[SC.LUG] Damn!

[Martyn Kinder]

<snip>
> - Setting a frequency reference accurate to about 1 part in 10^9
> accessible to both

Not likely ;-)

I think this is the easiest bit. Using GPS, you can generate a very accurate
10MHz reference signal that should be in phase no where you are in the
world...

> - Measuring the distance between the two locations accurate to within
> a very little bit.

My dad has a GPS receiver...

This bit is much harder. Ian Morrison at JB commented (joked and don't quote
me) that the most useful thing JB had ever done was to measure the distance
from two points (one was JB) to within a few mm. The benefit of a VLBI is to
provide the resolution required to give optical standards at Radio
frequencies. Hence we have to build the equivalent of an optical mirror for
wavelengths millions of times larger. To achieve reasonable resolution at
10Ghz, you really need to know where you are to within 1/10 wavelength -
approx 30mm....

> Its much easier (but still bl**dy hard by amateur terms) to do
> something at VHF or UHF, but I'm still willing to have a thin about
> it.

...I wonder what we'd get by just muxing our data? noise should cancel
...anyway, so we'd just be left trying to phase and frequency correct our
...data...

That is not a VLBI - that is a simple additive radiometer

There are a few people (amateurs) in the US experimenting with this. The
usual technique is to place a delay on the Rx that is "nearest" the source
and then combine the signals - you will probably need to draw this out and
you will see how you can cancel wavefronts to achive the resolution. Its
very impressive. The americans have done it (on a much smaller scale) by
running identical lengths of coax (also using a common local oscillator)
back to the receiver. Now this will still only work if the radio source is
at or near the zenith. Imagine that the source is at an elevation of 1
degree and due south.

I am making the assumption that we are not going to complicate this further
by trying to track and will rely on a drift scan.

Your Rx is 10km due north of me. The radio front will reach you a mere
10/300,000 seconds after it reaches me. Therefore we need a delay at my end
1/30,000 second. Not much you may say but you can't rely on the public
internet services to generate that delay and dynamically mix. Therefore we
need a common time source, again compensated for to allow for the distance
beween us recorded onto a second channel. i.e. the left channel of a stereo
pair. The right channel has the data. I do the opposite. Then you send me
the wav file, synchronise and compensate for the delay and then mix the two
data channels - pass it through the sound card using one of the DSP programs
around and analyse the data. Now this will give us the effective resolution
of an eliptical dish  radio telescope 10km major axis :-> and probably 600mm
minor axis :-<  So we need to do the same with another volunteer to the east
or west of us - now we are in buiness

...It'd probably suck, but it'd be fun to try :)

Oh I agree... in fact this discussion has fired me up to have a go at a
SBLI )i.e. Garden sized as a pilot. If we can get the  problems sorted here
it should be straightforward to scale it upwards.

Martyn