I bought a cheap NEO-7N based GPS module,

 https://www.amazon.co.uk/gp/product/B06XXGJPVM/

It just plugs in and works, and it accepts all the
regular UBX commands to set timing pulse width, or set
a timing frequency.

I've got it in frequency mode hooked up to a phase measuring
thing, with the reference coming from a Trimble GPSDOCXO.

It's only been running 20 minutes and the Trimble is still
warming up, but here's a first plot

 http://abelian.org/vlf/tmp/170710a.gif

After it has settled down, we can get an idea how good it
would be for a LF LO, by scaling up the phase by the
frequency ratio LO/9765.625.

The GPS modules is powered through its micro USB socket from
an RPi, which also fires the UBX config commands from it
using gpsctl > /dev/ttyACM0.   I've got the phase measuring
thing running on the RPi too - using the Octo audio card.
Sample rate jitter on the Octo is cancelled out in this
test, as the phase thing just looks at phase difference
between two input signals.

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Paul Nicholson
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