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alexh110
2004-Jan-06, 11:37 PM
Hi,
I wondered whether SETI radio telescopes can detect either terrestrial radio and television carrier waves, or television satellite uplinks bouncing off the Oort cloud, back to the Earth?

Also would it be possible to detect such reflected waves bouncing back to the Earth from local stars.
I.e. Could you detect terrestrial broadcasts from some years ago reflected off the ion cloud surrounding local stars? Or off the Oort clouds of local stars?

If so, how degraded would these signals be? Would it be possible to retrieve any information from them; or would you be lucky to detect just a carrier wave?

Also how does the solar wind and the interstellar wind affect terrestrial signal propagation in space?

Thanks,
Alex Hunter.

Sam5
2004-Jan-07, 12:09 AM
Hi,
I wondered whether SETI radio telescopes can detect either terrestrial radio and television carrier waves, or television satellite uplinks bouncing off the Oort cloud, back to the Earth?

Also would it be possible to detect such reflected waves bouncing back to the Earth from local stars.
I.e. Could you detect terrestrial broadcasts from some years ago reflected off the ion cloud surrounding local stars? Or off the Oort clouds of local stars?

If so, how degraded would these signals be? Would it be possible to retrieve any information from them; or would you be lucky to detect just a carrier wave?

Also how does the solar wind and the interstellar wind affect terrestrial signal propagation in space?

Thanks,
Alex Hunter.



I’m certainly not an expert on this subject, but I would think that any signal that we could receive reflected back to us from deep space, would be so weak, we couldn’t detect the signal above the background level of all kinds of other EM space noise.

alexh110
2004-Jan-07, 12:46 AM
What if you were, for example, 40 light years away from the Earth: would you be able to detect any Earth radio or TV broadcasts?

I.e. How easy would it be for an alien civilisation with our level of technology to detect signals from Earth?

Sam5
2004-Jan-07, 01:12 AM
What if you were, for example, 40 light years away from the Earth: would you be able to detect any Earth radio or TV broadcasts?

I.e. How easy would it be for an alien civilisation with our level of technology to detect signals from Earth?

You would have to have an extremely large dish antenna, that would gather as many EM waves as possible, and then you would have to amplify them a lot. And I still don’t think that someone could gather enough waves of old earth broadcasts to be able to override the background EM noise-waves of space.

This is a fundamental problem with the SETI project. In order to pick up any alien signals, the aliens would have to be beaming a very strong and highly focused signal toward us, actually, toward where we will be in the future, many years before we get there.

If they just used an old-fashioned open non-focused beam, transmitted from a tall tower, like a radio tower, the transmission power would have to be so strong, that would probably interfere with all their own local electronic equipment. So, first, they would have to focus the beam so it will go far out into space without losing much power, and then we would have to be lucky enough to stray into the beam as we move through space, and if we did, we wouldn’t be inside the beam very long. Maybe just a few minutes or less, depending on how narrow the beam is.

alexh110
2004-Jan-07, 01:26 AM
What about satellite uplinks?
These are directed into space, and are more powerful than terrestrial broadcasts.

Or perhaps our transmissions to the Voyager probes would be a better bet, as these are specifically directed into deep space, and are presumably very powerful in order to reach Voyager 1's rather small dish at 90AU's out. (Come to think of it, I don't suppose we're transmitting to Voyager anymore; just receiving? But anyway, we have presumably sent transmissions to the Voyager space probes in the past as far out as Neptune.)

Sam5
2004-Jan-07, 01:47 AM
What about satellite uplinks?
These are directed into space, and are more powerful than terrestrial broadcasts.

Or perhaps our transmissions to the Voyager probes would be a better bet, as these are specifically directed into deep space, and are presumably very powerful in order to reach Voyager 1's rather small dish at 90AU's out. (Come to think of it, I don't suppose we're transmitting to Voyager anymore; just receiving? But anyway, we have presumably sent transmissions to the Voyager space probes in the past as far out as Neptune.)


NASA sez:

“Since the nearest star to our Sun, Proxima Centauri, is about 4.22 light years away, that comes out to a total distance of over 260,000 AU to our nearest stellar neighbor.”

So, 260,000 / 90 = 2888.88 times the Voyager distance just to get to Proxima Centauri, and I don’t think the dishes on earth are aimed in the direction of Proxima Centauri. Even with a dish, the signal will spread out in deep space. And what are the signals anyway? Just a bunch of meaningless numbers. It’s not like a recording of Elvis songs.

Pinemarten
2004-Jan-07, 02:25 AM
I seem to remember two areas of search. The 'water hole' and the 'max distance' area.

The water hole is the quiet area where background EMRs are at a minimum, and the max distance is the area where the range vs signal loss are at optimum.

I may be wrong, as usual.

alexh110
2004-Jan-07, 02:59 AM
Do you happen to remember what these particular frequency ranges are?

Do they coincide with any terrestrial transmission bands?

Pinemarten
2004-Jan-07, 03:26 AM
No.
The SETI site doesn't have them either.
Terrestrial Hz lie in the 'noisy short range' zone.

My reference material returns from shopping soon, and I will try and 'narrow it down' then. :wink:

alexh110
2004-Jan-07, 04:18 AM
I wonder whether in fact we could send a strong enough signal to be received at Proxima Centauri; or at least for the carrier wave to be detected?

Pinemarten
2004-Jan-07, 04:28 AM
I don't think we need a 'strong' signal, but the ability to use our system as a carrier.
Can we modulate our solar activity and carry a signal with that?

ToSeek
2004-Jan-07, 05:29 PM
I don't think we need a 'strong' signal, but the ability to use our system as a carrier.
Can we modulate our solar activity and carry a signal with that?

It's been suggested. Some solar shades just big enough (and controllable enough) to systematically reduce the Sun's radiation in a specific direction even by one part in a million would still be enough to be detected thousands of light years away.

eburacum45
2004-Jan-07, 05:51 PM
I wrote a story about that a while back;
http://www.orionsarm.com/historical/Signal_Lamp.html

as far as propagation of tv signals goes, this page is informative...
http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html

apparently you would need a reciever several kilometers in diameter to pick up tv transmissions from the nearest star...

but high energy radar carries a lot further.

Tom
2004-Jan-07, 06:12 PM
I can't help it. This may be related:

Long Delay Echoes (http://www.violations.dabsol.co.uk/probe/probepart1.htm)

Here's the google search (http://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=long+delay+echoes)

Fascinating, nonetheless....

AGN Fuel
2004-Jan-07, 11:05 PM
It's been suggested. Some solar shades just big enough (and controllable enough) to systematically reduce the Sun's radiation in a specific direction even by one part in a million would still be enough to be detected thousands of light years away.

So those irregular variables are actually Marvin practicing his Morse Code? :lol:

Pinemarten
2004-Jan-08, 12:32 AM
I don't think we need a 'strong' signal, but the ability to use our system as a carrier.
Can we modulate our solar activity and carry a signal with that?

It's been suggested. Some solar shades just big enough (and controllable enough) to systematically reduce the Sun's radiation in a specific direction even by one part in a million would still be enough to be detected thousands of light years away.

And I thought it would be a difficult task.
Am I to understand that the further from the sun the shade is the smaller it would have to be?
Would it make much of a difference in the size of the shade?
My brain is too tired to do the math right now.
I can assume that once we detect a signal, or spot a nice watery planet, we will start the research/funding.
There will also be a push to build a generation ship at the same time.

Mokele Mbembe
2004-Jan-08, 07:38 AM
Not sure if this helps, but the "water hole" refers to the section of the EM spectrum taken up by the spectral lines of... water. I can't remeber where exactly that is, but, um... yeah. I *thought* it would help, but maybe not... ;)

ToSeek
2004-Jan-08, 03:50 PM
Am I to understand that the further from the sun the shade is the smaller it would have to be?
Would it make much of a difference in the size of the shade?

I'd think it would be the other way around. I read an article on this topic, in Analog, I think, that worked out the numbers. I'll have to see if I can dig it out.

ToSeek
2004-Jan-08, 03:52 PM
Not sure if this helps, but the "water hole" refers to the section of the EM spectrum taken up by the spectral lines of... water. I can't remeber where exactly that is, but, um... yeah. I *thought* it would help, but maybe not... ;)

1.42-1.64 GHz (http://setiathome.ssl.berkeley.edu/about_seti/radio_search_2.html), or around 21 cm. Bounded by the lines for neutral hydrogen and hydroxyl (HO).

Andromeda321
2004-Jan-08, 10:19 PM
You are always going to have RF interference with SETI work on Earth (some from the ground, some from satellites). I remember in the novel Contact Carl Sagan goes into detail describing things like the SETI astronomers accidentally detecting secret military satellites etc.
Something else I read on this topic (don't remember where) mentioned interstellar propogation. It has been proposed that, like with the atmosphere on Earth, there are occasional times where, due to unusual circumstances, signals would travel exceptional distances for a short period of time. It has been proposed that the "wow!" signal was caused by such propogation (some transmitter so far away that under normal circumstances there's no chance of hearing it). The problem with interstellar propogation is it would be very rare and difficult to confirm (you'd have to monitor a particular part of the sky for decades to confirm it).
Hope this helps!

Manchurian Taikonaut
2004-Mar-04, 06:28 PM
What if you were, for example, 40 light years away from the Earth: would you be able to detect any Earth radio or TV broadcasts?

I.e. How easy would it be for an alien civilisation with our level of technology to detect signals from Earth?

They say our broadcasts and radio signals have only been powerful enough in the past 50 yrs to beam their signals into space.

So if Aliens are sitting on a planet 100 light years away.... using radio telescopes beyond this 50 light years radius then they will never find us.

Well, not until 2054

Pinemarten
2004-Mar-09, 04:24 AM
Recently, I heard a theory new to me:

Supernovae are a popular place to 'look'.
If a planet with a nova 'at its back; transmits 180deg outward from the nova, then anyone 'watching' along that direction/vector (looking at the nova), may detect the signal.

Have we; through SETI, et al, ever focused on signals riding on, or remaining after nova?

Argos
2004-Mar-09, 01:31 PM
apparently you would need a reciever several kilometers in diameter to pick up tv transmissions from the nearest star...


Or you could use an interferometer: two equal dishes orbiting the fringes of a solar system, 180 degrees apart, in which case youŽd have an antenna the size of the solar system. That could boost your receiving power. I think that a highly developed civilization with communicative dispositions will take steps to ensure that the signals from the outer space are received, by setting up something like that.

Gerbil94
2004-Mar-09, 02:05 PM
What matters in this application is sensitivity rather than resolution, so just replacing a large, single dish with a couple of much smaller ones in a solar system-sized interferometer would not produce the desired results. You still need a total collecting area equal to that of the "several kilometers" sized dish.

Picking up "Alien TV" is not really practical, as the calculations eburacum45 linked to demonstrate.