I actually managed to find a good use for a Raspberry Pi
model 1 B.

Using my favourite neo7 GPS module

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

and this nice little touch screen

 https://www.amazon.co.uk/gp/product/B01M4KO567

I've been able to put together a neat little portable calibrator
for my VLF receivers.  It replaces the motley collection of
signal generators, attenuators, scopes and what-not that I
previously used.

The touch screen works real well, but is hard to photo!

 http://abelian.org/vlf/tmp/171109b.jpg
 http://abelian.org/vlf/tmp/171109c.jpg

A simple little C program for the Pi drives the display/touch
screen and sends UBX protocol commands to the GPS.   The GPS
can be told to generate any VLF frequency with integer Hz,
or can be set to 1PPS with whatever pulse width I want to use.

The PPS/frequency output of the GPS drives a TC4422 to produce
a 12V square wave for phase calibration by near-field coupling.
There are two other outputs for current and voltage injection
which I use for amplitude calibration.

My favourite way to do the phase calibration is to set the GPS
to run at 133 Hz and take the 12V output to a short wire which
radiates near the E-field rx, or to a loop near the H rx, or
both at the same time.  It produces a nice comb of GPS locked
odd harmonics all the way up the band with good S/N.

A command like

 vtread -T2017-10-30_16:30,+30m /test |
    vtnspec -f 48013 -r 266 -c -w 95760 > spectrum.dat

measures all the odd harmonics in one go, averaging over 30 mins,
and then it's easy to plot the phase response,

 http://abelian.org/vlf/tmp/171109d.gif

The spikes are very strong signals from GBZ, MSF and Inskip
on 81kHz.  I just have to remove these spikes before making an
eqmap file for vtfilter.

I hope to take the new E rx down this weekend for some more
alterations - to add a port to inject a calibration signal.
I'm also getting a lot more than expected phase shift from the
downlink cable and isolating transformers.

I ran two of the neo7 modules with their antennas side by side
for about 24 hours and logged the data from them.  Then I plotted
their reported separation

 http://abelian.org/vlf/tmp/171109e.gif

The average separation is 30cm.  The first hour was pretty
good - after that I altered the SBAS settings.  The first
hour averaged 4.7 cm.  The distance between the antenna centers
was 3.5 cm.

The RPi, the Neo7 and the touch screen all seem to work together
without any hassle at all.  Raises all sorts of possibilities,
especially if you throw in the 6 input AudioInjector Octo 8

 https://www.amazon.co.uk/dp/B06Y3PZ6MF

I have a nice little C library for doing UBX commands if anyone
wants a copy.

--
Paul Nicholson
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