Subject:
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Re: Evolution of Earth and moon (was: Couldn't resist)
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Newsgroups:
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lugnet.space, lugnet.off-topic.geek
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Date:
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Thu, 12 Jul 2001 16:22:54 GMT
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Viewed:
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8854 times
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In lugnet.space, Kyle D. Jackson writes:
> In lugnet.space, Larry Pieniazek writes:
> >
> > Would there be any difference in the accuracy of laser vs. radar? I'm not
> > enough of an EE geek to know, but thought "no" because they're just
> > different wavelengths of the same thing, right?
> >
> > Or does the wavelength difference (it IS many orders of magnitude in
> > difference) matter?
My first inclination is that there should not be a difference. When measuring
distances with light, the accuracy *can* be limited by the wavelength. I think
one can measure accurately down to roughly half the wavelength of the light
used. I would expect a laser in the visible spectrum (hundreds of nanometers)
to be used, because the atmosphere is quite clear in that range and the moon
obviously reflects some visible light. Radar wavelengths are much longer than
those of lasers, but both are insignificant compared to the distance to the
moon. I would expect that the technique used to measure the time for the
light to reflect would be a much larger source of error.
But...
> Isn't the theory behind rainbows (or light spectrums in general)
> that the different wavelengths refact by differing amounts, and
> so white light is "spread" into a spectrum of colours? If that's
> the case then radar and laser energy would refract by differing amounts,
> meaning one would end up being more likely to deviate from a
> straight line in our atmosphere, and add errors to the measurement.
> It would also presumably scatter more so the "signal" loses "power"
> more.
One could arrange their experiment so that their beam would travel straight up
through the atmosphere and eliminate any refraction. However, this raises an
interestiong point - refraction is caused by different wavelengths travelling
at different speeds through a medium. So, the laser and radar beams would
travel at different speeds until they left the atmosphere. Again, I think
any error introduced by this would be negligible compared to the timing
mechanism. It could be accounted for mathematically, anyway.
The amount of scattering also depends on the wavelengths. However, the loss
of power isn't really important, as long as there is enough power remaining
in the reflected beam to be measureable. A weak faint beam travels just as
fast as an intense one.
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.
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 think I would lean toward using a laser for the measurement.
Jeff J
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