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In lugnet.space, Jeff Jardine writes:
<snip>
>
> And aren't radio frequencies affected by the ionosphere? I seem to recall
> something about how AM and short wave radio frequencies bouncing off the
> ionosphere, allowing them to travel further along the surface of the earth.
I'll address this point to fill in a bit of a hole, as a lot of the other
concerns are past me.
Yes, radio waves are affected by gravity which is why they travel along the
surface, so in a sense they "bend". It's a phenomenon commonly termed as
"groundwave" and it's how AM mainly propagates during the day when there's
oodles more ions in the air. But as you increase the frequency, the waves
travel in more of a straight line - so they can leave the earth's curved
surface. Also, as you substantially increase frequency, they're not AM waves
anymore, which means the waves can go through the ionosphere.
Groundwave occurs all the time during medium/shortwave broadcast. But at
night the signal can travel further because at night the sun's energy isn't
usually making as many ions in our atmosphere. So the resulting ionosphere
layer is much thinner which makes its underside much higher in the air. So
as you increase the distance to i'sphere (assuming that its underside is
more or less flat thereby keeping the angle of reflection more or less the
same) the destination distance of the wave is also increased. Think of it as
a triangle: if you keep all the angles the same, and increase the length of
the two sides of a triangle, its base (the ground distance away from the
station) also increases.
> Finally, most existing radar technology has been designed to cover a range of
> space (although I sure someone will correct me on this). Lasers, by
> definition, travel in a straight beam and are not as affected by the inverse
> square relationship between distance and intensity.
I'm not sure I agree with that. (But so what? :) PCMIIW[1] It'd be nice if
everything stuck to its definition, but light, and thus any photon, can have
it's trajectory bent, even however slight, in a gravitational field. Yet
it's proven that higher frequencies travel in more of straight line than
lower frequencies. Radar, which is in the microwave part of the spectrum, is
a higher frequency wave than visible light, so it's less "bendable" than
visible light.
Assuming that we're talking about a laser of visible light, it would be
still subject to the same degree of bending and distance's inverse square
law as any other visible light because of the wavelength used. Individual
photons in a laser generally don't possess any different characteristics
than any other photons of identical wavelengths other than a common
trajectory. But the effects can be quite different.
> I think I would lean toward using a laser for the measurement.
What would be the best of both worlds is if you made a laser of radar waves.
But be careful of frying things :)
-Tom McD.
[1] Please Correct Me If I'm Wrong
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