[Laser] halos, pillars, and lasercomm?

[laser@mailman.qth.net]

I confess that I do not understand troposcater, less for light than uhf, but 
I have had some thoughts on light communication using clouds.  Below the idea 
is to use high thin ice crystal clouds, which are very common, for over the 
horizon communications.  For more information I suggest checking the atmospheric 
optics link on the web page for spaceweather, especially halos and pillars.  
(If I included the link, I don't think that my message would get past the spam 
filter.)

High thin clouds are often made of ice crystals in the form of hexagonal 
rods, some long and some short.  

The short ones seem to fall flat side down and are associated with weather 
phenomina called pillars and sun dogs.  A laser beam sent into such a cloud 
would be reflected with its energy distributed over an area that depends on the 
amount of wobble or tilt the crystal have.  The more wobble, the bigger the area 
of dispersion.  To use this for lasercomm, the geometry would be similar to a 
large horizontal mirror that reflects (less than 100%) and expands the beam 
(more wobble, more beam expansion).  Knowing the angle of the transmitted beam 
and the distance to cloud would allow calculation of the cloud height and the 
"footprint" of the reflected beam and approximate signal strength.  If the 
receive station is within the footprint, communication is possible.  The return 
path should be open, but I think that the signal strength will be better when 
the distance from the transmitter to the cloud is longer than the distance from 
the cloud to the receiver. 

The long hexagonal rods might also be used for lasercomm.  If the axis of the 
rod is perpendicular to the line of sight from the transmitter, the light 
striking one of the three faces toward the transmitter (ignore the other cases 
which are rare) will be refracted by an angle that is minumum 21.7 degrees and 
maximum 33.8 degrees (see the simulation on the formation of 22 degree ice 
halos).  Combine this with the skewing caused by crystals whose axies are not 
quite perpendicular to the line of sight from the transmitter and the result is 
another footprint with signal strength distribution.  There is a greater 
concentration of the light near the minimum refraction angle.  The 22 degree halos 
around the sun, (full or partial) are caused by this.  

Calculation of the geometry for the communication path using long hexagonal 
rods should be similar to the short rod example, but I expect three distinct 
differences.  

1.  The signal strength for the long rod best refraction directions should be 
less the short rod reflection best direction.  

2. The short rod has a single aim point above the line between the 
transmitter and the receiver, whereas the long rod geometry can use any aim point along 
an arc whose center is the transmitter to receiver line.  (The angle from 
transmitter to cloud to receiver will be close to 180 degrees minus 22 degrees, or 
158 degrees. )  

3.  The footprint of the short rod geometry is better for stable atmosphere, 
but should not affect the long rod case.  For short rods, the more wobble, the 
longer any given crystal will be away from the desired cone of best signal, 
but "wobble" for the other system will cause as many rods to go into the 
desired cone as out of it, because they were randomly arranged.


I imagine the equipment to use these clouds of crystals would be a 
transmitter that first serves as a LIDAR to get the range to the cloud crystals.  The 
azimuth and elevation angle of the beam combined with the range to the clouds 
and the location of the transmitter, would allow the computation of the 
footprint and signal strength estimates.  Slewing the direction of the transmitted 
beam would allow sending the best signal to the receiver as the conditions 
change.  If there is a similar system on the other end, and a liason communications 
link is available, it can be directed to the most probable direction of 
signal.  Once communication is established, if either system detects changing 
condition, it can send the information to the other side before it adjusts the 
transmit direction, reducing the need for separate liason.  Also, the receiver may 
need to be pointed independently.  The system could also be used to map the 
"real" signal strength compared to the predicted value.  In turn that can be 
used to improve the prediction.

Lots of things to work out for such a scheme -- field of view for the 
transmitter and receiver, can the LIDAR receiver be the same as the communication 
receiver, ........


I feel that the crystal would provide a stronger signal than troposcatter, 
but could be used less often.  The same equipment could be used for both, unless 
there are color advantages that I have not heard about.  It also seems that 
this system would provide interesting information on the ice crystals in the 
clouds at the same time it allows communication.

James
N5GUI


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