ELF: ZEVS phase and frequency accuracy?

[Markus Vester <markusvester@aol.com>]

I've been wondering whether the 82 Hz carrier from the ZEVS transmitter is accurate and synchroneous to UTC. So I set up a colour spectrogram on the DL0AO grabber
 http://df6nm.bplaced.net/dl0ao/VLFgrabber/vlfgrabber_dl0ao_test.htm
(scroll down to near the bottom), which shows the phase with reference to a 1pps harmonic on the other channel. The colour sequence is red-yellow-green-blue for retarding ZEVS phase (unfortunately opposite to the way it's done on LoranView).

During the last six hours, the colour has changed from red to green, indicating a 120° delay. This Looks like the carrier is ~ 20 uHz below 82 Hz (-0.25 ppm, quite a large difference on a relative scale). I am asking myself how it the transmit frequency is physically being steered, possibly "Alexanderson-style" by setting the velocity of a rotating machine? If the frequency offset turns out to be real and stable, it could be a means for DX'ers to differentiate the real ZEVS signal from possible local receiver artifacts. I wonder whether anyone else could verify the offset?

On the other hand, I wouldn't yet rule out a diurnal propagation effect, even though at a distance of less than a wavelength this should not produce such a large phase change. There could also be local effects on the receive site from variable humidity in the vegetation near the E-field probe. 

So the big question is whether or not the phase trend will continue tonight and tomorrow.

Best 73,
Markus (DF6NM)