SDG #031 GPSDO & Clock Update - BST Ends

The end of British Summer Time is a genuinely useful annual test for any GPS referenced clock project, and that is exactly what this short update covers. A GPSDO (GPS disciplined oscillator) recovers time of day information from the satellite constellation alongside its frequency reference, but GPS itself broadcasts UTC, not local time. Any daylight saving behaviour has to be implemented in firmware, which means the interesting question is not whether the timekeeping is accurate but whether the timezone and DST logic was written correctly and actually fires on the right Sunday at the right hour.

This is worth understanding before buying or building a GPS clock. Many cheap modules and displays either ignore DST entirely, expect an offset to be set manually twice a year, or apply a hardcoded US style rule that shifts on the wrong dates for UK and European users. A unit that rolls back automatically at 02:00 local on the last Sunday of October is doing real work; one that needs manual intervention is really just a very accurate UTC clock with a manual adjustment.

The broader conclusion is that GPSDO accuracy claims and clock usability are separate things. The oscillator side provides a traceable frequency reference for test equipment calibration, while the displayed time depends entirely on the software layer above it. Anyone evaluating a GPSDO or a home built clock should check the DST handling and the timezone configuration before assuming that the display will take care of itself.