From Milliseconds to 26 Nanoseconds: How a $20 eBay SFP Module Beat My Entire NTP Setup

A homelab enthusiast documents achieving ±26 nanosecond time accuracy using a $20 Oscilloquartz OSA-5401 GPS-disciplined PTP grandmaster SFP module paired with a Raspberry Pi 5. The setup uses ptp4l to discipline the Pi's hardware PTP clock, then chrony to discipline the system clock. Key findings include: a chrony 'trust' flag bug that prevented switching to the better PTP source, a measured 58.3 μs MCU delay in the GPSDO's PPS output path (corrected with a static offset), and honest analysis showing ±40–50 ns typical vs the headline ±26 ns post-filter figure. The result is ~45x better accuracy than the previous GPS PPS + NTP setup, at roughly $100 total hardware cost.