Essentially, all vessels over a certain tonnage are required to broadcast identification data (AIS) for safety regulation so other vessels can identify them. Many companies now exist that listen in on those public vessel broadcasts and aggregate the date. Messages are sent by the vessels constantly, and terrestrial antenna are installed by companies, or assisted by volunteer hosts to help collect the messages along coastlines. Satellite companies can aggregate as well, but are usually hindered by how often the pass over a certain area (10 minutes of visibility every 90 minutes, depending on the constellation, and the company).

All in, the larger AIS data aggregators collect about 500 million messages a day from around 150,000 vessels through terrestrial and satellite sources. Access to the full feed, with no delay, is sold for up to $500k/year, though typical deal sizes are around $75k/year. Access to online map-based products usually cost around $10k/year.

<== Used to work for one of the large AIS companies