What GMDSS Is Doing for You
GMDSS is not simply a radio system — it is an integrated distress and safety communications architecture. Every action you take within it is designed to alert, locate, and co-ordinate rescue as quickly as possible. Understanding why each step exists helps you make correct decisions under pressure.
The Sequence of Distress Communications
1. Initiate the DSC distress alert first. DSC (Digital Selective Calling) sends a digitally encoded burst on VHF Ch 70 (and/or MF 2187.5 kHz, HF DSC frequencies as appropriate for sea area). It transmits your MMSI, position, time, and nature of distress automatically. You activate it on the dedicated DSC controller — usually a guarded red button held for three to five seconds. The reason this comes first: it reaches all DSC-equipped vessels and RCCs simultaneously and without voice error.
2. Follow immediately with a Mayday voice call. On VHF Ch 16 (and/or 2182 kHz on MF), transmit the structured Mayday: MAYDAY spoken three times, vessel name three times, MMSI or call sign, position, nature of distress, number of persons on board, any other relevant information, then OVER. Voice confirms and expands on the DSC alert, and catches any vessel listening on Ch 16 without DSC capability.
3. Activate EPIRB if abandoning or if no response. The EPIRB transmits on 406 MHz to the COSPAS-SARSAT satellite network, providing MMSI-linked identity and a derived position to the RCC. Hydrostatic release will float it free automatically, but manual activation as early as appropriate accelerates rescue. Modern EPIRBs with integral GPS acquire position within minutes and transmit a homing signal on 121.5 MHz for the final search phase.
4. Deploy SART or AIS-SART when rescue assets are close. The radar SART responds to X-band radar transmissions, painting a line of dots on the rescuer's display. The AIS-SART transmits a synthetic AIS target. Both are survival craft homing devices — they are not primary alerting tools and are only effective once search assets are in range.
5. Maintain a listening watch and acknowledge. Once your alert is out, keep Ch 16 and DSC watch. If you receive a DSC distress relay from another vessel, relay it to the RCC if they are unlikely to have received it directly, then maintain radio silence unless you are the nearest vessel able to render assistance.
Sea Area and Equipment
Your vessel's GMDSS equipment fit is determined by its certificated sea area (A1 through A4). You must know your sea area, because it defines which equipment you are required to carry and therefore which communications paths are available to you in an emergency.