The APOLLO lunar laser ranging experiment uses a 3.5 meter telescope as a laser turret and manages to get about 2,400 photons back from those retroreflectors every half an hour, and it's a challenge just to find the things as the spot's a few km wide by the time it gets to the moon. Good luck.
Yeah, I was just doing some calculations on this. You'd think that with 3.5 meters you could do better than a few kilometers, wouldn't you? Is something wrong with their telescope?
I don't know what wavelength they're using, but at 555nm, 1.22λ/d would be 0.193 microradians, which, unless I'm doing the math wrong, works out to a 74-meter Airy-spot radius at the distance to the moon. At that sort of size, you'd think the majority of the photons in the desired wavelength band would be from their laser rather than stray Earthshine.
I was doing calculations based on λ = 350nm and a 500-mm reflector, and no retroreflector, and getting rather sad estimates of 3 joules of light transmitted per returned photon (per receiver). While that's clearly a feasible commnications system, it's going to be pretty limited in bandwidth. I'm not sure if C-band radio is better?
