[time-nuts] Timing Distribution in Mountainous Terrain
jimlux
jimlux at earthlink.net
Sat Sep 11 05:19:02 UTC 2010
Hal Murray wrote:
> [Lunar Laser Ranging]
>> Hmm.. the SNR isn't all that huge on the echo. The target is say, 1 square
>> meter, at a distance of 300,000 km.
>
>> The beam divergence coming back is about the same as the outbound (that
>> is, in order to cover 300km on earth, you need to have a spot on the moon
>> about 300km in diameter).. So the laser power will be spread out by a
>> factor of 71E9.. (about 110 dB). The power reflected back, if intercepted
>> by a 1 square meter aperture will have the same "loss" for a round trip
>> loss of around 220dB.
>
> I don't see why the outgoing beam has to be that big. Anything that doesn't
> hit the corner cubes is wasted.
>
> I thought the size of the corner cubes was picked so that diffraction would
> spread the beam enough so the telescope would be in the reflected beam
might be...
I seem to recall that the returned beam divergence was no narrower than
the incident beam divergence, so if you want a X km footprint on Earth,
you need a X km footprint on the Moon.
>
>
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