Vessels send AIS messages by radio. We have a position for a vessel only while a receiver hears her. This map shows where vessels were heard in the last 24 hours so you can judge the data for the waters you care about before you sign up.
API positions are available only in certain areas. A free account shows which.
Receivers go quiet and vessels sail out of range. The map is the last 24 hours and is worked out again about every 15 minutes.
Most of what we hear comes from receivers ashore so the open ocean and some coasts are thin. We show them that way.
Watch alerts say when a vessel was not heard and each report gives how well its call or period was heard.
The share of the last 7 days on which our sources heard vessels at each port, graded the same way as our port reports.
Near a coast, a river or a port? A small AIS receiver at your window or on your roof adds coverage where it is thin. Hosts get a free plan while their station keeps sending and we send free receiver kits to a few places we choose.
