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Author Topic: line of sight  (Read 3075 times)

Offline ukmicky

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line of sight
« on: 23/03/2006 00:48:03 »
i don't know enough about electromagnetic waves.:)

Radio waves etc travel in a straight line,and any signal coming from an alien transmitter would be coming from a small moving source.  so if the people at seti were to intercept an alien signal with there fixed dishes how do they keep their dishes centered on the signal when you consider the earth is spinning and  moving around the sun and our solar system is also in motion through our galaxy.
Wouldn't the earth very quickly lose line of sight. I know the signal is being transmitted in all directions from its source but considering the motion of our planet and solar system the distance and the motion of any alien source, the spin of the planet it was on and the spin of the earth wouldn't we very very VERY quickly in a blink of an eye lose the transmission if any were found.

Michael
« Last Edit: 23/03/2006 00:59:33 by ukmicky »


 

another_someone

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Re: line of sight
« Reply #1 on: 23/03/2006 03:41:33 »
It depends upon the transmitting and receiving antennae, and the distance between receiver and transmitter.

The antennae can be thought of like lenses, and the transmitter is like a torch, and the receiver like a telescope.

The more powerful the lens on the torch,  narrower the beam on the transmitting lens, the further you can project the beam with a usable amount of light, but it will mean that the beam will only illuminate a very small target.

The same is with the lens on the telescope: the more powerful the lens, the greater the magnification, but the narrower the view you get.

Radio antennae behave the same way, they can either have a wide field (even a 360 degree field) or a high gain with a narrow field.

If you are looking at a moving target, the the closer the target is to you, the faster it will appear to move across your field of view, and so you either need to tract it faster, or have a must have a lower gain antenna with a wider field of view.  A more distant object will appear to be moving slower across your field of view, so you can use a higher gain aerial (with a narrower field of view), and in fact you will need a higher gain aerial, because of the weaker signal you will get.

In modern terms, you can actually create synthetic antennae, by adding together the signals received by many antennae that are widely spaced apart, and by shifting the phases of the received signals when you combine them, you can make them behave (inside a computer) as if they were one giant antenna.



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Offline Soul Surfer

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Re: line of sight
« Reply #2 on: 23/03/2006 21:57:37 »
Angular pointing is no problem

The signal from a distant source is coming from a fixed point in the sky.  

The main movement is that caused by the rotation of the earth which is along the celestial equator 360 degrees in 24 hours (15 degrees in an hour or one degree every four minutes) at the poles it is zero.  It is quite easy to steer an optical telescope so that it maintins pointing accuracy of seconds of arc.

Radio waves are  much longer than optical waves and a very narrow beam antenna is a degree of arc so even if the beam is not steered a soutce will be in the beam for at least a minute and it is usuallly quite easy to steer the telecope so that any point in the sky is in view for as long as necessary.

A rather more subtle correction that needs to be done to confirm a signal is doppler shift particularly if the search is for a very narrow band signal.  The doppler shift on frequency of a radio signal is basically equal to the number of wavelengths of the signal path length change per second this is not usually too much of a problem on one observation but needs to be taken into account if you look for the same signal later and is a real problem with keeping in touch with long range space probes

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another_someone

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Re: line of sight
« Reply #3 on: 24/03/2006 04:34:11 »
Ofcourse, if one is talking about awkward aspects of radio transmission there is a nave assumption that we would be expecting to see fairly simply modulated signals.  If an alien civilisation were using radio transmissions in difficult situations, it would be very possible that they would be using spread spectrum techniques to overcome the hostile environment (e.g. doppler shifting, multipath signals, high noise environments).  We are already moving to spread spectrum techniques, and I could well imagine that it would be possible to develop some far more sophisticated spread spectrum techniques in the future that would allow better performance is ever more demanding environments.

Does SETI have any idea how to look for spread spectrum signals?  I doubt it one of the useful aspects of spread spectrum techniques for the military is that they can be very difficult to detect unless you know what you are looking for.



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Offline Soul Surfer

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Re: line of sight
« Reply #4 on: 24/03/2006 14:20:24 »
Unknown potential spread spectrum signals may be more difficult to detect and process than simple carrier wave based modulation systems but it can be done, and it is getting easier all the time as processing speeds improve. I have spent much of my life inventing, developing, detecting and countering spread spectrum systems (both communications and radar) for both civil and military systems using initially analogue and later digital processing techniques.  I have also carried out theoretical research in a wide range of complex modulation and reception techniques that take correllation based processing just about up to the theoretical shannon limits of signal density provided you are prepared to desgn receivers of vast complexity.

I am not sure if SETI is using this sort of analysis technique in its algorithms but it could.

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« Last Edit: 24/03/2006 14:24:14 by Soul Surfer »
 

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Re: line of sight
« Reply #4 on: 24/03/2006 14:20:24 »

 

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