I think NPL would have something to say about that if it was really there!.
Droitwich is defined as a secondary national standard; the carrier is
derived from a rubidium source which is regularly compared / checked
against a caesium reference.

Apart from the intentional phase modulation carrying the elec co's.
timing (and other rented out data broadcast ?), any phase shift you
see will almost certainly be due to ::

1)  Propagation :  skywave changes at dawn/dusk with groundwave interference.
2)  Any detuning of a high Q antenna will shift the phase which
ripples though the PLL and upsets the output stability
3)  An imperfect PLL with noise interference leaking into it.

I've never used a Droitwich locked reference, but have made three
different MSF 60kHz ones (see the website).  Those suffererd
propagation induced errors of a few PPB at dawn / dusk;
antenna-detuning-induced phase shifts created an audible wobble  when
used for a 10GHz reference with a PLL time constant < 20 seconds  , so
must be several tens of PPB

A longer PLL T/C of  several minutes  in the later model and a good
quality OCXO,smoothed things out somewhat, but performance was only
just adequate for microwave use.   However, it was made in the days
before low cost GPS modules, so was all that was available at the
time.

Andy
www.g4jnt.com


> phase of one waveform "walking" relative to the other shows frequency
> differences down to the parts-per-billion level quite easily. It seems the
> 198kHz carrier has some kind of cyclical drift in phase which occurs over a
> period of a minute or two (the receiver PLL seems to filter out the faster
> phase modulation data), so the direction of "walking" changes from time to
> time.
>