[time-nuts] Timing Distribution in Mountainous Terrain

Hal Murray hmurray at megapathdsl.net
Fri Sep 10 22:10:55 UTC 2010


[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 
footprint after Earth's rotation had moved it from the transmit position.  My 
back-of-envelope says that's about 1 km in dia.

If you have to work with the existing corner cubes, I don't see how to start 
with a pulse at one site, bounce it off the moon, and get it back to another 
site that isn't nearby.





-- 
These are my opinions, not necessarily my employer's.  I hate spam.







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