TLDR
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Builder upgrades a GPS-disciplined Raspberry Pi desk clock (O-1) to a cesium CSAC GPSDO unit (O-2) for sub-microsecond UTC accuracy without GPS.
Key Takeaways
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O-2 uses a Symmetricom SA.45s chip-scale atomic clock paired with a Jackson Labs CSAC GPSDO board and u-blox GPS chip.
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Once disciplined, the cesium oscillator holds better than 0.3 ppb stability with no GPS signal, staying within a few microseconds of UTC per day.
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DARPA BAA-01-32 (2001) and BAA-08-32 (2008) funded CSAC development; Symmetricom (now Microsemi) commercialized the SA.45s surface-mount cesium package.
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Interface complexity: RS-232 to TTL level translation for NMEA, 5V to 3.3V for 1PPS; two serial ports on the GPSDO serve different functions.
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O-2 doubles as a stratum-1 NTP server on a local LAN alongside three other GPS-antenna-dependent stratum-1 servers.
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