[Laser] Lunar downlink

[Stewart Nelson]

Hi all,

This is not my field, so I may be putting my foot in my mouth here.
However, the experiment as described all makes sense to me:

> I'm not sure that the tale of the observatory location being off by
> 100m is credible. If the beam is already 2 km on the moon and 18 km
> on the return, a translational error of 0.1 km seems insignificant.

The laser pulse lasts only a fraction of a nanosecond.  The object is
to measure the distance to the target it within an inch or two, but it
is already known within, say, ten meters.  To eliminate most background
noise, the receiver is gated on for only about 100 nanoseconds.  If
the terrestrial position is wrong by 100 meters, the return pulses
will be outside the window and will all be lost.

http://physics.ucsd.edu/~tmurphy/apollo/timing.html

> 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.

There is a huge amount of noise, mostly from earthshine.  However,
even if it amounted to a million photons per second, the chance of
a noise pulse landing in the 100 ns window would be only one in ten.
So, if you get one return photon for most pulses, your S/N is
pretty good.

73,

Stewart KK7KA