My Stratum-0 Atomic Clock

Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Harrison designed and built a series of chronometers which he named H-1 through H-5. But O-1 has a flaw: it is only as good as its signal from the GPS satellites. Should the GPS transmissions be unavailable, O-1 would drift away from UTC. What I needed was a better, more stable oscillator: an atomic clock.

Atomic clocks are extremely stable frequency sources tied to fundamental physical properties of the universe. In 2001 and 2008, DARPA funded research into chip-scale atomic clocks (CSAC). A CSAC is a cesium-133 reference oscillator packaged in a surface-mount device. This opens up vast possibilities for mobile devices requiring precision timing.

For O-2, I used the Symmetricom SA.45s CSAC integrated with a Jackson Labs Technologies board to create a CSAC GPSDO. This provides time via GPS and phase locks the cesium oscillator. Once disciplined, it maintains better than 0.3 parts per billion stability without a GPS signal. O-2 runs on a Raspberry Pi 3, receiving NMEA sentences via serial and 1PPS via GPIO. Like O-1, O-2 doubles as a stratum-1 NTP server on my local network. While O-2 cost thousands of dollars in materials, the project was an awesome learning experience.