AIS is one of the best safety tools modern sailors have, and also one of the most misunderstood. Most boaters assume that the AIS targets on their chartplotter and the ones on their phone app come from the same place and mean the same thing. They don’t. In this article, I walk through how AIS actually works, how ships end up on the internet, why some targets disappear, and the critical difference between AIS as a safety system and AIS as a tracking system. If you use AIS for navigation, this is one distinction you really need to understand before it matters.