[Laser] reply to Tom regarding low paths over the sea

[I]

Tom,


You wrote this a few days ago and I have just caught up with it.........

"I had a conversation with a laser buddy about atmospheric absorption 
some time ago, specifically about what to expect in optical ducting over 
water, like ghost-shipping mirages.  When this occurs, I understand, 
reflections under a thermocline extend the apparent optical horizon to, 
sometimes, great distances.

We questioned what absorption to expect when the path is very close to 
the water surface, what is the nature of the thermocline, cooler-above 
or cooler-below, and how that plays with relative humidity and, thus, 
absorption close to the surface.

Does anyone have experience with low over-water paths?


Tom
Cape Coral"



In reply, I must point out that our record UK distance contact over 117.6km
(modest by world standards) is interesting because it fits your request. When
analysed using profiling software, it is NLOS due to the low altitude of the 
ends of the path. Geometrically, the central 40km of the path is under the sea.
Of course we were aided by atmospheric refraction, but nevertheless, this
means that the central third of the path must have been skimming the waves.
The red patch seemed to be about 0.3 degrees above the horizon, which
ties in with the theory.
We had completed a 90km contact over land just the week before, with a
very strong signal that was very bright to observe with the naked eye. The 
over-sea path was a completely different story, being much, much weaker
in fact at the limit of naked eye visibility. Through the binoculars, the red dot
was smeared out into a fuzzy patch. I think the layer of saturated air just
above the water attenuates the signal as it bends it around the curvature of
the earth. I have noticed an effect when viewing distant islands over the sea 
(usually from a cruise liner!), the higher parts of the islands seem much 
clearer than the waterline, so much so that the island seems to "float" above
the horizon. I conclude that this low layer of air, although helping our contact
in bending the light, hindered it by attenuating the signal so much.


Stuart, G8CYW