[Laser] Tx lens diameter.

[J. Forster]

You need a fairly large FOV in the Rx, unless you actively track the spot
position.. Cloud bases are not fixed in altitude, and as they move up and down,
the spot moves.

You can build a tracker using a quadrant photocell (like a monopulse tracker in
radar) and two axis actuator, but it's non-trivial.

-John

> Yes. I agree.  The idea I was getting at is that, for a moderate distance,
> with the cloud base about in the middle of the path (neither station in a
> "beacon" mode pointing straight up, etc.), the spot on the cloud base from
> the Tx can be set to about the size of the fov of the Rx.  All the Tx light
> is then available to the Rx.  Flutter fading might be less and alignment
> would be easier ... except a larger fov at the Rx might not be desirable due
> to stray light, etc.