[Laser] NLOS trial

[I]

Reading Clint's recent report of his short, but non-line-of-sight (NLOS) contact reminded me to report a similar recent event using a different propagation mechanism here in the north-east of England.


I have noticed that when operating my baseband optical receiver from home here on a hill that overlooks miles of the locality, that all sorts of QRM becomes audible (one of the main reasons for going to subcarrier FM as I have previously posted). In particular, strobes of varying kinds result in loud clicks coming out of the receiver loudspeaker. When operating this recently, I noted that aircraft strobes could be followed as far as visibility allowed. Upon tracking an aircraft coming in to land at the local airport 10 miles away, I noticed that I could still hear the clicks even when the aircraft went out of my direct view when stopping at the terminal. This signal was then joined by various other clicks which I presume were the various ground handling vehicles with their rotating beacons which give a softer sound than the Xenon tubes on aircraft.


Since then I have noted other instances of clicks from beacons or strobes still being audible when not being directly visible. I have likened this to the occasion when you can see headlights from an oncoming vehicle the other side of a rise in the ground. You see the beam in the air by forward scatter which is most obvious when there is a little mist present on a cool evening.


One of our group some while back used a strobe to identify their position for a 65km optical contact on FM and SSB (very successful). This prompted Keith, G4MSF to construct a focussed Xenon beacon for future location identification. This beacon uses a 45Js tube right up to its power limit.  Assuming the flash takes 1 millisecond, this is a peak power input of 45kW at the most optimistic, and the light output is then focussed by a lens into a 5 degree beam maybe adding another factor of as much as 30dB when calculating the effective radiated power. The startling result is that this device can project the image of the tube on to the lower side of any clouds that may be overhead, resulting in some amusing comments about this. In fine rain or drizzle (common in the north east of England, we have as many words for rain as Eskimos have for snow) you get a nice sparkling light show as well.


I began to wonder if we could use this behemoth to make a NLOS contact over the hill between my QTH and that of Keith, some 10 miles away.


Last month, Keith attempted to send an opto signal from his QTH to mine, given that the hill is between us. We both aimed at the point above the hill on a straight line between us, and Keith used GB3NT, the local 70cm repeater to relay to me the sound of his inverter charging up the Xenon tube capacitor after the previous flash. On setting up my receiver and aiming at the correct point, amongst other QRM I could hear a click, immediately followed by the sound of the inverter, repeating at about 3 sec intervals. I am 90% sure the clicks were from the flash as I copied a click followed immediately after by the inverter whine for six consecutive cycles in one trial, and five times in another. I replied on NT reporting the sound and timing I could hear and Keith said I had got the sequence and timing exactly (we had not previously shared this info). Obviously I cannot prove it but I am pretty sure hence the 90% figure. This is further borne out by the nature of the clicks I was hearing, from aircraft the sound changes when the path is obstructed and it was the obstructed type of click I was getting. To go further would require a computer recording and some sort of display to visualise this, we might do this later.


We are next going to try using some cw from my 200mW laser which is modulated at 19kHz and can be keyed, aimed VERY carefully at the same spot.To receive this, Keith will use his very sensitive LED transceiver and transverter system connected to his FT817 reported earlier on these pages. 


Stuart G8CYW