I've rewritten the frequency and offset correction code to be entirely digital and restructured the system clock to be a three layer structure with a free-running monotonic timescale, a frequency disciplined timescale, and an offset disciplined timescale.  The three-layer clock allows for independent disciplining of frequency and timing offset and improves overall tracking accuracy.  The system is now capable of frequency steering with a resolution of 0.232 ppb.

In addition, I've restructured the GPSDO and NTP code into a single unit to make GPS/NTP fail-over more seamless and improve code reuse.  NTP source selection and health tests are still a WIP, but I've also implement support for select chronyc queries:

• sources
• sourcestats
• ntpdata
• tracking

This allows for viewing fairly detailed status information with the chronyc utilty.

A large benefit of this massive rewrite is that the frequency error measurement is now both precise and accurate enough to expose the presence of hysteresis loops in the tracking error.  Further analysis is needed to isolate the cause, but they still make some interesting patterns.  The loop widths are less than 30ppb,  and they were previously buried in the measurement noise.