Hi Paul, Nick,
thank you for your interesting and helpful comments.
In VLBI a very high stability frequency standard (hydrogen masers
etc.) is used as a phase reference on each receiving station and these
timing signal is inserted on the same tape as the received signal.
...
A low cost amateur solution could be to record a commonly received
signal such as Loran-C, MSF, GPS etc. on the other channel of the
sound card, while the actual DX-LF signal is recorded on the other
channel.
Yes, I thought about using an adjacent signal like DBF39 in the same channel.
After some prefiltering, one might then use squarelaw-detection to get the
audio heterodyne, pushing aside all receiver stability issues. You would end
up with something like optical holography, where the reference wave is
distributed over the air as well. But, as you say, this wouldn't solve the
difficult part:
With an array spread over 2000 km, would in theory produce a beam
width of a few arc minutes. However, since the propagation
characteristics are not stable on LF (compared to empty space), such
narrow beams would be more or less useless.
I agree its hopeless to geometrically calculate absolute phase differences
over the array. But thats where the adaptiveness comes into play: Using the
carrier itself as a reference, the pattern will focus on the source by
itself, thats the heart of the idea. It would even follow changes in the
ionospheric waveform distortions, as long as these are not faster than the
300 s timescale. An analogy would be a guide star in the field of view of an
adaptive-optics telescope.
... using low cost generally available
hardware and software (which is an absolute requirement for any
coordinated effort between several hams :-).
Well most of us already have all we need: An LF receiver, connected to a PC
with soundcard and internet. As a lot of dynamic range is gained in the FFT,
the ADC requirements will be no different from standard qrss reception with
Argo or Spectrogram. The receiver has to be stable enough for 3 mHz
resolution which means a drift rate better than 10^-8 / 5min, not too hard
for a good xtal oscillator.
It may be advisable to do some data reduction on the audio data before
sending them to the common processor. The filtering-and-decimation technique
used for the speedup of qrss could result in data rates of a couple of
bytes/s per receiver. Then we could even replace the offline internet
connection by 100 quasi-realtime shortwave links, a la PSK 31 or using 10 Hz
bandwidth analog FM ...
Looking at submarine sonar techniques and producing an analogue for radio
would also be very useful.
Yes I'd like to read more about that. Could you point out some (unclassified)
references or URLs?
73 de Markus, DF6NM
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