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Automatic Identification System (AIS)
Automatic Identification System is a maritime vessel reporting systems which lets other vessels see where others are.
It was generally introduced as a backup or supplementary system for radar, which does not work well where there are
The AIS-system is also used by authorities and military as a way of surveillance.
AIS operates by sending out a small message on VHF (161.975MHz or 162.075MHz). This makes it possible for hobbyists to
receive these with a VHF-receiver (either a maritime VHF or a radio scanner if it's legal in your country). There are different software out there which makes it possible to decode the transmission on your personal computer.
You can see live AIS-data from the Trondheim-area here.