Inspired by this graphic which demonstrates how far our radio signals have reached
Assuming our best scientific receivers, and assuming anyone out there is using the most powerful transmitters we use.
How far would they have to be away before we couldn’t detect them?
I’m just trying to get a grasp of if we could actually detect a civilizations radio signals assuming they have similar technology to us.
The simple answer to your question is that a receiver can detect signals as long as the signal-to-noise ratio is significantly over unity, i.e. we can detect the signal beyond the background radio environment and receiver noise, SNR = P[SUB]r[/SUB]/N, where
P[SUB]r[/SUB] is the received power of the comm link and
N is the receiver system noise.
P[SUB]r[/SUB] = P[SUB]t[/SUB]⋅A[SUB]et[/SUB]⋅A[SUB]er[/SUB]/(d⋅λ)[SUP]2[/SUP]) where
P[SUB]t[/SUB] is the transmitted power,
G[SUB]t[/SUB] is the gain of the transmitter antenna,
G[SUB]r[/SUB] is the gain of the receiver, and
λ is the wavelength of the RF signal.
The gains are directly related to effective aperture area of the transmitter and receiver by G = 4⋅π⋅A[SUB]e[/SUB]/λ[SUP]2[/SUP]. So the larger your transmitter or receiver, the more gain you get.
Now, these assume the SNR a basic single pulse width modulated system at a single amplitude (could also be phase or frequency modulated but let’s keep it simple). A more complex modulation scheme or a longer duration pulse width may make it possible to make a signal carry more information per unit bandwidth or detectable closer to the noise level, but for a given noise floor on a receiver the SNR depends fundamentally on aperture. Ambient RF ‘noise’ could also play an issue, but except for the microwave background space is mostly empty of RF (or more correctly, space is permeated by RF but at such low levels that it isn’t a consideration) unless you are pointed directly at another RF signal like a star or black hole.
Most dedicated deep space communications systems use X-band (f= 8 GHz to 12 GHz) or S-band (f = 2GHz to 4 GHz), transmitted by a directional (tight beam) antenna. The typical frequency bands used for television, VHF and UHF (30 MHz to 300 MHz, 300 MHz to 3 GHz) and radio (540–1610 kHz for US AM) have lower frequencies and correspondingly larger wavelengths, which also means more divergence of a tight beam, but since TV and radio signals are broadcast with a omni-directional transmitter (more or less) the signal power can be assumed to just reduce at the normal r[SUP]2[/SUP] rate.
What this essentially means is that we couldn’t even detect standard NTSC or PAL television signals at Lunar orbit, notwithstanding attenuation of Earth’s atmosphere. Similarly, commercial and private radio signals, except for the lower half of microwaves, don’t penetrate Earth’s upper atmosphere and ionosphere, a quirk that shortwave and ham operators used to transmit beyond line of site around the world. You might be able to get some FM and high end of VHF frequencies in low orbit, as well as higher frequency ham signals during favorable times (night, low solar activity) but to get a useable signal in upper orbit would require a very large deployable receiver. Getting useable broadcast signals at interplanetary, much less interstellar range is practically impossible.
The only way we could detect a radio signal from another star system is if it were directed specifically at us at very high (gigawatt or higher) broadcast power. Laser is more likely (and would be more detectable owing to the coherence and monotonic frequency of the signal) but we’re not currently looking for visible signals and we could only detect X-ray or higher beams above the atmosphere. However, if a hypothetical alien race were using nuclear fusion for propulsion or radiant power production, the spectrographic lines would be distinct from natural fusion processes. I could only hazard a wild ass guess as to how far we could detect the characteristics of a fusion plume but I’d guess dozens or perhaps even hundreds of light years depending on the power output.
Stranger
If we are trying to detect signals from a planet going around another star you tend to be pointed directly at that star.
Also detecting things like TV broadcasts from far away gets to be a problem because there are probably more than one station broadcasting on a given frequency. On earth that is not an issue because one station will be much farther away. So the signal from the close one drowns out the signal from the far one. But if they are both about the same distance away then there is not a stronger signal.
ETA: shoulda refreshed; I thought I was answer #1.
It’s been awhile since we did this. But the short answer from the experts is something around the orbit of Saturn.
The point is that just counting years since the first broadcasts in 1920 and converting the time to distance in light years is only a small part of the battle to actually receive a signal.
The vast majority (99+%) of what we’ve transmitted since then was deliberately aimed to keep the transmission down on the surface where the receivers are. Only a tiny fraction of the energy was emitted outwards. Then that gets further attenuated by the ionosphere.
So there’s only a tiny & weak signal leaving the upper atmosphere. One that pretty quickly drops below the noise floor when compared with all the radio noise coming out of the Sun.
Consider that for any observer even as close as Saturn the Earth is never more than about 30 degrees from the Sun. At Pluto it’s more like 1 degree (estimated by me, not calculated). Viewed from the nearest other star Earth & Sun are always within a tiny fraction of a degree of each other.
This is clearly not true, since we’ve gotten transmissions from Cassini, New Horizons, Pioneers 10 and 11, and Voyagers 1 and 2. And those were all using extremely low-power transmitters, not the most powerful ones we have.
You’d look offset for a moving signal relative to the star. But yes, for a star at significant distance, the distance between the central object and a planet orbiting it will be less than the divergence angle, and trying to determine the source by parallax from radio signals at that range is an exercise in futility. Hence, why laser would actually be the better method of interstellar communication.
We are currently maintaining contact with Voyager 1 from DSN Madrid Station 63 (the 70mm dish) on the S-band or X-band frequencies, and commands received on Voyager 1’s 3.7 m antenna sent on S-band (2.1 GHz). Voyager 1 is well beyond design and original mission lifetime and substantially past the point where engineers expected to be able to maintain communications. Despite some onboard software and mission control problems, Voyager 2 is also still in communications as it passes through the heliosheath. At this point, it is expected that loss of power due to decay of the RTG power supplies below the functional threshold of the transmitter will be the cause of loss-of-signal rather than attenuation through distance. A space-based solar orbiting telemetry and communications system could potentially allow communications with spacecraft out to the furtherest extend of the Kuiper Belt and to the inner portion of the Oort cloud (about 2000 AU). Going beyond that, a wide spread array the size of a large moon would probably be required for ‘practical’ communications to even the nearest star system.
Stranger
So on the International Space Station, could they pick up any commercial radio or TV signals? (Assuming they tried, and had an appropriate antenna – I expect they probably have better things to do.)
Signals can be detected with S/N ratios well below unity. GPS signals, for instance, are about 26 dB below the noise floor on Earth’s surface. That’s a S/N of around 0.002.
The bit rate is low, of course, but that’s to be expected. Shannon’s Law has no lower limit to the S/N as long as it’s strictly positive.
After reading Stranger On A Train’s before his last one on this thread so far, I’m relieved that Carl Sagan was wrong in “Contact”. I’m glad aliens won’t be picking up Hitler’s 1936 Olympics television broadcast.
Only intermittantly and irregularly.
This is true, which is why I qualified the response as ‘the simple answer’; a SNR that is under unity requires an FIR filter to extract the underlying signal with high fidelity.
Stranger
Just wanted to clarify, since we are talking about the weak signal case.
For those watching, an analogy:
Suppose someone wants to send you a single bit of information. They’re allowed to send you a number, but before it gets to you, someone adds in the value of a 100-sided die (that’s the “noise”).
It would be easy for them to just send you the number 1000 for a yes and a 0 for a no. The *no *might be corrupted to anywhere from 1 to 100, and the *yes *from 1001 to 1100, but even so these are easily distinguishable.
What if the source is only allowed to transmit up to a value of 50? Well, the *no *can again range from 1 to 100. The yes ranges from 51 to 150. Uh-oh! There’s an overlap. If we get a number from 51 to 100, the bit is ambiguous.
However, if our source does this several times in a row, then we have a pretty good shot at receiving the bit unambiguously. Each time we have a 2/3 chance at an unambiguous bit, so after 4 or so tries, we’ve got a 99% chance at getting the right bit.
What if the source is only allowed to transmit 0 or 1? Well, now the problem gets pretty tricky! We have to capture lots of values before we can be confident in our answer. We could either wait for a 1 or 101 value, or average together all of the values and see if there’s a bias. In any case, we need hundreds or thousands of values to be sure of the result.
Radios aren’t exactly like this (the noise is typically Gaussian instead of flat, for instance), but I think this conveys the gist of it. Having a high S/N makes things easy, but it’s possible to transmit information reliably even when it’s below 1.0.
Here’s a short paper by Alexander Ziatzev, detailing the most detectable transmissions we have sent out so far.
Highly powerful and focused radar transmissions have been sent out towards numerous targets, and these transmissions have been sent to numerous places throughout the night sky. If there were receivers located in the sky at any of these points and were by some coincidence listening then they could have detected our transmissions briefly; they might have seen a little burst, perhaps something like a Wow! signal. Would that tiny anomaly be enough to alert the aliens to our existence? I don’t think so, somehow.
IMO, there is a tremendous difference between the situation the OP asked about: “How far away can random commercial broadcasts, Earth-based radar, etc., be received?” and the question you’re posing an answer to: “How far away can a system designed for long range communication work?”
assuming our current most powerful transmitters and receivers are similar to whats out there…
how far out would our most powerful radio signals be detected?..rough guess will do…1 light year? 5? 200?
http://lightyear.fm/ something that may be of interest. It is a visual tour of the solar system and beyond based on what songs would be playing out there. Think of the intro to the movie Contact. I’m not sure how scientifically accurate it is, but it is cool to watch.
Have you read this wiki article?
It lists some of the deliberate signals that have been sent to various locations, and the dates at which those signals will arrive. None of them have arrived yet, but (if I am not mistaken) at least some of those signals could be detected by the sort of receivers we currently have.
I don’t have a definitive answer nor recent assessment, but CP-2156 NASA Conference Proceedings on Life in the Universe, 19-20 June 1979 has some interesting pieces of information to consider. The astrobiology is worefully out of date (in 1979 astrobiology was considered just above crackpot pseudoscience as a serious field of study) but I think the comments on radio leakage and detection are still largely correct. It notes that the Ballistic Missile Early Warning System (BMEWS) was at the time the most powerful source of radiation leakage and estimates that it could potentially be detected up to 15 light-years distant from Earth by a detector as sensitive as the Arecibo 305 meter radio telescope. However, because of the shifting frequencies actually verifying acquisition would be challenging at best. The more consistent radiated frequencies that can penetrate the ionosphere are TV carrier wave (optimum at 500 to 600 MHz according to the proceedings) which could be detected “from distances as large as 1/10 those discussed for the BMEWS radars.”
One of the issues with Arecibo is that it is a fixed telescope and so can only scan a very small fraction of a steradian of the sky, which is constantly shifting as the Earth rotates. The Green Bank Radiotelecope is the most powerful steerable telescope but has only 10% of the collecting area, so reduce the estimates for a steerable telescope accordingly, i.e. 1.5 light-years for a transmitter as powerful as BMEWS or the more current PAVE PAWS or X-Band Radar systems, or about 100000 AU for television carrier signals. A solar orbiting radio observatory with a receiver of the same aperature and sensitivity as the Arecibo telescope, or the equivalent of the Very Large Array or Square Kilometer Array could probably achieve on an order of magnitude or slightly more greater sensitivity, so a reasonable upper bound on detection distance would probably be on the order of 50 light-years for television carrier signals, assuming a high power narrow band signal with a fixed modulation and constant transmission. In reality, practical ability to recieve and interpret a useable signal would likely be much less, even with a larger or more sensitive detector.
Stranger
Just for kicks, I looked up the number of star systems within 50 light years. There are about 1400, but most are red dwarfs. 133 of them are visible with the naked eye. There are 2000 stars in that volume when binary, etc star systems are counted.
That’s not what the OP asked. The original question was “Assuming our best scientific receivers, and assuming anyone out there is using the most powerful transmitters we use.”. I don’t know where you got the idea that she was asking about random commercial broadcasts or the like.
You’re absolutely right.
I dumbly keyed off the one sentence referencing the diagram which is simply the radius in light years since the year Earth started broadcasting radio & ran with that. The rest of the OP clearly supports your (and others’) interpretation.
Say enough stuff and I’m bound to say something stupid once in awhile. D’Oh :smack: