steps 3-4. The Pi 5 is the cheapest board ($60) that supports the full chain.
Open Questions
- Is the budget USB GPS option sufficient for SMPTE 2110 (<1us requirement)?
- Can the GPS GM coexist with the SERVER-PC GM during transition, or must BMCA handle failover?
- Where to mount the GPS antenna in a home lab setting?