[Laser] Lunar Laser EDM

[Tim Toast]

Hi Pat and all,
It seems almost impossible to detect a signal that weak. I don't think those setups are even 
capable of carrying any modulation or at least detecting it. I think most of these experiments 
are done with under 10 watts of average laser power. One in particular i remember used 2 
watts. A tiny fraction of those 2 watts illuminating a 1 square meter retro-reflector on the 
moon... yet that's enough to get those really good distance measurements. Is it possible to 
do a rough calculation of the signal to noise ratio of this experiment? i was wondering about 
what it would be roughly.
 
Now imagine illuminating a small crater - say 20 miles across, with about 100 watts of 
modulated laser light. The receiver used would have a field of view roughly the same as the 
transmitter spot - about 20 miles wide. 
Unlike the experiment above, it is not a tiny fraction of the hundred watts hitting the crater, but 
the Entire Amount - (minus what is absorbed by the atmosphere). And this doesn't need 
extreme optics to do - a small telescope could do it. The chosen crater is a relatively bright 
one with an albedo of 20 percent or so - for math purposes. The crater is in the dark but is 
illuminated by a bit of earthshine and some starlight. If you know about how much earthshine 
is hitting the area, you can do a rough calculation to see what the signal to noise ratio would 
be. It would be interesting to compare this with the lunar ranging ratio to see if it's at least 
the same or better (or worse). Yves (F1AVY) did a rough calculation one time but i'm not sure 
if he ever posted it in here for everyone to go over. I could search the archives for it but i 
thought a second go around wouldn't hurt. Yves??
-toast
 

> Message: 1
> Date: Thu, 19 May 2011 16:03:49 -0700
> From: Patrick Barthelow <apolloeme at live.com>
> Subject: [Laser] Lunar  Laser  EDM
> To: <laser at mailman.qth.net>
> Message-ID: <SNT115-W420420D6B33E04C2365D48DF8E0 at phx.gbl>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> 
> Hi Everybody,
> 
> 
> Does anyone know how big the laser spot size on the moon is, that they send via 
> an Arizona telescope to the moon to measure distance using the 
> retroprism arrays left by the Apollo missions?   The can resolve distance 
> apparently to near mm levels.
> 
> The Apollo era  retrprism arrays are on the order of a meter or two, 
> rectangular, and have a lot of I think, 1" to 2.5 " crystal 
> retroprisms.   The Arizona station only 
> receives 10s of photons per second on the return.    If you only had a single 
> 2.5" retroprism on the moon, would that severely limit return signal 
> compared to an array 
> of say of 100 prisms?
> 
> Best Regards,  
> Pat Barthelow; Echoes of Apollo
> apolloeme at gmail.com  apolloeme at live.com
>                           
> 
> ------------------------------
> 
> Message: 2
> Date: Thu, 19 May 2011 19:49:26 -0400
> From: Tom Becker <GTBecker at RighTime.com>
> Subject: Re: [Laser] Lunar  Laser  EDM
> To: Free Space LASER Communications <laser at mailman.qth.net>
> Message-ID: <4DD5AC86.3030600 at RighTime.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> "Earth's atmosphere distorts the beam so that it is expanded out to 
> 1.25 
> miles in diameter when it hits the Moon. Only one in 30 million of the 
> original photons in the beam actually will hit the retroreflector. By 
> the time the light makes it back to Earth, the beam will have expanded 
> to 9.3 miles in diameter. Of the returning photons, only one in 30 
> million will hit the telescope on Earth."
> 
> http://www.spacetoday.org/SolSys/Moons/LunarProspector.html
> 
> 
> Tom
>