[eli5] How vessel tracking work ? Who provide the data to them ?
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If you are talking about ship tracking there is a system called Automatic Identification System which is required to be mounted to all larger ships. This system broadcasts information about the vessel such as its name, registration number, status, destination, position and speed over VHF radio. The purpose of this is so that other ships in the area can receive this information and make navigation decisions based on it. You get a lot more information then with a radar and it is much cheaper and reliable.
But since this information is broadcast in clear anyone within visual range of the ship can pick up these broadcasts. It can then be logged and shared with others. The receivers are very cheap so you can have a lot of them in coastal areas. And for years now a lot of satellites have been fitted with such receivers as well which extends coverage to the entire globe.
Aircraft have a very similar system called ADS-B. It works on the same principles but use different frequencies and different type of data.
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