[Laser] Lunar downlink

[Glenn Thomas]

Hi James!

At 05:11 PM 2/22/2006, you wrote:
>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.

My sincere apologies if I gave the impression that I thought you were 
the "creative" (that's a nice word) source. Your responses have 
generally be thoughtful and credible, no matter my skepticism. My 
experience is that such "creativity" is often reserved to journalists 
working to a dead line. In my career there have been several 
instances where projects I was working on were reported in the press 
with details that had those of us actually doing the work literally 
rolling on the floor laughing.

>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'm not sure how the radius of the earth figures into this. A 
position error of 100m at a range of 240,000 miles translates into a 
pointing error magnitude of approximately 0.0542 arc seconds. This 
translates into a position error on the moon of, strangely enough, 
100m. This assumes that the position error vectors are orthogonal to 
the slant range vector. If they're not parallel, the apparent 
pointing error will be less. Parallelograms are like that.

However, someone on here pointed out that a range gate capable of 
discerning range changes in the millimeters may not be able to 
acquire the return signal with a 100m error. To acquire the return 
signal either the gate would either have to be open for more than 
.333 usec (the time for a 100m range error) or a shorter gate time 
would have to be slewed over .333 usec or more to acquire the return. 
Unfortunately, I don't know how long the gate was open or how long a 
time span they searched in this case. However, the range gate seems 
like it might be a credible reason that a 100m position error could 
prevent reception of the return pulse, again depending on what timing 
parameters they used.

>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.

I think I've seen that same report. Receiving 0.01 photons per pulse 
means that they got one photon for every hundred pulses. I don't 
think there's any such thing as a fraction of a photon that remains a 
photon! It's still not clear what the potential contribution of 
thermal noise is to this system. A good test would've been to shut 
the laser off and see how many photons (thermal radiation from the 
atmosphere, the telescope or even the mirror itself) they got then! I 
don't know if they did something like this.

>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.

An even better approach would be to track the earth via one of the 
water based infrared spectral lines. They aren't affected much by night & day.

>Interesting thoughts.

I agree! Thanks for the interesting conversation.

73 de Glenn wb6w
 > James N5GUI


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