Has anybody while in space or a satelite experiment tried to pickout ordinary (uhf)tv signals or domestic radio signals and if so would they be still readable?
If they are readable at distance then it makes a project like SETI viable? otherwise they are going into it blind ,if they cannot get a readable signal from Earth
Begs the Question.....When Man reaches the Stars will they be able to recieve TV pictures and radio broadcasts from Earth? (albeit a few hours or years old?)
It was stated(speculated?) on a TV Documentary that Marconi's "S" signal (first ever broadcast) and old "Lucy" shows could be picked up at 50-100 Light years away
But nobody have proved that yet
Nearer home they can just pick out the Voyager spacecraft signal of a few watts, but it barely has gone a fraction of a light year.
Nearest possible civilised system could be over 100 light years away? or more? the latter is the more likely answer. I should imagine that they would have to have had a form of Communication at least 50 years before ourselves for us to detect them or vice versa? or did they go down a different path? or never develop a communication system? Lots of things to think about?

UHF (Ultra-High Frequency) & VHF (Very High Frequency) TV signals are, as the names indicate, high frequency radiation. High frequency radiation is easily scattered by small particles, such as the dust in space. This is one reason that SETI does not use UHF or VHF. Instead, SETI uses a lower frequency, which can travel through this dust without being scattered. They also chose this frequency because it is the same frequency as is emitted by hydrogen, the most abundant element in the universe. Any civilization exploring the universe is bound to be looking at that frequency to do so.

I don't know exactly how far UHF, VHF, and radio signals can travel before being attenuated.
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07-18-05, 01:44 PM
mensaman
As a radio engineer I can tell you all VHF and higher radio signals penetrate the ionosphere and travel into space. But, as you can imagine space is very vast. A radio signal suffers a 6 decibel loss in power for each doubling of distance. For example is an FM station is 20 db above the noise at 10 miles, it will be 14 db above it at 20miles. Now, could TV signals be received at even the edge of our solar system. Well, maybe, but no information would be available such as I Love Lucy. You do the mathematics, using the inverse square law. In fact unless the receiving antenna was enormous and the receiver had a very low noise figure, probably nothing would be detected. Now, ordinary FM and TV carry information. Most of it as of this time is analog. The bandwidth for an analog TV signal is broadband. About 4 megahertz wide and at that, definition not terribly good. Unfortunately noise is proportional to bandwidth and the amount of information a signal can carry per unit time, proportional as well. That means very weak signals take a very long time to communicate information. A TV signal with its information spread across a large bandwidth would be undechiperable. Recall when some of NASA deep space antennas failed to open and the number of pictures had to be decreased. It also took much longer to receive just one picture. Deep space antennas are MONSTERS, very large. They are high gain and look at a very tiny part of the sky at one time. That's why SETI hasn't really looked at the whole sky yet.

So the upshot sadly is this. Do not count on radio ever being a real time rich information communication medium from space. Transmitter powers even at enormous levels grow dim very quickly.

The good news is digital transmissions provide a number of systems that can detect information buried in noise.
This is great of earth bound transmissions, but... space is very deep.
A signal that traveled only 2.5 light years to perhaps Alpha Centauri would be -400 db below where it would have been if it just crossed town. That's a tiny tiny amount of radio photonic power arriving at Alpha Centauri.
-400 db down is a very dim signal indeed and probably buried in cosmic noise.

In the 1960's pulsars were discovered and it was not known what that met. The LGM or little green men (people) theory seemed almost acceptable, until many more were found. After that physicists became aware pulsars were powerful noise beacons radiated by fast rotating nuetron stars.

This message has been edited. Last edited by: mensaman, 07-18-05 06:35 PM

07-18-05, 03:28 PM
babthrower
We are bombarded with radio waves from space, for example from the center of our galaxy. How is this possible given that, as Mensaman says, "A radio signal suffers a 6 decibel loss in power for each doubling of distance" ?

07-18-05, 03:41 PM
bedstor
Thanks for that good reply
Gives us something to mull over Wink
I imagine colonists being somewhere local to the Solar system and they have to deploy 20ft sized or larger dishes just to access the once per day main news feed from Earth (and have to track the signal) logistics setup and costs are going are going to be astronomical!
Welcome to Answerpool BTW

07-18-05, 05:07 PM
mensaman
We are bombarded with radio waves from space, for example from the center of our galaxy. How is this possible given that, as Mensaman says, "A radio signal suffers a 6 decibel loss in power for each doubling of distance" ?[/QUOTE]

Here is the answer. The natural radio signals are generated by a large number of very broadband sources, such as, stars, whole at galaxies etc. This broadband enery is essentially unmodulated and carries no information. i.e. hiss or cosmic noise.
Very sensitive radio telescopes see their noise floors go up when the telescope is pointed at a sourse. But, these signals are random noise, not digital pulses as a function of time. I suppose some very slow phase modulation giving a binary one or zero might someday be so detected. But, don't expect alien voice, music or faxes.

07-18-05, 05:57 PM
babthrower
Cool. Thanks. Smile

07-18-05, 06:18 PM
mensaman

quote:
Originally posted by babthrower:
Cool. Thanks. Smile



Just for throughness here is one other point. Radio transmitters of non natural design transmit a carrier wave. The carrier is a very narrow band signal that has a defined frequency. For example BBC could be tuned on carrier frequency of 5 megahertz. There would be no frequency or amplitude modulation.

SETI looks at frequencies to see if anything like a pure carrier wave can be found in the noise. As you know they get plenty of false positives.
Although a carrier wave might not have any modulation or carry any information except its oscillatory frequency, such a find would excite SETI folks. The reason being nothing in nature produces such narrow band radio signals.

07-18-05, 10:27 PM
Professor
Great posts, mensaman! Welcome to AP Smile

I assume that the 6 db loss of signal with doubling of distance is based on 1/R2 attentuation of a spherical wave spreading out through space.

In addition, isn't there some dispersion (bandwidth smearing) with propagation over time, something to do with differential phase shifts in the frequency domain? So analog signals not only get weaker and noisier, but lose their content as well? (These are questions -- I'm pretty shaky on stuff that I haven't studied for 35 years. Frown )

In Carl Sagan's fanciful sci-fi novel Contact, aliens positioned "world-sized" dish receivers throughout space to listen for intelligent signals (where "intelligent" is in the technical sense meant to include reruns of "Mr. Ed" Smile ). The automated receivers then re-encoded the signals and beamed them back to their source. On Earth, the SETI scientists received their first e.t. video image -- of a Nazi swastika! -- because of early experimental TV broadcasts from Germany in the late 1930's.

My question: If the receiver is large enough at a range of, say, tens of light-years, is this scenario even physically possible?

07-19-05, 07:47 AM
mensaman
I assume that the 6 db loss of signal with doubling of distance is based on 1/R2 attentuation of a spherical wave spreading out through space.

Yes, that's why the inverse square law works without fail in space, but can fail on earth because antenna height, sight to horizon, is what vastly curtails terrestrial VHF and up communications. Including cell phones.

In addition, isn't there some dispersion (bandwidth smearing) with propagation over time, something to do with differential phase shifts in the frequency domain? So analog signals not only get weaker and noisier, but lose their content as well?

Right, which makes information extraction from any interstellar radio communication even more problematic.

In Carl Sagan's fanciful sci-fi novel Contact, aliens positioned "world-sized" dish receivers throughout space to listen for intelligent signals (where "intelligent" is in the technical sense meant to include reruns of "Mr. Ed" Smile ). The automated receivers then re-encoded the signals and beamed them back to their source. On Earth, the SETI scientists received their first e.t. video image -- of a Nazi swastika! -- because of early experimental TV broadcasts from Germany in the late 1930's.


Recall from your communications education, more gain means decreased volume of sky seen. Such very large antenna might gather more photons, but you'd need some sort of complex phased array to see more than a tiny slice of sky.
There are some modern radar systems use this technique and I hear some progressive cell companies also.

My question: If the receiver is large enough at a range of, say, tens of light-years, is this scenario even physically possible?[/QUOTE]

I recall Sagan in a 1960's book stating 300 watts and a dish the size of the Cornell dish in Porto Rico would be detectable with a similar system at 50,000 light years.
As a student I thought that estimate rather naive. Carl wasn't an engineer.

But, to answer your question. With very large antennas and enormous power, even 300 light years would smear most of the intelligence from even a modestly broadband analog signal. Digital phase modulation with very slow data transmission rates might yield some simple message. Say, phase vertical polarization equals a 1 and so on. But, polarity of radio signals can shift in such long transmission ranges. I suppose a probability algorithum of it most like was a one or zero, could extract some information. It's really naive to think the old Lucy Shows or even Howard Stern would be recoverable.
Maybe that's all that's needed, just making another intelligent civilization recognizable to another. Carl thought prime numbers repeated in sequence might be a universal code any advanced civilization would choose. Its interesting to speculate why a civilization would go to so much trouble to bring attention to itself. After all its entirely possible the Borg may be listening.

This message has been edited. Last edited by: mensaman, 07-19-05 10:01 AM

07-19-05, 11:05 AM
Professor
Thanks again, mensaman, for your insight.

I rather like Sagan's idea of using prime number sequences as an unambiguous marker of intellect.

It has long been assumed that, given the distances, interstellar communication would have to exploit electromagnetic signals traveling at the speed of light, especially in the 21-cm window of the hydrogen spectrum mentioned by methos above. However I read recently (though I can't find any articles at the moment) that some SETI theorists now think that contact between us and "e.t." might be better accomplished with material spacefaring artifacts bearing densely packed information, given all the difficulties with radio discussed above. For example, the Voyager I and II spacecraft of the 1970's, now just beyond our solar system, bear plaques with diagrams and even an audio phonograph record of sorts, as I recall.

Of course the trick is to then find these bottles adrift in the cosmic ocean. It's all so speculative as to be almost pointless...

07-19-05, 12:08 PM
babthrower
Wouldn't the audio record be pretty well wrecked by cosmic radiation? After all, the message-in-a-bottle approach would have to survive a huge amount of travel time. Roll Eyes

07-19-05, 01:26 PM
mensaman
[QUOTE]Originally posted by Professor:
Thanks again, mensaman, for your insight.

I rather like Sagan's idea of using prime number sequences as an unambiguous marker of intellect.

I thank you for stimulating my space communications cortex. And, yes, I think Sagan's idea about primes very good, but also, I suspect an equally good context for the message might be some base for a numbers system. Say base three or what ever base the civilization might use.

I suppose the so called searching for a needle in hay stack would be a challenge whether it be a capsule or radio signal.
There have been a few who speculate lasers might be more effective in interstellar communications. The over all spreading of the signals is less since focus devices would yield by analogy antenna gains far in excess of what might be practical at radio frequencies. Frequencies would be in some band where natural occurrence of the monochromatic signal was improbable. Sending digitally encoded messages by lasers might be practical or maybe not. I don't know if any deep space experiments have been tried.

This message has been edited. Last edited by: DorianGreyed,

quote:

Originally posted by babthrower:
Wouldn't the audio record be pretty well wrecked by cosmic radiation? After all, the message-in-a-bottle approach would have to survive a huge amount of travel time.

According to this site from NASA:

quote:

The Voyager message is carried by a phonograph record -- a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. The contents of the record were selected for NASA by a committee chaired by Carl Sagan of Cornell University. Dr. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind and thunder, birds, whales, and other animals. To this they added musical selections from different cultures and eras, and spoken greetings from Earth-people in fifty-five languages, and printed messages from President Carter and U.N. Secretary General Waldheim. Each record is encased in a protective aluminum jacket, together with a cartridge and a needle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate how the record is to be played. The 115 images are encoded in analog form. The remainder of the record is in audio, designed to be played at 16-2/3 revolutions per minute. It contains the spoken greetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago, and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth, there is an eclectic 90-minute selection of music, including both Eastern and Western classics and a variety of ethnic music.

But this doesn't say much about its longevity in the presence of cosmic rays, solar wind, micrometerors, and other space nasties. Who knows?

Just my opinion, the so called message from earth capsule was basically a promotional gimik for NASA. Nobody serious thinks in the history of time this capsule will ever reach another intelligent civilization.

But, the gimick did work as PR among the earthlings of the late 20th century.