Explanation: 406 MHz EPIRBs are the international standard for transmitting distress alert signals via the Cospas-Sarsat satellite system. They transmit a digital message containing unique vessel identification and position data, directly alerting rescue coordination centers worldwide. This makes them the primary and most reliable method for initiating a satellite-based distress alert under the Global Maritime Distress and Safety System (GMDSS) regulations. In contrast, while 121.5/243 MHz EPIRBs were historically used, satellites no longer monitor these frequencies for initial distress alerts. They now primarily serve as homing beacons to guide rescue aircraft to the distress location once an alert has been received by other means. S-Band and X-Band are typically associated with radar frequencies (e.g., Search and Rescue Transponders or SARTs operate in X-band) and are not used by EPIRBs for transmitting primary satellite distress alerts.