The system is based on AIS. Receiving stations cover the entire range of 40 nautical miles (about 75 km) and periodically receive information from some more remote vessels. Read more about AIS on Wikipedia.
It is difficult to determine the maximum coverage area, but it can be imagined in this example: a vessel with an AIS transmitter connected to an external antenna located at a height of 15 meters above sea level will be able to exchange a signal with the coastal AIS station at a distance of 15-20 nautical miles (approximately 28-38 km). The higher the AIS stations are, both shipboard and shore, the further the radio signal will spread.
Want to know more? See our helpful Guide to learn how AIS maps work.