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 / ts2phcthe linuxptp daemons: distribute IEEE-1588, slave system clock to PHC, steer PHC from GNSS/PPS
≥ 2 grandmastersGNSS-disciplined GMs with BMCA failover; holdover oscillator (OCXO→Rb/CSAC) sets coast time