Explanation: An EPIRB (Emergency Position-Indicating Radio Beacon) is a crucial piece of GMDSS equipment designed to automatically or manually transmit distress alerts. When activated, it sends a signal on 406 MHz to the COSPAS-SARSAT satellite system, relaying vital information, including the vessel's identity and position, to rescue coordination centers worldwide. This makes it the primary means for a vessel to transmit its locating signals globally in an emergency. A Radio Direction Finder (RDF) is a receiver used to *locate* other transmitting signals, not to transmit one's own position. A SART (Search and Rescue Transponder) operates on 9 GHz (X-band radar) and helps rescuers pinpoint a distress scene via radar, but it's not the primary global alert system and does not transmit on 406 MHz. A Survival Craft Transceiver is primarily for voice communication and DSC alerting, not for automatic, satellite-based locating.