[Laser] New UK optical communication distance record 117.6km

[I]

New UK optical communication distance record 117.6km


In the early hours of Tuesday 12th April Stuart, G8CYW/P located west of Alnwick, Northumberland in IO95CJ made contact with Rob, M0DTS/P on Danby Beacon, on the North York Moors, west of Whitby in IO94NL. The contact was over a distance of 117.6km on a difficult path involving 70km over the North Sea where the beam was at a low altitude over the waves. Signals were weak at 51/52 but easily readable on SSB.


A word here about the path, the website "Heywhatsthat" was used to check the path and visibility and although there was an indication that the path was possible in one way (the "visibility cloak"), the path profile looked decidedly dodgy since it indicated that no less than 40km of the path was literally under the sea!, from 45km out to 85km from Danby Moor the line plotted was below sea level, and from 85km to 90km there appered to be land obstructions. Some help from refraction in the atmosphere was definitely needed here. Fortunately, the atmospheric pressure was above normal and so it was thought to be worth taking a chance.


Later analysis of the path using more accurate software showed that at k = 1.0, the path was indeed obstructed, but just clear (far station 0.1 degree above central bulge in the path profile), at k = 1.33, the usual value used for radio signals.
The path clearance at this value of k was shown to be just 50m at a distance of 50km.


On arrival at the Alnwick end of the path at 11pm, Stuart and Gordon contacted Rob on 70cm who was still driving to his vantage point. The view from the Alnwick site was indeed good, several lighthouses at intervals down the coast were busy doing their stuff, and distant streetlights could be seen. The path is just to the seaward side of the three North-East conurbations of Tyneside, Wearside and Teeside and the orange glow in the sky was intense. Stuart tried to get a bearing on his compass in the dark with the "aid" of a far too bright LED torch and noted the path was within a degree of a particular lighthouse. The Alnwick location was actually literally on the "wrong" side of the B6341 road to Rothbury, fortunately Gordon only counted six cars that passed late that night necessitating turning the beam off while they passed.


When Rob arrived and set his gear up, he put out a light on the correct bearing, which paradoxically, was spotted immediately, slap bang on the correct heading. Maybe all this recent practice on 3rd April's successful 90km record attempt, and the first G to GM contact on 7th April had finally paid off. Rob's light was seen at an estimated 0.3 degrees above the central bulge in the path profile (estimated using the given field of view of a pair of binoculars). 


Stuart then switched the powerful LED beacon on to its 2.5Hz setting and sure enough, Rob immediately radioed that he could see it at Danby Beacon. The LED beacon was then switched to 20kHZ and Rob tuned this in on 3.602 MHz. Rob was using the optical transceiver and separate RX and TX heads. Stuart continued the use of the LED transceiver that had performed so well over the past few days. Both stations noted how weak the red dots were this time, and spent time lining up, but to no avail. It must have been something to do with the marginal path and the opto signal having to travel just above the waves as the signal was bent by refraction around the curvature of the earth (or should that be sea?) Neither station was particularly high above sea level, Rob just short of 300m asl and Stuart less than 200m asl. The local horizons as calculated from each station just do not quite meet.


Rob's "red dot" as seen by Stuart and Gordon was barely visible to the naked eye, and not really a dot but in the binoculars a blurred out patch of red, probably due to having to travel just over the water in the humid air. It was estimated that the optical signal could not have been anywhere near a tenth of what was observed a few days ago over the 90km path when it was the brightest thing on the horizon. The red dot was even dim compared to streetlights in the Teeside area. This would be a stern test of the gear, it was hard to imagine whether a usable signal could be recovered from such a dim red splodge of light lost in all the amber streetlight glow. It was now past midnight.


Rob went on transmit using LSB and Stuart switched the LED trasceiver to receive, and made small adjustments to the aiming of the 0.3 degree beamwidth optics that were much narrower than the 2 degree LED beacon. Rob's voice then duly appeared out of the noise. All the QRM had raised the noise level in the receiver to about S5 but as Rob spoke, the AGC action of the FT817 was apparent. This amounts to an interesting test of the sub-carrier system in use by all in the North-East group. Much discussion had centered on how the system would perform in weak-signal mode. The answer seems to be that it can return readable signals right at the bottom of the S meter range. As Rob spoke the noise level dropped just as in "normal" rf communications, to gradually return in the longer pauses as the AGC recovered. Rob passed all his details and went to receive. Stuart then replied on light, repeated all Robs details and added his own. Rob then confirmed reception to complete the contact, made at 12.10am on Tue 12th April. After a little more chat, Stuart recorded a sample of Rob's signal as Rob had done earlier on the formal exchange, completing this at nearly 12.30am.


Due to the weak optical signal, FM was not used. This finally shows the benefit of being able to use SSB, the group have never before had to cope with such a weak signal that SSB was actually necessary. 
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