Woud an extrasolar civilization have to be crazy or stupid to send pulses?


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metiman
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Message 1180316 - Posted: 23 Dec 2011, 22:32:12 UTC
Last modified: 23 Dec 2011, 22:38:15 UTC

I was just reading this planetary society article on the Astropulse project and I couldn't help but notice the following:

"Amir Alexander" wrote:
In other words, if you wanted to communicate across interstellar space you would almost certainly use the exact same technology that has worked so well for us in the much shorter distances of Earth: continuous narrow-band radio transmissions. Wouldn't aliens do the same?

I can think of some reasons why they might not.

"Amir Alexander" wrote:
But then again, maybe they wouldn't. Perhaps the aliens, for their own reasons, would choose to communicate using a very different type of signal. For example, instead of sending a continuous narrow-band transmission they might choose to send distinct broad-band pulses. These would stand out against the background noise not because they are precisely centered on a particular wavelength, but because they are very short and punctuated bursts of energy. Why would the aliens choose this method over our own? Who knows, after all we are not aliens and cannot begin to imagine the technological choices they face.

Is the idea of sending short, low duty cycle, periodic pulses really so silly? So silly that the only explanation we can come up with for why they might do so is: "Well, they are aliens. Who knows how they think?" So what makes it so ridiculous? The greater bandwidth? The interstellar dispersion? I'd love to know.

Couldn't the bandwidth problem be mitigated by using somewhat longer pulses and a gaussian filter for a more gradual pulse power gradient so that the wave modulation is not so square? Something like a cross between a sine wave and a square wave? Does anyone have a link or an explanation on how to do bandwidth calculations for short pulses with varying shapes?

As for interstellar dispersion could this not be mitigated by using higher frequencies? If I had to transmit from the surface of a planet (as opposed to a space based observatory) through a nitrogen-oxygen atmosphere like ours I would transmit at just under 10 Ghz or at 77 Ghz to mitigate atmospheric attenuation as well as interstellar dispersion and scintillation while allowing the highest possible EIRP for a given antenna diameter. From a space based dish I'd either transmit on the highest frequency possible which would still have rf properties to communicate solely with other space based dishes or I might hedge my bets and transmit at around 77 Ghz or 10 Ghz to penetrate nitrogen-oxygen and water vapor atmospheres at the highest possible frequencies.

The cost of building a dish is proportional to its diameter. Even more so for a space based dish which has to be launhced from the planet surface piece by piece. So you would expect them to choose as high a frequency as possible limited only by their ability to create mathematically perfect parabolic surfaces. For this reason I would consider 77 Ghz transmissions to be even more likely than 10 Ghz ones. And depending on how optimistic they are about our abilities they might even be transmitting at 150 Ghz or more, possibly with the (not unreasonable) assumption that we have the technology to build space or lunar based radio telescopes so that we don't have to worry about atmospheric absorption. This would also have the side effect of filtering out the most primitive technical civilizations like us who don't yet have orbiting or lunar millimeter wave telescopes actively listening for transmissions. Something they may or may not find desirable.

So would sending 75 Ghz pulses with pulse durations between 1 and 5 microseconds using something like amplitude shift keying, pulse position modulation, or pulse duration modulation really be so crazy or stupid of them? I guess that somewhat depends on how much more bandwidth pulses actually use compared to modulated or unmodulated CW, whether they have figured out a way around that bandwidth limitation, and what their energy costs are.

I was recently pricing out transmitters that might be suitable for sending interstellar messages. Pulsed transmission can be many, many orders of magnitude cheaper both in initial cost and in use. A 35 kW Toshiba CW traveling wave tube (TWT) costs around $150,000 USD. Klystrons with even less power output start at around $100,000 USD. A pulsed 75 kW magnetron can be found on Ebay for something like $150 USD. A new 350 kW CPI pulsed magnetron costs around $6000 USD. Pulsed klystrons and gyrotrons can reach much higher powers than (commercially available) magnetrons and have much better frequency stability. So you would probably expect at least the alien government (or equivalent organization) to use those designs in preference to the cruder magnetron, but what about amateur alien METI? For all we know transmitting to other planets may be taboo in that world (as it sort of is here) and only a few rogue amateurs ever attempt it. They might not be able to afford a klystron or gyrotron or the electricity to run a CW device.

In terms of cost of electricity, a 350 kW CW transmitter with a 50% efficiency would use up 504,000 kWH per month. Around the world most electricity costs between 7 US cents and 20 US cents per kWH. Some places are slightly less expensive and some places are slightly more. So the monthly cost to operate the CW transmitter would be somewhere between $35,280 and $100,800 USD. Or between $423,360 and $1,209,600 USD per year. And that's just for a measly 350 kW.

In comparison, consider a 50% efficient 2 megawatt pulsed magnetron with an average pulse duration of 3 microseconds and an average pulse frequency of about 1 pulse per second resulting in a duty cycle of about 0.000003. The peak input power of 4 MW becomes 4000,000 x 0.000003 = 12 watts average power usage resulting in 8.64 kWH and a monthly cost between 60 US cents and $1.73 USD per month or between $7.26 and $20.74 per year.

So a pulsed transmitter with an output power of 2 MW is nearly 60,000 times less expensive to operate than a CW transmitter with only 350 kW. By going pulsed you not only might save a lot of money on the initial purchase depending on the tech, but you can also afford to broadcast at almost 6 times the power for almost 1/60000 the operating cost.
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Message 1180355 - Posted: 24 Dec 2011, 2:17:03 UTC

The web-link provided by the poster is not working correctly.

Could you fix this error since the rest of the contents is not at all bad.

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Message 1180356 - Posted: 24 Dec 2011, 2:17:32 UTC - in response to Message 1180316.

The problem is a black hole can create short pulses. I think there is a good chance they would broadcast on the frequency of the hydrogen line in space, because hydrogen is a very common element in the universe and they would use normal radio signals because sending 2 or 3 normal radio signals at least 3 minutes apart would never be caused by anything else in space. If there power resources are scarce that might be a problem, but an advanced planet probably has figured a way around that.

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Message 1180361 - Posted: 24 Dec 2011, 2:46:02 UTC
Last modified: 24 Dec 2011, 2:48:44 UTC

When listening to "space signals" provided on YouTube, you are in fact listening to sound.

Sound could be supposed as having been created by numbers (on a piano you have both white as well as black keys). When you press the "C" key in the middle, you get a sound back which could be thought of as a wave. This wave has a uniform shape since it is only one single tone. It is analog, not digital.

I am still of the impression that you can not get numbers from something that is being represented by means of an analog signal or tone, it will have to be a digital one.

If a space signal should be thought of as being a gaussian or gaussians for that matter, can someone explain the subject of "modulation" of such a signal, like possibly frequency modulation, or the like?

How do you transform a gaussian score into something that is supposed to be audible in nature? Are there any particular numbers needed for such a transformation?

Please explain this to me.

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Message 1180364 - Posted: 24 Dec 2011, 3:28:26 UTC - in response to Message 1180361.
Last modified: 24 Dec 2011, 3:29:36 UTC

To give a simplified expanation:

Communication involves a "Carrier" that is at a constant center frequency. It is usually at a frequency that is easily transmitted and propagated over long distances. The Carrier is "Modulated" either by varying the frequency of the Carrier slightly at the Audio rate or in Amplitude also to carry the audio message. It could also be switched on and off or otherwise shifted in phase and amplitude to transmit data digitally as a representation of analog or digital information.

It might be interesting to try to figure out what scheme might be used by others or even ourselves to transmit an intelligent message to the cosmos

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Message 1180376 - Posted: 24 Dec 2011, 5:03:37 UTC - in response to Message 1180356.

The correct link to the article is http://www.planetary.org/programs/projects/setiathome/setiathome_20080827.html. I'd like to fix the original link, but I can't seem to find an edit button.

Except for the fact that light is not supposed to be able to escape the gravity well of a black hole (that's why it's called a "black hole") it would certainly seem plausible that black holes would create pulsed signals like pulsars since neutron stars are sort of the little brother to the black hole and both tend to spin very fast. But in the case of either stellar object any pulses would have precise and equal time intervals. There is the occasional strange pulsar with changing pulse durations and frequencies. Maybe those are being artificially modulated by an ETI. Whether we should be listening to those particular pulsars with an eye towards detecting some kind of modulation is a good topic for another thread. Also in the case of these stellar pulses it is a lighthouse effect of a rotating beam. I believe that will give you a different kind of pulse signature from one created artificially and sent directly toward you. Also I don't believe we have observed many pulsars with pulse durations or pulse positions or amplitudes which vary in an interesting way.

The way that I used the term 'modulation' could be a little confusing. In order for an electromagnetic wave to contain any kind of specific information other than "I'm here." it has to be altered in some way. And that alteration is what modulation traditionally refers to. I was also using the term to describe how a pulse of voltage and current from an energy storage bank of capacitors and inductors (a pulse forming network) is transferred to the magnetron/klystron/gyrotron which transmits the actual waves to the antenna. If you plot the rise in voltage/current with time on the horizontal axis a nearly instantaneous rise/fall will give you a square wave. That square wave 'modulates' the amplitude of the sine waves being created by the oscillator by turning the device on or off.

The problem with that system is that something about a square wave modulating a sine wave gives you a large bandwidth. That's something I really have to look into more deeply since I don't quite understand it, but square waves + sine waves = bad. A more bandwidth efficient system is to modulate the sine wave with another sine wave. This is what gaussian filtered minimum shift keying (GMSK) attempts to do. Basically you would try to put the square voltage pulse through a (low pass) filter which rounds off the square wave before it modulates the carrier wave. Pulsed communication is itself really a form of amplitude modulation. Except that it doesn't convey any information. Further modulation has to be used in order to send a message. The problem with using sine wave shaped pulses to control the transmitter is it can make designing a receiver to detect the signals more difficult. It also means that your pulses stay at full power for less time. Instead of transmitting at full power for 4 or 5 microseconds, it may only transmit at full power for 1 microsecond before the power starts dropping again.
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Message 1180465 - Posted: 24 Dec 2011, 16:09:52 UTC - in response to Message 1180376.

Here's the fun part of sending a signal out. If you don't know the direction the galaxy is spinning and don't account for the speed of the star you are attempting to send you message. Then you'd most likely wind up sending a message to empty space. Lets also not forget that we aren't attempting to contact the star but a planet that may or may not be spinning around that star. Talk about hit or miss.
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Message 1180546 - Posted: 25 Dec 2011, 0:16:59 UTC

To my knowledge we are not sending a signal to the stars other than what radiation escapes from our radars. radio and television. Let's hope that if there are other civilizations in the Galaxy that they are sending such a signal.

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Message 1180555 - Posted: 25 Dec 2011, 0:54:08 UTC

61 Cygni is a double K class orange star in the constellation Cygnus. It is 11.36 light years away and one of the best nearby candidates for life in the northern hemisphere. It has high proper motion with a tangential velocity of 316,800 kph. It's also traveling towards the earth at 231,480 kph. Every year its declination changes by 3.26 arcseconds or 0.0009 degrees of arc and its right ascension changes by 4.16 arcseconds or 0.0016 degrees. So if it will take 11.36 years for the message to reach them you would have to aim not where it is but where it will be in 11.36 years. In this case you would adjust your declination by 37 arcseconds and your right ascension by 47.26 arcseconds so that your message beam intercepts their solar system. Obviously you would have to continually update your coordinates to account for the movement of the star with respect to you.

But it gets worse. If you are on a planet orbiting a sun then you also have to deal with the parallax created by the planet oscillating back and forth every year with respect to the star. If the communication is planet to planet then you have two parallaxes to deal with. Luckily this variation, which is proportional to the orbit of each planet, can generally be ignored due to the relatively small distances involved.

As far as having to aim at the exact planet, you could simply aim for the 'goldilocks zone' or the distance from the star necessary for liquid water to exist, but at frequencies under 10 Ghz this probably isn't much of a concern. The beam spread at large distances is quite large. An RF beam with a 3.25 cm (9.3 Ghz) wavelength transmitted from a 20 meter dish would have an angular divergence of .00163 degrees or 5.87 arcseconds. With a bit of trig (11.36tan(divangle/2)*2) it seems that the 61 Cygni system would be hit with microwave radiation in a disc with a diameter of 3.23 x 10-4 ly or 20.4 AUs. So if my calculations were correct such a transmission sent from 61 Cygni to us would cover the solar system out to about Saturn. Using a smaller transmitting antenna or a longer wavelength would give you greater coverage. For a closer system like Alpha Centauri you really would have to aim a bit more at that frequency. Transmitting in the water hole should negate this issue, but you would reduce your EIRP significantly.

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Message 1180559 - Posted: 25 Dec 2011, 1:09:19 UTC - in response to Message 1180546.
Last modified: 25 Dec 2011, 1:11:26 UTC

William Rothamel wrote:
To my knowledge we are not sending a signal to the stars other than what radiation escapes from our radars. radio and television. Let's hope that if there are other civilizations in the Galaxy that they are sending such a signal.

http://en.wikipedia.org/wiki/Active_SETI

According to wiki a message will be reaching the Gliese 581 system in 2029.

I'm hoping to start transmitting myself sometime in the next 3-5 years. Unlike previous efforts which only lasted a short time due to the scarcity of large telescope time, I hope to transmit continuously for at least a year to each candidate. I see METI more as something amateurs or amateur organizations with smaller homemade dishes should do on their own. It's really too controversial for governments to spend a lot of money on anyway. It's hard enough to get funding for traditional passive SETI.

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Message 1181006 - Posted: 27 Dec 2011, 21:04:53 UTC - in response to Message 1180559.

William Rothamel wrote:
To my knowledge we are not sending a signal to the stars other than what radiation escapes from our radars. radio and television. Let's hope that if there are other civilizations in the Galaxy that they are sending such a signal.

http://en.wikipedia.org/wiki/Active_SETI

According to wiki a message will be reaching the Gliese 581 system in 2029.

I'm hoping to start transmitting myself sometime in the next 3-5 years. Unlike previous efforts which only lasted a short time due to the scarcity of large telescope time, I hope to transmit continuously for at least a year to each candidate. I see METI more as something amateurs or amateur organizations with smaller homemade dishes should do on their own. It's really too controversial for governments to spend a lot of money on anyway. It's hard enough to get funding for traditional passive SETI.


Ok, good luck with your future broadcasts. One thing though, how do you know that, by our comparisons, you will connect with an Abraham Lincoln, rather than a Mr A Hitler? And what about Mr intestellar H's ability to arrive here PDQ, having the knowlage that we are still using primitive radio communication?

Godwins Law should be suspended in this case, I hope. :-)

Regards,

A

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Message 1181027 - Posted: 27 Dec 2011, 23:01:08 UTC

According to wiki a message will be reaching the Gliese 581 system in 2029.


Sadly I won't be around then. If we get a reply, would you answer it for me?

Thanks.

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Message 1181075 - Posted: 28 Dec 2011, 3:17:02 UTC - in response to Message 1181006.

I think the chances of any nearby star systems having life are astoundingly low. I'm not really expecting to reach anyone. But I still want to try because:

1. It's exciting. I would like my life to be more like a science fiction novel and this was the only idea I had. It's super cool and would add a dose of awe to my daily life.
2. Being the first person to transmit to a particular star is kind of like being the first person to step foot on another planet. I'd love to walk on Mars or Titan or even the moon, but it's just not gonna happen.
3. I could try to do passive SETI on frequencies that the SETI community is not currently listening on. Like say 77 Ghz. But I can't possibly build a dish big enough to even remotely compare to any of the government owned ones. It seems like there are enough people already doing passive SETI. Although if I couldn't convince any of them to listen to my targets after the message round trip time. I'd have to listen for myself anyway. Unfortunately most SETI installations aren't really set up to listen to the frequencies I'd be transmitting on: between 9 and 10 Ghz.
4. It pisses off people like David Brin who don't believe I have the right to speak for the entire planet or expose everyone else to the risk of alien invasion and planet-wide extermination. But I see it as a freedom of speech issue. You know, human rights. Haha. I wonder if he would advocate having me killed or imprisoned to stop me from transmitting.

Despite all this it looks like I may not be able to do amateur METI after all. I hadn't realized how wideband short pulses really are and CW transmitters are super expensive. Received noise is proportional to receiver bandwidth. I see this as a kind of cosmic censorship. So I may be stuck. Or I may transmit anyway and just hope that the aliens have greatly superior receiver technology that can bypass the wideband SNR problem. At the moment I am torn between that option, giving up on the project, or learning to design my own high powered microwave devices, which I have to assume would take at least 10 years of intense study. I am somewhat leaning toward the latter.
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Message 1181207 - Posted: 28 Dec 2011, 22:20:18 UTC

Given a sufficiently advanced receiver technology at the other end, almost any VHF or higher frequency transmitter at our end might suffice, provided its signals could penetrate the atmosphere. There have been serious scientific proposals to use the gravity of the Sun to focus radio waves from the cosmos. The gain would be immense. What we can contemplate, other civilizations might well accomplish. The trick might not necessarily be to have a technically conspicuous signal, but to transmit content likely to be of particular interest to cosmic anthropologists studying our (relatively) primitive planet. Michael

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