The automatic identification system (AIS) is an automatic tracking system that uses transponders on ships and is used by vessel traffic services (VTS). Originally designed as a terrestrial communication system, satellites are now also used to passively capture the signal traffic. With a few exceptions, AIS traffic is in the form of continual status announcements of a sender to all interested parties in its proximity. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport, since AIS relies on the accuracy of information transmitted by other vessels, whereas radar independently detects objects. Although technically and operationally distinct, AIS is analogous to the ADS-B system which performs a similar function for aircraft.