How can I see AIS targets over the horizon

[Haddock1]

All,
I have an AIS B system on my boat which is a transponder which is linked to my plotter. It's a great system and shows closest point of approach and a vector line. Beats hand held compasses like in the old days. Anyway, when in use I can see vessels that are transmitting AIS data via VHF from my masthead aerial which give a range of circa 20 miles. Easy to understand. What I don't understand is that if I zoom out on the plotter I can also see vessels which are over the horizon/line of sight of the aerial (i.e. sailing from Palma, Mallorca, I can see vessels on the coast of Spain). So the question (drum roll) how is this possible???? Also, when on the internet, I can track friends who are still out in the Med and have the same system as me, through services such as Marine Traffic. Clearly tucked up at home, many miles away in the UK, how am I able to see their boats?
Any insight most welcome.
Confused Haddock.

[StuM]

Answered in the other thread where you asked the same question.
(You may like to read the Forum Rules:
http://www.cruisersforum.com/forums/...ork&page=rules
i.e. "Do not post the same discussion more than once or in more than one forum.")

AIS signals are received by satellites and relayed to services such as MarineTraffic. So you can see world wide AIS traffic over the internet.

As for long range reception by an AIS receiver, this has been discussed at length in another thread over the last few days. Atmospheric conditions can result in "tropospheric ducting"

https://en.wikipedia.org/wiki/Tropospheric_propagation

"Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) away are not uncommon when the refractive index of the atmosphere is fairly high."
...
"Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, Strait of Florida, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1000 to 3,000 miles (1,600 – 4,800 km). A US listening post was built in Ethiopia to exploit a common ducting of signals from southern Russia."

[Haddock1]

Opps, I posted it twice as did not see there as a specific marine electronics forum.
Apologies