What causes the SART to begin a transmission?

The SART, or Search and Rescue Transponder, is designed to assist in search and rescue operations by responding to RADAR signals. When it receives a RADAR interrogation signal from a vessel or aircraft searching for a distressed vessel, the SART activates and begins transmitting a series of radar reply signals that help rescuers locate the distressed vessel on their RADAR screens. This characteristic is essential, as it ensures that the SART is only activated when it is needed for search and rescue purposes, making it efficient and effective in emergency situations.

While manual activation by the operator is a valid way to start a transmission, the key function of the SART is to respond to RADAR interrogations automatically, which is what sets it apart as an emergency signaling device. The other options, such as detecting significant vessel movement or automatic detection of emergency signals, do not accurately describe the primary trigger for a SART transmission. The core functionality relies on interaction with RADAR, which drives the SART to broadcast its location to assist rescuers effectively.