[Laser] Lunar downlink

[TWOSIG at aol.com]

I admire healthy skepticism.   And I think that secretly part of  me is 
flattered that you might have thought that I was creative enough to make  this up.  
I assure you, I am not that creative.
 
 
In a message dated 2/21/2006 6:57:28 PM Central Standard Time,  
glennt at charter.net writes:

>I'm not sure that the tale of the  observatory location being off by 
>100m is credible. If the beam is  already 2 km on the moon and 18 km 
>on the return, a translational error  of 0.1 km seems insignificant. 
>
>73 de Glenn wb6w
 
 
I would agree with you, with a beam spread of 2 Km that a translational  
error of 100 meters would be insignificant.  Please consider that  the team that 
was pointing the telescope probably calculated the line from  the center of the 
Earth to the center of the Moon, then calculated the the  offset from that 
line to the location of the reflector on the Moon and  the  location of the 
telescope, adjusting each for the rotation of each body with  time.  After some 
hand waving and head scratching, they came up with a  direction to point the 
telescope.  
 
I suggest that the direction to point the telescope was based a line from  
the center of the Earth to the presumed location of the telescope, which was off 
 by 100 meters.  That translates to a very small, but  measureable, angle.  
If the telescope was pointed in a  direction with an error caused by 100 meters 
at the surface of the  Earth, then at the distance of the Moon, the error 
there would be  in the ratio of the distance to the Moon to the radius of  the 
Eath.  That ratio is about 240,000 / 4000.  Roughly 60.   The resulting error 
would have been about 100 m times 60, or 6,000 m.   6 Km is, I submit, not 
trivial to a beam 2 Km wide.
 
 
I recently check some sources on the web.  The initial testing to the  Apollo 
XI reflector was done at the Lick Observatory 3.1 m telescope  (  about 122 
inches )  Roughly 36 times the area used for the long term data  collection.  
One source claimed the average number of photons received per  pulse was 0.01.  
 
 
 
There was also a suggestion that a Lunar downlink lock onto an  Earthbased 
beacon for tracking.  I had never considered that to be  necessary.  The Earth 
itself should provide a much easier to find object  for tracking.  It will 
change in brightness as it goes through phases (  which will compliment the phases 
of the Moon ), but the edges will be easy to  separate from deep space.   My 
idea was to send a one milliRadian beam  down, so tracking by the use of 
redundant Earth edge detectors should  serve.  That said, a tracking beacon might 
be needed if you try to produce  narrow beam tracking, say with a beam only 500 
meters wide for commercial  communications.
 
 
Interesting thoughts.
 
 
 
 
James
N5GUI