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 23:31:21 GMT
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Viewed:
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351 times
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In lugnet.space, Jeff Jardine writes:
> 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 most of this sounds reasonable, but I'd guess that laser still
follows the inverse square "law".
ROSCO
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