[Laser] Cloud Bounce, Comm or Distance

[Dave]

On 2010-09-17 12:00 PM, laser-request at mailman.qth.net wrote:

> Date: Thu, 16 Sep 2010 20:48:36 -0400
> From:<n5gui at cox.net>
> Subject: Re: [Laser] Cloud Bounce, Comm or Distance
> To: mike1 at mgte.com, Free Space LASER Communications
> 	<laser at mailman.qth.net>
> Message-ID:<20100916204836.KLZU0.1855127.imail at eastrmwml31>
> Content-Type: text/plain; charset=utf-8
>
>
> ---- Mike<mikecouture at bellsouth.net>  wrote:
>>
>> Do we have any interest in cloud bounce for either communications or
>> distance measuring using low to medium power lasers or LED's?

There is an instrument designed to measure cloud height.  These date back,
at least, 70 years, if not more, and were mostly used for aeronautical 
purposes.

http://en.wikipedia.org/wiki/Ceilometer

> Your question got me to thinking ( which is usually dangerous ) about a couple of things.

Me, too!  :-)

> The first is perhaps a subset of cloud bounce.  The classic idea of cloud bounce, to me at least, is that you see a cloud in the sky, you point your light beam at it, and then someone sees your "spot" and decides to reply.  I have frequently seen an advertising searchlight hit clouds so this does not take a lot of imagination.

The tricky part is that the cloud of interest must be visible to both 
the transmitter and
receiver.  More specifically, the portion of the cloud having the signal 
bounced off of it
must be visible to both the transmitter and receiver, since it's 
possible for the cloud to
be between the transmitter and receiver, such that an illuminated spot 
on one side isn't
visible to the other side.

> There are atmospheric conditions other than puffy cotton ball clouds that might be used.  One such condition causes rainbows or fogbows.  I am thinking more of thin ice clouds.  Often in winter there are thin ice crystal clouds, usually observed with sun dogs or halos, or their equivalents from bright moonlight.  These ice crystal clouds are hard to see directly.  The ice crystals form different shaped prisms or flat hexagonal plates which reflect and / or refract light at predictable, but narrow angles.  ( That was a hint for the curious to investigate the atmospheric physics involved. )  A beam of light, whether from the sun, moon, or optical communication equipment, comes away in a cone defined by that angle.

There are certain angles at which this happens, and these angles are 
somewhat
colour/frequency specific (which explains rainbows).

> In order to use the reflections/refractions for communications you would need considerably more skill, or perhaps extraordinary luck, than the more typical cloud bounce.  I would not suggest anyone start on a project to use ice crystal reflections instead of, or before the puffy cloud bounce.  But maybe it is the next step ( or the next challenge after ).

It may be possible, although the attenuation would be rather high.

One wonders if other atmospheric objects could be used for reflected light
communications?  We already know that aircraft can sometimes reflect 
UHF/microwave
signals, so they should work for light, too (although, I certainly 
wouldn't recommend
anyone to fire a laser towards an aircraft, lest they get probed by 
Homeland Security in a
most unpleasant way!).

Helium balloons or hot air balloons may be another possible reflective 
target.
What would the possibility be of tethering an Aluminized mylar, Helium 
filled, party
balloon at 1000 feet or so [1], and bouncing an optical signal off of 
it?  I've done a
very little empirical work with this over the past few months.  In 
windless conditions,
it's not too hard to tether a party balloon at an altitude of 100 feet 
or so, and then
reflect a beam off of it.  Higher, of course, would be better.

[1] Make sure it fits into the FAR 101 exempt category, lest the FAA 
give you a similar
most unpleasant probing.

> The second thing was listening practice for cloud bounce, for either the puffy cloud version or the ice crystal reflection/refraction version.  In the densely populated areas, streetlights and other manmade lights pulsing at 60 Hertz and various harmonics, should be detectable on every cloud overhead.  I live in Kansas, so there are places with some pretty dark skys, so I should be able to find places where only limited sites can illuminate the clouds, puffy or icy.  Predicting and then confirming the cloud bounce, seems to be a useful practice before trying to communicate.  I am not currently working on such a project, but I thought I would pass the idea along in case someone else could use it.

I'm located in rural Kentucky, so I have some pretty dark skies, too.

Maybe what is needed is some beacons that could be listened for?  It 
shouldn't be too
hard to program a microcontroller to send an ID string over a LED, and 
have that
periodically send out a burst of light towards the sky.

As for the noise from street lights and such, most of these (well, at 
least in North
America) produce noise at a 120 Hertz frequency (double the power line 
frequency),
although there is a 60 Hz component, too, since they aren't necessarily 
perfectly
symmetrical.  Additionally, they produce a high harmonic content, too, 
since the
current through the Sodium vapour lamps isn't even close to a sine wave.

One interesting idea I had a while back is that one could build a device 
which would
observe the light from a particular phase of the power line (Since the 
power distribution
system is three phase, and since the lights are driven from one of the 
phases, and
since the utility company tries to keep the phases balanced, there 
should be a roughly
equal number of lights on each phase, although they may be 
geographically distributed.).

> James
>   n5gui

I'm currently working on a small prototype LED based system (although 
it's going
VERY slowly).  I just got the lenses in for a single LED system mounted 
in a piece
of PVC pipe.  With no optics, I have well over a quarter of a mile range 
(Yeah, I know;
that's nothing compare to what some of the guys here have done, but you 
have to start
somewhere.).  I'm hoping that the optics will allow me a five mile 
range.  We'll see,
eventually.

Dave
WA4QAL