GNSS · Global Navigation Satellite System
The satellite time reference a grandmaster clock disciplines to; because it can be jammed, spoofed, or lost, holdover oscillator quality is the real timing-plane design decision.
Current numbers
ptp4l / phc2sys / ts2phcLinux PTP daemons: ptp4l distributes IEEE-1588, phc2sys slaves the system clock to the PHC, ts2phc steers the PHC from GNSS/PPS
≥ 2 grandmastersGNSS-disciplined grandmasters with BMCA failover, ideally fed by independent antennas