[Laser] One way 3 mile laser NLOS contact using PSK31

[TWOSIG at aol.com]

Good to hear of your success!
 
 
I had two ideas after reading about your setup.  Don't know if they  are 
worth trying.
 
First, you are using a narrow beam from your beam expander.  After  
establishing a contact and getting an idea of how strong the received signal is  if you 
slew the receiver along the narrow beam, what would the effect be of  letting 
the beam spread out?  The beam intensity would drop, obviously, but  the area 
of the beam, as seen from the receiver, would be greater.  Perhaps  you could 
get a stronger received signal.  If you have excess signal for  the 
communications path, then perhaps, it would be worth doing to make aiming  less 
critical.
 
 
The second idea, is intended more for a protocol for establishing a  contact. 
 You had a pre-arranged contact setup.   You sent a 260  Hz signal out.  If 
you assumed that there was another station also  transmitting on 260 Hz, and 
you received such a signal, even if you could not  yet decode it, then you shift 
to another frequency, say 310 Hz to indicate that  you had received a 
possible contact.  If the return shifts frequency, it is  an echo.  If that happens, 
or the return stops, you return to 260 Hz and  continue scanning.  If you 
receive a 310 Hz return, then you know the other  station has detected you, so you 
should respond at another frequency, say 370  Hz.  This protocol does not 
depend on who is east or west and runs full  duplex.  It also does not depend on 
being able to decode the message that  is sent, only the carrier frequency.
 
The protocol idea probably has lots of holes, but maybe it will spark a  
better idea.
 
Good luck on the continuing experiments.
 
 
James
N5GUI
 
 
 
 
 
In a message dated 12/13/2004 9:45:20 A.M. Central Standard Time,  
kbanke at qualcomm.com writes:
Last evening Chuck (WB6IGP) & I made our  first one way NLOS laser contact 
using PSK31 between our homes which are  about 3 miles apart. The 1W 910 nm 
laser with 4" beam expander was located  at my home and precisely pointed in 
the azimuth  direction of Chucks  home and a couple of degrees above the 
horizon.  There was slight haze  which I have found to be great for 
producing scatter signals.  At  Chuck's home the 260 Hz carrier was readily 
audible using the 4" lens ahead  of the K3PGP pin detector/preamp 
configuration.  Looking at the signal  with SpectrumLab showed the carrier 
to be steady at about 35 dB above the  noise with a 1 Hz BW. A test message 
was sent with perfect copy. The  narrow  laser beam could be followed with 
the detector over maybe 30  degrees elevation  with the signal slowly 
increasing towards the  horizon. In this case we were limited near the 
horizon by a Sodium vapor  street lamp so could not view the bottom maybe 5 
degrees.  My recent  propagation experiments have shown the following with 
this  configuration:

On totally clear, cold nights following a rain, there is  no useable scatter 
at 3 miles out (at least for levels required for PSK31) .  Most 
evenings  there seems to be a cone of useable signal  for  both back scatter 
and forward scatter with the signal dropping off as the  receiver approaches 
90 degrees to the transmit beam. Of course clouds  produce an amazingly 
strong spot or line. Light rain  also produces  great scatter.

The next step is to finish the equipment to do a two way  contact. Chuck has 
assembled a second  working receiver & I'm in  the process of building a 
second  laser transmitter. We'll then be  ready to see what kind of two way 
distances are possible and will also try  the LaserScatter software as the 
hardware has been built with this in  mind.
- Kerry N6IZW -