Marine AIS is the Automatic Identification System used by commercial ships to broadcast their position and other data and receive that information from other ships in order to avoid collisions. True AIS uses a special VHF frequency and transceiver. Sailors and other boaters on private vessels can also equip their boats to gain from the security of AIS data using various kinds of equipment. Also available are at least four different apps for Apple and Android that purportedly show the location of AIS ships on your smartphone or device. But how far can you trust these?
Private online AIS apps work in a different way from government-regulated radio AIS systems. The radio broadcasts are picked up by unofficial shore stations from which the data is moved into an online system that displays ship locations on a map (not a nautical chart). Private vessels using one such system may also send their data to be visualized on that system's app - but not necessarily not in the app from a different system. That's one limitation to online AIS apps. Another issue surfaced in my comparative testing of different apps: one app may show a ship in one place while simultaneously a different app shows the ship somewhere else! Obviously one of them must be wrong, but there's no way to quickly rush around checking boat after boat to see which is where an app says it is or which app may at a certain time be wrong as a result of unknown technical issues. That's pretty scary. Keep a good lookout, use radar and other classic seamanship in fog, and don't risk your boat and your life based on a commercially developed app that might seem to be as reliable as true radio AIS - but isn't.
