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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