Hard Light Productions Forums
Off-Topic Discussion => General Discussion => Topic started by: General Battuta on March 07, 2011, 03:33:54 pm
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How would you define and describe a radio signal from near or within the solar system, omnidirectional, very complex and containing intricate but not necessarily artificial patterns? Invent a notional signal that could meet these parameters. You don't need to worry about a source, but it would be nice if it could be mistaken for something natural.
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Snuffleupagus. :P
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"weird"
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I don't think that there are many natural omnidirectional signals.
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I don't think there are many omnidirectional signals that come from a local point... unless you are inside the transmitter.
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I don't think there are many omnidirectional signals that come from a local point... unless you are inside the transmitter.
dun dun dun
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What does "omnidirectional" mean in this context?
As in the source is emitting to the whole solid angle, or that the receiver sees the whole solid angle?
I would guess source emitting to the whole solid angle, but need to confirm this.
EDIT: Also do you need a mathematical description of that kind of signal?
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Well I notice your main question is asking us to "describe and define" the radio emission, with less focus on the nature of the source itself.
I'm not quite sure what sort of answer you are looking for.
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How would you define and describe a radio signal from near or within the solar system, omnidirectional, very complex and containing intricate but not necessarily artificial patterns? Invent a notional signal that could meet these parameters. You don't need to worry about a source, but it would be nice if it could be mistaken for something natural.
I dunno.
How about an intrasolar mosaic beam composed of fenestrated stellar radiation?
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What does "omnidirectional" mean in this context?
As in the source is emitting to the whole solid angle, or that the receiver sees the whole solid angle?
I would guess source emitting to the whole solid angle, but need to confirm this.
Yes, source emitting at the whole solid angle.
I need good convincing-sounding technical terminology that you might hear between a couple radio astronomers, though perfect fidelity isn't necessary. If the question's too broad, think of it this way: I would like it to be a signal which would draw attention as something very weird, but which would not bear any unmistakable marks of artificial origin.
This might be too stupid and broad, and if so I apologize.
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A pulsar or rotating black hole might do it.
Here's a glossary of radio astronomy terms.
http://images.nrao.edu/glossary.shtml
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How would you define and describe a radio signal from near or within the solar system, omnidirectional, very complex and containing intricate but not necessarily artificial patterns? Invent a notional signal that could meet these parameters. You don't need to worry about a source, but it would be nice if it could be mistaken for something natural.
.....buh?
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How would you define and describe a radio signal from near or within the solar system, omnidirectional, very complex and containing intricate but not necessarily artificial patterns? Invent a notional signal that could meet these parameters. You don't need to worry about a source, but it would be nice if it could be mistaken for something natural.
The magnetospheres of planets tend to produce pretty interesting radio signals with intricate but not artificial patterns. Magnetospheres of gas giants tend to produce a lot of such signals, being so big.
Examples here:
Jupiter (http://www.youtube.com/watch?v=e3fqE01YYWs)
Saturn (http://www.youtube.com/watch?v=hjUf69q4Bvk)
Earth (http://www.youtube.com/watch?v=eHvdZdsIZxg)
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I was using Jupiter's magnetosphere as a placeholder; maybe I should hold on to that.
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Some ideas to consider:
-What is the frequency of the signal?
-What is the peak intensity?
-Any type of modulation (eg, amplitude modulated?)
-Is it a point source or diffuse?
-Location (right ascension, declination. Galactic coordinates (http://en.wikipedia.org/wiki/Galactic_coordinate_system) are also often used for radio sources.)
-Any proper motion? (http://en.wikipedia.org/wiki/Proper_motion) (Rate at which source moves relative to background stars).
-Parallax? (http://en.wikipedia.org/wiki/Parallax#Stellar_parallax) (apparent change in position over 6 month period due to earth's orbital motion). I'm not sure though if this works for radio sources, I only know of its use for stellar objects, as the exact position of a star is much easier to determine IIRC.
If the source is freely orbiting our own sun then you would expect to see high proper motion, and over a short period of time (weeks to months) you could determine its orbit. If its much farther out then this would be difficult (if the signal does not correspond to a visible entity like a star or galaxy) and would require some intuition on how strong is the source, given its characteristics, compared to how strong is the signal you are receiving.
I don't know enough on radio astronomy to go into much more detail than this, but perhaps you may find the following APJ article useful:
http://iopscience.iop.org/1538-4357/660/2/L121/pdf/1538-4357_660_2_L121.pdf (http://iopscience.iop.org/1538-4357/660/2/L121/pdf/1538-4357_660_2_L121.pdf)
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Yes, source emitting at the whole solid angle.
I need good convincing-sounding technical terminology that you might hear between a couple radio astronomers, though perfect fidelity isn't necessary. If the question's too broad, think of it this way: I would like it to be a signal which would draw attention as something very weird, but which would not bear any unmistakable marks of artificial origin.
This might be too stupid and broad, and if so I apologize.
I think this is a rather good question. It is not easy to answer though. Here's some thoughts, maybe it gives some body else ideas.
I had to browse through some faint distant memories of how does a radio telescope actually see something. The antenna collects radiation, and the signal is then amplified and processed, either by analogical or digital means. The processing can either use amplitude modulation or frequency modulation, and both have their own acceptable bandwidths; the filtering can be done digitally or analogically, and they remove the higher or lower frequency components from the signal - while the antennae length will also do some preselecting. I would guess that there are multiple stages in the signal processing, where the signal is modulated and multiplexed to different frequencies. The above is valid for a single measurement, the bandpass filters would likely kill the signals outside the frequency spectrum. This means that the telescope is not able to detect signals that are outside it's operating bandwidth, they don't exist in the measurement result. But this doesn't limit measurement results that are collected over time, there could still be some signals with much lower frequencies with sufficient amplitudes can be seen there (more on this later) that work by some other mechanism that is visible within this wavelength range.
I would expect radio telescope beam widths to be comparatively narrow, which means that the source that emits the signal should be within the telescope field of view. There is a possibility of the signal of entering the telescope through some side lobes (it's hard to get rid of these in radio frequencies), but the signal strength would likely need to be rather high, which would mean that there is a high probability that the source would then be directly in the field of view of some other telescope. It would be seen as rather bright, and would likely lead to quick detection. I then thought that if those telescopes were ground based, atmosphere could distort the signal partially. Unfortunately this doesn't happen (much) with radio waves. If the signal suddenly disappeared in certain wavelength, that would likely be detected immediately, but it would be very difficult to say what was wrong. The first thing those poor guys would do would be to check, verify and recalibrate the instrument. In order to make signal disappear or considerably weaken it by a destructive interference; it would require some knowledge of the signals that the telescope sees if it weren't interfered - or pure coincidence, but that's far fetching.
Then, (you might want to verify this), I have a faint memory that there might exist some periodical signals coming from the sun within certain intervals. At least I recall seeing this in incoherent scattering radar measurement data, though it never became clear to me if this was just some coincidence for that particular time, or whether there really are this kind of cycles. Nor did the source of such a signal ever become apparent to me, but I didn't investigate much further than reporting it to the assistant back then. But if such cycles really exist, could the signal slightly alter it, or be very close in frequency and in phase so that it would take time to see those two signals to go out of phase?
Sources for signals that the radio telescopes see should be rather large (in order of metres), the wavelength of the radiation depends on the size of the source. I was thinking of a charged comet, or a comet that has an antennae in it with a weak transmitting power? Or there could be polarization modulation in the signal, I don't know whether polarization modulation is detectable by current instruments. The instrument would likely then see a new signal, and decrease of the new signal when the moving object is outside the FOV. Finding the source of the signal again could be difficult, if it was small and moving.
EDIT: Redefinition of some parts
EDIT^2: Added some details in the text
EDIT^3: Crap, this starts to take my sleeping time. Moving faint object needs some additional data to be written there.
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an artificial signal from within the solar system would need to be somewhat focused and directed to make the distances involved in even intra-solar communications. a broadcast type transmission would be quite odd indeed, since it would need to be way more powerful than a directed signal to have the same kind of range. so a very powerful broadcast from within the system would be very strange.
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Sources for signals that the radio telescopes see should be rather large (in order of metres), the wavelength of the radiation depends on the size of the source. I was thinking of a charged comet, or a comet that has an antennae in it with a weak transmitting power? Or there could be polarization modulation in the signal, I don't know whether polarization modulation is detectable by current instruments.
I did wonder about something like a rogue comet with a high iron or other magnetic material core, i suppose it would depend on how predictable the existence of the signal would be for example a magnetosphere's output would be of a predictable strength where as an unexpected signal/signal change would probably require the presence an uncharted source possibly from outside the system
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an artificial signal from within the solar system would need to be somewhat focused to make the distances involved in even intra-solar communications. a broadcast type transmission would be quite odd indeed, since it would need to be way more powerful than a directed signal to have the same kind of range.
Exactly.
Mika, thank you, that was a good read. If it's not too much bother, what would you say if the recipient was a distributed antenna, as in radio interferometry?
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i think one thing something like seti really misses is that we still pretty much stick to the basics of radio transmission (fcc pretty much makes sure of that). transmissions are keyed to a carrier wave of a singular frequency or a singular amplitude. typically we just add the data (analog or digital) to known simple carrier wave (usually just a simple sine wave) of a specific frequency. but if you look at lots of technology that uses rf signals, like wifi or military grade systems, anything where security is a concern, the data are scattered on a multitude of frequencies (spread spectrum) so that anyone listening to any of those frequencies will just hear unintelligible gibberish. without having all the information about that signal, its impossible to read the transmission. but its still just using basic carrier channels, so its possible to know there is traffice because you have a bunch of waves active simultaneously.
now my concern is that: what if there was a more subtle way to communicate over rf frequencies without tipping off someone that youre doing it? such a transmission would be a great military asset and would likely be developed at some point in a civilizations tech tree. im thinking something like a fractal carrier wave that changes form as a function of time. such a signal would sound like space noise. anyone who knows the parameters for the carrier wave can key it into the demodulator and receive the data it carries. you might detect the source of the signal if it was powerful, so a space military would try to mask the signal from know enemies by hiding the signal in the shadow of a star, and then transmitting in intermittent bursts to appear as if the signal was a solar flare to anyone who wasnt looking for it. now this is all scifi since im not sure if such a carrier wave is feasible in the real world. it makes you think, if theres a way to subtly send rf signals, it will at some point be discovered and applied.
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How would you define and describe a radio signal from near or within the solar system, omnidirectional, very complex and containing intricate but not necessarily artificial patterns? Invent a notional signal that could meet these parameters. You don't need to worry about a source, but it would be nice if it could be mistaken for something natural.
Not a radio astronomer (I've so far stuck to optical in my journey to my degrees), but I did stay at a Holiday Inn Express last night! By which I mean, I had to do a radio lab for my astronomy lab last quarter. Anyway, I would describe it as having very complex modulation with a packed frequency spectrum, lots of power everywhere.
This will not be mistaken for something natural, IF it is as you describe it (more on that a bit later). No way in hell if it's as complex as you say, especially if it keeps repeating. All the natural solar system radio sources look like a big burst of noise, even if they do have periodicities in them. In astronomer speak, their power spectra tend to be very broad, no definite sharp peaks anywhere. This signal would have a definite peak that stood out above the others, the carrier wave, which would be the strongest. However, like Nuke mentioned, you can easily make an artificial signal almost completely indistinguishable from natural sources if you apply enough modulation to it; most modern radio signals are this way, such that they look like random noise until you apply the correct demodulation. Since this is a high tech artificial device, I would imagine that it would be much the same way, and indistinguishable from natural sources.
But what is the frequency(ies) this is broadcasting on? In certain bands, it would be a dead giveaway that it's artificial, because nothing natural (that we know of) emits at those frequencies. For example, if it emits at 21 cm, it'll be immediately known that something funny is going on, because the only natural source is neutral hydrogen, and the only appreciable emission is on a galactic scale. Seeing high flux from inside the Solar System would raise immediate "WTF?" looks and exclamations. If, on the other hand, it emits around 20 MHz, well, Jupiter's magnetosphere, the Sun, the Milky Way, and quite a lot else radiates at that. So it could be disguised for a long time if the modulation is such that it can fight across the noise produced by all the stuff around it.
In any case, Mika provided a good overview of some radio telescope basics; I'll expand on what he's written. A radio telescope doesn't have appreciable side lobes by design; to achieve as much resolution as possible without resorting to interferometry, the current design of a big dish and shielded antenna kinda necessitates itself. The dish also obviously helps gather more radiation. A typical radio telescope will have multiple antennas, each of which is called a "feed," each of which is tuned to a different frequency range, along with a single polarization. The signal processing is fairly simple; the data is read in from each feed, amplified, down-converted, and separated out by Fourier analysis to get the amount of power at each frequency, and also to read the radiation's phase. Since this is usually all radio astronomers care about, this is where it usually stops. If it's a transient search, more Fourier stuff can be used to get power spectra over time, etc.
Mika, thank you, that was a good read. If it's not too much bother, what would you say if the recipient was a distributed antenna, as in radio interferometry?
Really, not much changes. With interferometry, you get better resolution, and that's really it. Unless you are using VLBI (Very Long Baseline Interferometry), you won't be able to tell if it's a focused tight beam or an omnidirectional emitter; depending on its distance, maybe not even then. If the source is on Jupiter and you are standing on Earth, even if the source is focusing the radiation as tightly as possible (a radio laser, if you will), the beam will still probably cover the planet. Basically, you need to be able to observe at significantly different angles with respect to the source. This may not be possible except with ships, and they probably won't be set up for the wavelengths that this thing would use to disguise itself, so they may not even be able to see it.
By the way, if you want me to vet any astronomical dialog or anything once it's written, I'd be glad to help.
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I've so far stuck to optical in my journey to my degrees
That's the best place to get stuck at.
The artificial signal that does frequency hopping is rather close to the background noise indeed. But it is also very difficult and unlikely to be detected if the hopping frequencies are not known beforehand - or suspected that something like that might be at play; I would think that the current capabilities wouldn't allow for detection of that kind of signal across the frequency spectrum.
If the radiation source is small (in relative terms) and close (in angular terms, not in distance terms) to a distant star, and the emitting power is adjusted according to the power coming from a distant star, the signal would be interpreted to be coming from a star if the signal source and the star are inside a telescope's resolution limit. Assuming that the object would otherwise be difficult to detect within other wavelengths.
Even though I have sat in front of a white light interferometer probably for a year (hey, it at least felt like that), I can't help with stellar interferometry that uses radio waves. I'm not familiar with the used techniques and synthetic aperture imaging, I only know that it allows for a better resolution, but I don't know the theory behind it.
Wait a sec, did I just write there how Battuta's question could be solved?
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You guys rock. I can get a lot of good stuff out of this.
The signal would be very very brief. How brief can I get away with? At the moment I have about 2.5 seconds down but that may be ridiculous.
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Before going to pulse length, there are two more things that need to be considered if the source is to be masked with a background object (galaxy or a star). First is the parallax, the apparent motion of the source is different from the background. But, if the source remains inside the angle that the full background object is stationed when viewed from Earth, it shouldn't be possible to separate these two with parallax if the source is not detectable with other wavelengths. The other thing is the red shift, that should be accounted by the mysterious source, if it is to lie in front of a distant
source object undetected. Or that could provide a mechanism to detect it, I don't know. "That's weird, the red shift is slightly different from last time" "Can't be, recalibrate the instrument" etc...
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What kind of pulse do you mean?
A pulse of length 2.5 s that is never again repeated, or repeated with some frequency?
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My initial thought was never repeated, but the nature of the story is such that I can tweak that if needed, though not to a total elapsed interval between first and last pulse of more than a minute or two.
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Who's listening? Is anyone? If it's at an unused band, it is very unlikely that anyone will have their receivers both tuned to that, and, in the case of a radio telescope, pointed at the source. So it may go entirely undetected if the signal does not repeat after those 2.5 s. If it is actually detected, something 2.5 seconds long could be worthy of followup, but it may easily be lost in the noise of whatever source people were observing. Just by statistics, you expect to see a few weirdly high noise peaks in a couple hours of data, and it may simply be ignored as one of those. So I suppose the question is: do you want this thing to be detected and taken seriously or not?
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I'd like to use the bands recently opened by the European LOFAR, so 10-250 mHz? And it's actually good if it would primarily be ignored, as I'd like it to pop out only to a certain character but not to the broader community.
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There's a lot of terrestrial noise there; lots of people transmitting and receiving on pretty much all portions of it. You could easily create a signal that one specific character would have a subconscious flash on or something and be ignored by everyone else as noise or a blip or what have you. Here's (http://upload.wikimedia.org/wikipedia/en/c/cc/VHF_Usage.svg) a current map of the VHF band; the HF band below it is mostly used for amateur radio and long range terrestrial communication (reflects off the ionosphere). I imagine these bands would only get more crowded in the future, since they make for convenient antenna lengths.
Astronomically, this range is mostly marked by bright, compact sources; there's not a whole lot of diffuse galactic emission there. A big survey array like LOFAR might pick this up, and it could be dismissed by just about everyone. Maybe your character (presuming he's a radio astronomer, here) could notice that funny thing in the blip that everyone else misses. That's more plausible than you might think. However, he wouldn't be able to do a whole lot with it; without a repeat of the signal, or better yet, several, getting a wide spectrum detection is kind of a pipe dream (maybe he gets really lucky, and someone was observing that the same night he was in a different band). And obviously, if it repeats too much, others will notice.
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You're awesome.
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In a total coincidence, this month's Physics Today happened to have an article on LOFAR and how it works. Turns out it's a big-ass survey array, so it has a huge field of view and is specifically designed to observe its entire bandwidth at once. So if something like this is operating in your story, not only would it plausibly see the signal, it would see it in every band. Someone would definitely take notice of the signal were it to be detected by this kind of array.
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That's okay - just as long as it looks like statistical junk to most of the people watching.
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Well, I thought I had seen a picture of a radio telescope without a central dish...