[Laser] modulated retroreflectors

[James Whitfield]

Tim

I think this is an interesting idea.

If you want get some idea of the range of such a system, I suggest that you 
consider the following:

Start with the "base" transmitter, which can be idealized as a point source 
that sends a beam out in the desired direction.  For an example, let's 
assume that the beam expands in a cone so that at a distance of 10 miles, 
its "spot" is 100 feet across.  By geometry at a distance of 1 mile, the 
spot is 10 feet.  Likewise at 5 miles 50 feet, 20 miles 200 feet.

Assume all of the energy of the beam is evenly distributed over the spot.

The receiver that you use at the "base" has a 16 square inch aperture and 
collects a reference unit of signal at 10 miles.  By ignoring minor effects 
such as atmospheric absorbtion we get a mathematical relationship that the 
signal strength will be proportional to the square of the distance.  A 20 
miles the signal is 1/4th the standard strength.  At 1 mile 100 times.

Now imagine that the beam is projected on wall at a distance of 5 miles. 
Think of the retro-reflector as a hole in the wall.  If the retro-reflector 
has an area of 20 square inches, then imagine a similarly sized and shaped 
hole in the wall.  This allows the beam to continue out to 10 miles.  It 
will only be 80 square inches, but if the receiver is placed in that beam, 
it will receive the signal at standard level.

The beauty of a retro-reflector system is that it is supposed to center the 
return beam on the source.  Therefore to apply the analogy of the hole in 
the wall, the receiver will need to be within the 80 square inch area around 
the source.

As a practical matter, the retro-reflector cannot maintain the "tightness" 
of the received beam, so it will tend to expand the beam more than it would. 
Think of this as a lens that expands the beam as it passes through the hole 
in the wall.

If you think of the intensity of the beam, or as the reciprocal of the area 
over which the beam has expanded, you can get a rough first order of 
magnitude estimate of the range.  It is a matter of trading the aperture of 
the receiver against the expansion of the beam.

James
 n5gui