[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.
> 
> 




More information about the time-nuts mailing list