View Full Version : deep space communication

04-21-2011, 10:45 AM
I would love it if some people more knowledgeable on the subject of radio communication could waffle a bit about this subject.

Supposing there were intelligent creatures on a planet orbiting the Sun's nearest neighbour, and supposing they used radio communication.

Based on my knowledge, even if you had patience worth four-and-a-bit years for your message to get there (and the same for the reply--huh, they'd better not think too long about it), you could never communicate with them because the signal is just way too weak.

Now, supposing they took themselves off their planet and came this way, how close would they need to get before anyone could conceivably, give or take a bit of newfangled technology, communicate with them? This assuming that we know where the approaching ship is.

My admittedly uneducated guess is--hmmm--about 5-6 light hours?

Apart from the distance, it will depend on:
- signal strength and focus (I'm assuming signals would be focused)
- antenna size
- wavelength? (I'm not sure)

Can someone here waffle a bit about what I'd need to consider. I'm not afraid of formulae. In fact, a formula or two would be great.

04-21-2011, 01:05 PM
Good question. A close-to-home example: your local radio station is an unfocused blast of a few megawatts of radio waves; a cheap radio can pick up only a few microwatts of power, amplify the tiny current, and hurt your ears. Our big radio telescopes are outrageously sensitive. We can still (just barely) detect signals from Voyager 1 and Voyager 2, at the edge of our solar system, with I'd guess less than 100 watts of power available for communications. Voyager 2 is 94 AU away, about 8 billion miles, 13 light hours.

See http://en.wikipedia.org/wiki/Voyager_2#Communications

Signal power decreases as the square of the distance. So if Voyager 2 was twice as far away, the signal would be 1/4 its current levels. Or flip it around: if it had 4 times the power, we could still detect it at 50 light hours away. If it had 10,000 times the power, we could detect it 13*100 = 1,300 light hours away (54 light days, about 1/7 of a light year).

I don't agree that we couldn't get a signal to Alpha Centauri. First, use tightband (focused or beamed) signals, to avoid wasting energy in every conceivable direction.

You may have gotten stories crossed. The story about signals getting too weak to detect was relevant to that old joke about aliens watching our TV signals -- those do go out in every direction and it's hard to imagine them being detectable more than a LY away.

Choice of wavelength is tricky. We'd want to use a wavelength that has some well-known physical connection (like the 21cm line of Hydrogen) BUT those are also the frequencies with the most natural noise. Otherwise, guessing at which of a billion possible frequencies is a huge hurdle. As a result, I believe SETI monitors millions of frequencies simultaneously. But, once we find a signal, we could focus more resources on tuning into it.

Just some scattered thoughts. Hope it helps.

04-21-2011, 01:35 PM
Scattered thoughts are good.

The Voyager and other deep space probe articles is where I figured that there probably is a distance limit to realistic reception/separation of signal from background noise, even with focused beams.

I was under the impression that we'd lost radio contact with the Voyagers.

I also understand that the volume of data you can transmit diminishes significantly with distance? Could it be that although you might still get a simple signal across, there might not be enough reception for more complex functions?

My best guess is that the wavelength would be in the microwave range.

Can anyone elaborate on antenna requirements? I know about the huge dish network, of which we have one here in Canberra but I presume three of these things are needed because the Earth rotates, and not for any other reason?

I attended a talk by Jim Benford at Worldcon where he said that there are gaps in the frequencies monitores by SETI. I'd have to look up my notes to see if I wrote down what they were.

04-21-2011, 10:07 PM
Patty, check for information on long baseline interferometry radio telescopes.

Probably one of the best designs for long-distance radio telescopy would be a cluster (anywhere from 2 on up) satellites set to station keep on an extremely long baseline (measured in AU perhaps). Every satellite watches the same point in space or tracks the same point in space. Information is beamed to a receiving station or stations on the Earth or moon where interferometry signal matching and boosting is accomplished. It would be conceivable to set up a centralized receiving station that then beamed the information back to Earth, but that adds an unnecessary extra step in my opinion.

The antennae design is as simple as a coil of copper tubing mounted on a plywood platform. A simple backyard helical radio antenna like this was proposed by John Kraus way back in the Big Ear days. Another design might be to utilize several hundred TV dishes scattered across several square miles, but even a single dish can collect radio signals.

I have a design that uses those holiday popcorn cans mounted to a sheet of plywood.

Antennae are simple. It's the signal matching, amplifying, and interpretation that makes things complicated.

Then there's decisions regarding what wavelength are you going to monitor? How many different wavelengths are you going to try to monitor simultaneously? How do you monitor the same area 24 hours a day? What do you do if you receive a Wow! Signal?

04-21-2011, 10:41 PM
More randomness: Visible light. X-ray laser. Binary data. Prime numbers. The simpler the message, the easier it is to send/receive/decode. A light flashing on and off on a non-random, unnatural manner says, "We're here."

THEN you can focus a tight beam communication. But having a conversation, such as: "Hey, we're having a sale on ray guns." "How much?" "Potatoes. We're all out of potatoes" . . . Unlikely.

Such communication would have to be on the order of the data sent to and from our distant robots. And of course, we and the robots speak the same language. :)

04-22-2011, 10:35 AM
All these thoughts are all very good. Thanks especially, Greg. I need some technical terms to feed into google to get me information I can use. I've already been reading up on radio telescopes.

Some further information:
The interested party has pinpointed the location of the ship, making a tighbeam focus possible (not going in too much details so as to stay within the subject of this thread).
They also know who's aboard. In other words--they're not trying to negotiate alien-human barriers and know what frequencies the ship is likely to use.
While they have sporadic access to pretty good listening antennas, they lack anything much in the way of returning a reply.
I envisage the very few sporadic messages getting across consisting of code.

04-22-2011, 11:17 AM
There's this thing:

I read something many years ago, I don't have a reference for it, that this antenna could send and receive signals to a similar one a distance of 15,000 light years away.

Of course the dish is fixed to the Earth and rotates with the Earth, and the only other aiming is moving the antenna thing at its focus around, which can only shift the aim a few degrees or tens of degrees, so it can only sweep a fairly narrow section of the sky. It can only aim at something for a few minutes (while the top part is moving to compensate for the rotation of the Earth) and there are areas of the sky it just can't point at.

I also found this giant telescope page which may give other leads (google the names of interesting entries):

04-22-2011, 11:49 AM
I read something many years ago, I don't have a reference for it, that this antenna could send and receive signals to a similar one a distance of 15,000 light years away.

Terrific. If only Neolithic humans had had this technology, we might be receiving a response any day now.