Do satellite TV and radio signals carry into space as well?

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Pete

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Message 94704 - Posted: 4 Apr 2005, 9:57:42 UTC

A corollary to my other question:

Do satellite signals "escape" from Earth orbit? Or are they so precisely directed that they don't bounce around outside te Earth atmosphere?

As in my last question, just wondering if our TV and radio (or other) signals will always be sent off into space, even if we're not trying to do that. And whether they'd be detectable at edge of the solar system type distances.

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Pete
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Message 94714 - Posted: 4 Apr 2005, 11:33:18 UTC - in response to Message 94704.  

> Do satellite signals "escape" from Earth orbit? Or are they so precisely
> directed that they don't bounce around outside te Earth atmosphere?
>
> As in my last question, just wondering if our TV and radio (or other) signals
> will always be sent off into space, even if we're not trying to do that. And
> whether they'd be detectable at edge of the solar system type distances.

I'll prefix this with my "I'm not a physicist" phrase. However, all the signals are just sitting at different electromagnetic frequencies, of which (of course) visible light is simply one type. So you can use the analogy of a torch. When you turn your torch on in the dark, you can see that it's very bright where the bulb is (kids, don't try this at home with megawattage lights), but when you look at the light coming away from the torch, you can see it gets wider and dimmer as the light is further from the light source. For example, try shining a torch on a wall. First do it really close to the wall, then move further and further away and you can see that the light radius on the wall gets bigger, but the light on the wall gets "lighter".

From *distant* memory, it follows an inverse square law

As you can see from the torch example, you can't do much to keep the light in a narrow beam. You can try putting a cardboard tube around the light, and it will artificially hold the light in a more narrow beam for part of the way. However, one can't do that with satellite signals. :)

For a satellite signal to remain in Earth's orbit, it would require some type of force to "keep it there". From memory (again), there is no force that would do this (madly trying to think of forces, like weak electromagnetic force....)

Now watch all the physicists knock me down!!! :) :)
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Message 94732 - Posted: 4 Apr 2005, 13:04:21 UTC - in response to Message 94704.  
Last modified: 4 Apr 2005, 13:15:07 UTC

> A corollary to my other question:
>
> Do satellite signals "escape" from Earth orbit? Or are they so precisely
> directed that they don't bounce around outside te Earth atmosphere?

The signals FROM the satellites could not be detected. Take for example the satellites used for DirecTV. The transmitters themselves are 120W. That gives 1.2 picowatts (pico=one trillionth)of power over an 18 inch diameter dish, and that's only because the beam is aimed at the United States and therefore expending all it's energy in this direction. Your dish has to have a special low noise amplifier right on it to even receive these signals, and that's just from 22,000 miles away. Imagine how weak a signal would be millions of miles away.

The uplink signals (from the ground to the satellite) are quite another matter. Since they too are focused, I imagine (I have no numbers to put to this) that the signal stays relatively intact and strong as the lion's share of the power passes the satellite targeted and heads into space. I suppose one of the thousands of signals sent this way could be detected far out if an alien passing by was lucky enough.

Still, if you think about the size of all the satellite antennas on the earth added together and realize that they cover an insignificant fraction of the entire Earth, then imagine this same area compared to a sphere the size of the solar system, you can see that it's still VERY long odds that a passing spacecraft on the outer edges of the solar system would ever be in the right position to get a hit of RF. Even considering beam divergence (like a flashlight beam, the signals spread out some as they go out), that's still a lot of real estate with very very little struck by our signals.

I tried putting some numbers to a more conventional signal, one that is omnidirectional, using an insanely large transmitter on a planet orbiting Alpha Centauri, in another post:

Do you think a signal will be detected in your lifetime?

Realize I'm not an engineer, so I don't really know if my numbers are accurate.

More threads along these lines:

What if they're hostile?
Hostile Humans
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Message 94782 - Posted: 4 Apr 2005, 15:18:08 UTC - in response to Message 94704.  

> A corollary to my other question:
>
> Do satellite signals "escape" from Earth orbit? Or are they so precisely
> directed that they don't bounce around outside te Earth atmosphere?
>
> As in my last question, just wondering if our TV and radio (or other) signals
> will always be sent off into space, even if we're not trying to do that. And
> whether they'd be detectable at edge of the solar system type distances.
>
> Thanks,
>
> Pete
>

Yes. The various range of uplink signal frequencies are sent off into deep space. The downlink signals are too weak to be sent off into deep space.
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Message boards : SETI@home Science : Do satellite TV and radio signals carry into space as well?


 
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