Re: SETI Satellite dishes & Seti League


David Woolley (david@djwhome.demon.co.uk)
Sun, 25 Jul 1999 19:15:05 +0100 (BST)


> > I would expect to see frequency datasets at or above the hydrogen line
> > and below the hydroxol line. Not so.

It appears that the current data is nominally on the hydrogen line.
It may be rounded slightly. This makes sense if you have a narrow
range, as you treat the hydrogen line as a reference frequency, and
then allow for positive and negative Doppler shifts and the possibility
that a beacon is deliberately offset.

If you read the technical description of the system, you will find that
the basic receiver is actually 200MHz wide, but they are using 2.5MHz
centred on the hydrogen line and breaking it down into 256 sub-bands
(the last number in the work unit name). I suspect the full 200MHz is
biassed above the hydrogen line.

Statistically any of the 9,765 Hz wide sub-bands could be sent with equal
probability.

One of the FAQ answers says that, if they can get more funding, they
may continue the project using more of the 200MHz bandwidth.

> >
> > Question: I have processed 30 datasets of SETI data. The sampling
> > frequencies have been below
> > the "water hole" and not within the water hole for the most part. Why
> > would this be? See below.

They are all exactly on the edge of the waterhole for some transmitting
site. As such, they are not really monitoring in the waterhole but on
the exact hydrogen line frequency.

Calculating back from the sub-band frequency and number, I get
1,420,000 as the base frequency. It looks like the work unit sub-band
number is really an 8 bit two's complement number, i.e. 0 to 127 are
positive offset, and 128 to 255 are negative offsets, with 128 being
most negative. This means that 1,420,000 is the band centre, and the
extreme limits will be about 1,418,750 and 1,421,250.

You should get 1,420 for both average and median, if you had a
statistically significant number of samples.

NB some frequencies in the waterhole are extrememly noisy, because of
man made satellite sources.



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