Hi David,
I appreciate your sceptical response, and additional notes
I kind of claimed that, if ET is smart, that asteroid (or
out-of-home-planet)
radar is pretty much the only application which we might be able to
detect at a distance of 100 or 1000LYs.
You added weather radar and deep-space probe comm-links to that,
and corrected some of the detection factors (making the target range
of detectable ET radar larger than I assumed).
Notes below on some interesting issues in this continued quest
to speculate on the most likely ET signals...
Rob
"David Woolley" wrote in message
...
In article ,
Rob Dekker wrote:
[....]
The SETI Institute believe that the Allen telescope will be able to
detect analogue TV carriers from nearby stars even though it has a
smaller effective capture area than Arecibo; that's a result of analogue
carriers being a very ineffective use of the fraction of a Hertz that
they occupy and of the Allen telescope allowing extended SETI
observations.
Although I totally encourage (an fund) the Allen telescope project,
I dont think we should have any hopes for detecting Analog TV carriers.
Apart from the fact that we (as our only example) generate most
analog TV carriers below 1GHz (where gallactic cyclotron radiation makes
interstellar detection unlikely), there is another convincing reason
which reduces chances of finding analog TV carriers :
According to Drake's formula and the most reasonable assumptions
of its variables, if analog TV carriers exist for only 100 years in a ET
civilisation, there would be only 100 existing in the Galaxy today.
So the nearest one would be at 5,000 LYs or so.
Not much chance of detecting that.
I indeed assumed that ET is more intelligent that we are, and thus
creates near-optimal signals for their applications (radar or
point-to-point)
for remainder of their (hopefully 10's of thousands of years, otherwize
we will never find them) existence.
[.....]
We are not avoiding looking for leakage signal. The problem is that
they tend to be either undectable or unverifiable.
I'm not sure about the latter.
I don't think we actually made an effort to set-up detection algorithms
which would focus on radar (or not-often-repeating) signals.
One thing with radar signals (and also energy-efficient beacons) is
that they dont appear very often, but they do on average, increase the
detection probability of 'some' signal in the direction of origin.
I dont think that with seti@home we covered each star often enough,
or covered enough bandwidth, to actually talk about probability density
of detected signals, but a repeating signal in a certain direction
should at least trigger interest...
[......]
radar signals... That puts the recent discarding of signal SHGb02+14a
(which
can easily be a radar signal) in a whole new light....
As noted before, the very fact that this is detected at multiple times
is a contra-indication to radar. The other problem with this signal is
the wide range of chirps. Whilst our CW planetary radar is pre-chirped
for
the round trip. The acceleration along the line of site of interesting
Still, why do you think that ET would want to pre-chirp ?
There my very well be another reason to 'modulate' a radar signal
with some frequency modulation.
We (humans) are not sending out beacon signals (around
the water hole), but still we look for them. Yet we ARE sending out radar
signals (quite a few of them), but if we detect one which is not
exactly pre-chirped, we discard it as a Rio 0. And 'because' it repeats
it cannot be a radar, and if it would not repeat, it would not even
be logged as anything interesting.
This seems a self-fulfilling prophecy, targeted as finding a single
possible ET application (a constantly transmitting beacon).
That unnecessarily reduces our chances of finding ET signals.
We should really think about how to prove or disprove that/if
the galaxy is filled with radar (or other not-often-repeating) signals.
[....]
An ideal spherical, metal object would create a factor 2*(ro^2 / 4R^2)
because it would reflect all incoming power back into the 180degree cone
it
came from.
Even one much larger than the wavelength wouldn't do that. it would
scatter more back towards the sender than any other direction, but near
the limbs, it would scatter almost in the original propagation direction.
The overall formula may or may not be right - I'd have to set up the
integral to check that - but the logic is wrong.
I actually thought that the logic was quite creative 8-)
It might indeed not be absolutely correct logic for the pure metallic spere,
(although I'm pretty sure the formula still remains correct),
but it must be correct for a 'fractel'-like rough surface.
The integrals would become to complex to calculate, so I just looked
at a real-life rough surface which reflects EM radiation. The moon.
I found that the moon, wether full or almost crecent, still radiates
the same amount of light per exposed surface. So each surface
area on the moon must be an omnidirectional transmitter of light.
That would also be the only explanation for the R^4 radar signal
power drop-off.
If the object behaved as a mirror, instead of omnidirectional,
it would show only a R^2 drop-off.
[...]
Also, broadband signals will require a higher signal to noise ratio
because
of not knowing the background noise levels (for narrowband you can use
adjacent
channels as a reference).
I never understood this remark (you made it several times). Isn't it true
that any
signal, broadband or narrowband, always has 'adjacent channels' ?
I mean if a signal is 1Hz wide, and we conclude that because the adjacent
1Hz
channels show lower (noise) power, then is that not similar to a signal of
1MHz with
adjacent channels of 1MHz showing lower (noise) power ?
Why would broadband signals require higher SNR ?
Same thing with time (pulses are detected by checking adjecent time slots
showing
lower energy..).
[.....]
For CW planetary radar, you will be near break even, or negative. For
pseudo-random asteroid radar, you will be 15 to 30dB negative.
That's too bad. Means that the assessments of radar range go up to
real astroid radar (from near-home-planet radar).
Still, I'm not discarding broadband military radar as you do : there
has to be a way to obtain a better ratio for these signals.
After all, since they are broad-band, the receivers will be
broad-band, which lets in a lot of noise, so power requirements
have to go up (for same target range)...
That is all incorporated in the formula I gave, which is why broadband
radar is still positioned as the most likely one to be detectable. Just
because of it's massive power requirements.
I DO agree that with pseudo-random codes you need lower SNR's, but
energy/bit remains the same. Military broadband radar just wants to
detect a lot more than one bit, and that is why they go broadband...
[....]
by a factor of 10 at most, but that still requires an absurdly large
target
range for point-to-point transmissions (in the range of 65 AU). Only an
We have point to point systems operating over more than 90AU.
I assume you refer to the Pioneer 10 comm-links.
Indeed, these should be detectable with a square-mile telescope at 100LYs.
And because the Pioneer 10 receiver/antenna is rather small, the
transmission
should be rather powerfull, so ET can probably detect it with an
Arecibo-sized
dish at 100LYs (with all tricks described above). But not much further than
that....
Problem is of course that these (space probe) comm links do not happen
very often (unless ET has massive amounts of probes which need constant
instructions). But if they do occur, they will be probably be narrowband
(slow
data transfer is acceptable), and accurately targeted.
So they should remain in our antenna beam for extended time,
They will not (have to) be 'pre-chirped' though, assuming some intelligence
in the probe's receiver....
)
[....]
Basically it is the weakness of most communication signals and the
non-repeatability of radar that make beacons the most likely candidate
for a signal that can be distinguished from statistical noise variations
and independently verified. But what we look for is independently
verifiable signals which look artificial, and have a very low probability
of being statistical artefacts when all the evidence is taken into account
(although individual observations will almost certainly fail the last
test).
I second that.
Still, there should be more ways to analyze the data we receive.
Maybe probability analysis (of non-recurring signals from a certain
direction)
could tell us more about the possibility of ET transmissions.
That's just one blunt idea. I'm sure there are smarter ways to find
statistically interesting directions....
Bottom line : We humans are sending out many thousands
of times more radar signals (detectable at 100's of light years) than
we are sending beacon signals. Why would ET do differently ?
Thanks David !
Rob