[Laser] Lunar downlink

[Glenn Thomas]

At 07:22 PM 2/21/2006, you wrote:
<snip>
>Glenn Thomas wrote:
>
> > [snip]
> > Likewise a return signal measured in single photons seems
> > questionable when you consider the effect of noise due to thermal
> > radiation from the atmosphere and even the telescope itself.
>
>I think this is not correct. I remember seeing some video of the experiment. I
>think a cooled PMT was used (typical dark count under 1 per second) 
>and the thing
>was range gated. Every time a photon was detected in the right time 
>slot it'd ring
>a bell. The dings were roughly 1 in 3 pulses. The laser was pretty 
>powerful and
>the optics pretty large. There may well have been subsequent experiments, of
>course.
>
> > Perhaps
> > someone with a better grounding in physical optics than I can
> > comment. Not having worked through the math, I might believe 100's or
> > 1000's of photons!
>
>I don't think it was anything like 100 or 1000 photons per 
>received  pulse.-John
>
> > 73 de Glenn wb6w

Not knowing the wavelength used, it's difficult to compute what kind 
of noise level can be expected due to thermal radiation. I'm not 
aware of any steps taken to discriminate between photons due to 
atmospheric thermal radiation and photons due to the lunar return. 
The laser PRF and receiver gating characteristics also figure into 
this calculation of course. Does anyone have access to this 
information? Boltzmann's constant I can look up, these others I can't.

73 de Glenn wb6w


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