Builder upgrades a GPS-disciplined Raspberry Pi desk clock (O-1) to O-2 using a Jackson Labs CSAC GPSDO with a cesium-133 chip-scale atomic clock for sub-microsecond UTC holdover.
Key Takeaways
The Jackson Labs CSAC GPSDO pairs a Symmetricom SA.45s cesium CSAC with a u-blox GPS chip and ARM microcontroller; once GPS-disciplined, it holds 0.3 ppb stability without GPS signal.
CSAC technology traces to DARPA BAA-01-32 (2001) and BAA-08-32 (2008), targeting a cesium-133 reference oscillator in a surface-mount package for mobile military and civilian use.
O-2 reuses O-1 software nearly unchanged; the main hardware complication was RS-232-to-TTL and 5V-to-3.3V level translation between the GPSDO and Raspberry Pi 3.
Both O-1 and O-2 serve as stratum-1 NTP servers on the author’s LAN; O-2 cost thousands of dollars versus hundreds for O-1.
CSAC units are available used on eBay for $12K-$15K; new suitcase-sized telecom cesium clocks run tens of thousands.
Hacker News Comment Review
Commenters noted that multiple GPS antennas are unnecessary for multi-device stratum-1 setups; a GPS signal distribution box from eBay is cheaper and splits one antenna to many devices cleanly.
There is open curiosity about whether averaging four stratum-1 clocks improves precision beyond a single disciplined source, with no clear answer surfaced.
Notable Comments
@geerlingguy: GPS distribution boxes (e.g., Time Machines, GPS Source) can split one antenna signal to many NTP devices, making antenna arrays redundant and cheaper.
