You may have some altitude restrictions for airplanes, but you also have to consider helicopters, especially the police and EMS ones. These can be flying at any altitude, and usually very close o the ground. But, in general, I'd say 500ft is a good target for your calculations.
I think airplanes are restricted to 1000ft AGL when over a city, and 2000ft over mountains (if memory serves me well). But, again, there are other considerations. If the airplanes are approaching an airport, they can, and will, get lower as you get closer to the airport (they have to land or take off, right?).
If you're around 10NM from the airport, and depending on the airport, airplanes should be around 1500-2500ft AGL, at least if they're on the instrument approach :roll:
What airport are you close to?, if you don't mind my asking, that is
All in all, if you shoot for a 500ft target, you *may* have no problems with the FAA. MAY being the operative word. However, that target may induce a high divergence. Also, you state blue is a better choice for atmospheric scattering, but I think green would be a better choice, seeing as we're a lot more sensitive to 532nm than 573nm. You may get away with a lot less power from green than using blue. Blue is nicer, though
Any way, just my 2 cents
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