This chart is tracking SV TENACIOUS.

Vessels have an AIS transponder on board, which transmits their position, speed and course, among some other information, such as vessel’s name, dimensions and voyage details. Whilst the AIS transmits on a frequent basis (approx every 20 minutes), the position of the ships can only be shown if within range of an AIS receiving station (usually 15-20 nautical miles), though this can be much further depending on the elevation of the receiving base station. In addition, they transmit the position regularly via a satellite system for when out of range of land.

JST

JST