Re: TV Detection Ranges (was: What do we expect of SETI, though.)

>"David Woolley" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] wrote:
>
> > If the Arecibo radio telescope could be transported to Alpha Centauri,
> > scaled up in size and then pointed back towards Earth, then a telescope
> > with a diameter of over 33,000 kilometers would be required to
> > detect UHF television pictures leaked from Earth. This is equivalent
>
> The figure that you should be using is the 0.3 LY carrier detection range,
> not the 2.5 AU full channel detection range, making the required diameter,
> keeping all other assumptions the same, about 15 times that of Arecibo.
> The 2.5 AU case does have some relevance to digital systems, with no
> distinct carrier.
>
> > http://www.computing.edu.au/~bvk/astronomy/HET608/essay/
>
> Whilst I'm quoted here, I didn't choose most of the assumptions.  My main
> contribution was to revise the noise level and detection threshold
> assumptions.
>
> One particular assumption is that the time bandwidth product is one.
> That's reasonable for the 0.1Hz carrier, as it means an observation
> time of 10 seconds, which is consistent with drift scan searching on
> a narrow beam, but, for the 6MHz full channel, it might be better to
> to still use 10 seconds, in which case the detectable signal level
> is reduced by sqrt (60,000,000) or 7,750 and the necessary receive
> aperture diameter reduces by a factor of 88, although that is still
> somewhat large, and an Arecibo type feed would need to be moved
> rather fast to keep the source in beam.  (This increases the full
> channel detection range for Arecibo to a little over a light
> day.)
>
> The project Phoenix people reckon that they will be able to detect TV
> carriers from several stars using a fully populated 1 square km array.
> In part that probably reflects an assumption of observing a single
> source for a lot more than 10 seconds.
>
> Once you have found a signal, analogue TV has a lot of redundancy, which
> means that you can probably recover a workable picture when the signal
> is below the noise level (if you look at a grainy distant transmitter,
> the grain is the noise, but you can integrate the image from frame to
> frame and, even if there is movement, you can use what you know about
> the nature of the image and what preceded it, to make a good prediction
> of what was actually transmitted).  You cannot get your 7,750 times
> improvement from averaging over 10 seconds, but in reconstructing
> the grainy picture, you are averaging the same point over many frames.
> One probably only gets about a factor of 3 to 5 improvement in antenna
> diameter over your figure from this factor (which can't be combined
> with the 88 one above).
>
> One other point.  The calculations assume a particular illumination
> efficiency at the receiver.  That is a bit optimistic for an ordinary
> low noise receive dish but may be very optimistic for Arecibo, as it
> doesn't use a lot of the available surface in any given direction.
> 0.1 might be closer to the truth.  (This is something I learned
> fairly recently, but, probably, ought to be fed back into the
> sci.astro FAQ).  As the other people involved with this section
> of the FAQ are more likely to be found in sci.astro.seti, I'm
> adding a cross-post to that group.

Another point to be raised is that the SNR need not be so "high"
as shown in the FAQ. In fact, an SNR of 3 might be ok in many
cases. I note that Kraus in the Radio Astronomy reference, used
an SNR of 3 in his example ...
Al