From: MarkusVester@aol.com
Message-ID: <df.103f1961.27b946f3@aol.com>
Date: Mon, 12 Feb 2001 09:02:27 EST
Subject: Re: LF: Very large adaptive rx array
To: rsgb_lf_group@blacksheep.org
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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