Compliments to Ward, Jay, as well as Mike and Paul. Great work!

As Stefan says, it may not always be easy to avoid subtle couplings from the 1pps fed to one soundcard channel into the receive antenna circuitry on the other channel. The problem is clearly visible on my 21 uHz spectrogram at home. But a large 0.1 Hz offset would put you out of range of many already running grabbers, so I'd tend to stay within a couple of milliHz of 8270 Hz.

Best 73,
Markus