Explanation: An EPIRB (Emergency Position-Indicating Radiobeacon) transmitting on 406 MHz is designed to broadcast a distress signal and location information via satellite to the COSPAS-SARSAT system. This system then relays the alert to a Rescue Coordination Center, making the EPIRB the primary means for a vessel in distress to *transmit* its location globally for initial alerting and search efforts. A Radio Direction Finder (RDF) is a *receiver* used by search and rescue to locate a transmitting distress signal, not a primary transmitter of locating signals itself. A Survival Craft Transceiver is for short-range voice communication, not a primary long-range locating signal. A SART (Search and Rescue Transponder) operates on X-band radar frequencies (9 GHz), not 406 MHz, and provides a local homing signal when interrogated by a rescue vessel's radar; it's a secondary, short-range locating device.