Optical cloudbounce trans-Bass Strait record

[G4TUT]

Optical cloudbounce trans-Bass Strait record

A new NLOS (non line-of-sight) optical communications distance record of 288km has been set with a one-way transmission between VK7 and VK3.

The night-time contact on 27 October saw Joe Gelston VK7JG assisted by Paul Godden VK7KPG fire up a narrow beamwidth 60 Luxeon LED transmitter to scatter red light off the clouds.

The possibility of bridging Bass Strait by optical communications using weak signal technology now been proven with a distance of 288km.

More information on this story can be found in the news section of the Amateur Radio Victoria website www.amateurradio.com.au (http://www.amateurradio.com.au/)

[W8MLD]

I am curious, what kind of data/transmission did they send using this method?

[KE4IKY]

Friendly questions.

When you say "narrow beamwidth" how narrow are you talking about. Narrow for optical systems or narrow for ham radio antennas?

Also, was pulse modulation used?

How powerful was the light?

Luxeon is a maker of led's and not a power level.

Thanks

Joel

[N5SO]

havent there been laser eme contacts? if so, then is this an optical qrp story?

[KD6HDX]

Would a green laser be better for that?

[K3KO]

Yes, astronomers have been using lasers to mearsure the distance to the moon since 1985 with an accuracy of 1.5 ". These are one way QSO's too.

http://www.lpi.usra.edu/lunar/missions/apollo/apollo_11/experiments/lrr/

73 de Brian/K3KO

[KE4IKY]

I think the point was that it was a trans-straight record, not an ultimate record.

Joel

[AB1GA]

The interesting part of this test was that it was a non-line-of-sight contact, which in some ways is more challenging than laser EME.

Laser EME signals travel further, but they get some help. First, the power density of the laser is probably orders of magnitude higher than the 60-LED array used by the Australians. Second, the last time I checked the laser distance measurement used a retroreflector array placed on the moon by one of the Apollo missions, but this may have changed since I last read about it. Using mirrors to direct the beam back the way it came, with roughly the same divergence as it had coming in, is much easier than relying on diffuse scatter from the moon's surface. The last "benefit" is that much of the path traveled by the laser beam is through free space, which does not absorb light. True, the light has to pass through the atmosphere twice on its round trip, but the density of the atmosphere declines roughly exponentially with altitude; the 288km low-altitude path across the Bass Strait may actually absorb a larger fraction of the beam than the EME "vertical" path, but I'm not sure of this.

But as I said above, this is a NLOS pass, i.e. this optical signal is propagating through forward scattering and refraction, like we see with HF signals. The challenge is that the scattering particles are also strong absorbers, so the weather conditions have to be just right to allow for enough bending of the light beam while letting enough get through to detect on the other side. I believe there was a German op who liked to do 10GHZ in the rain, using raindrops to scatter is signal beyond line of sight, and this is the analogous process in the optical regime.

A nice experiment, I'm impressed.

:)