Explanation: COSPAS/SARSAT is a satellite-based system specifically designed to detect and locate distress beacons (EPIRBs, ELTs, and PLBs) transmitting on 406 MHz. These satellites receive the distress signal, process it to determine the beacon's position (often aided by embedded GPS data), and relay this critical information to ground stations and Rescue Coordination Centers. This direct, dedicated function of distress alerting is central to the Global Maritime Distress and Safety System (GMDSS) for initiating search and rescue operations. Inmarsat provides satellite communication services for GMDSS, including SafetyNET broadcasts and general distress communication, but it doesn't primarily detect initial beacon signals. GPS is a navigation system providing location data, which distress beacons often transmit, but it is not an alerting system itself. Iridium offers global voice and data communication but is not a dedicated distress beacon detection system under GMDSS.