The GMDSS is designed to perform 9 functions:

1. Transmission of ship to shore distress alerts by two separate and independent means.
   The details for this function are contained on the individual ship’s radio safety certificate. For example, a ship in area A1 would use VHF DVS equipment as the primary means and the EPIRB as the secondary. Ships in are A4 would use HF DSC equipment as the primary means and a 406 MHz EPIRB as a secondary means.
2. Reception of shore to ship distress alerts.
   If a ship sends a distress signal via an EPIRB or INMARSAT C satellite terminal, any ship in the vicinity will not be aware of the distress until the shore authorities relay the distress details by sending a DSC call and/or a satellite call to all ships within a defined area.
3. Transmission and reception of ship to ship distress alerts.
   A ship in distress can alert other ships in the vicinity by sending a DSC distress alert on VHF and MF and follow it up with a distress voice message on channel 16 or 2182 MHz. HF DSC is reserved for long range work and is intended for alerting shore-based authorities.
4. Transmission and reception of SAR coordinating communications.
   This is to enable ships to perform SAR coordination functions described in the International Aeronautical and Maritime Search and Rescue (IAMSAR) manual. It includes the use of radio telex (called Barrow Band Direct Printing or NBDP) between ships involved in the SAR.
5. Transmission and reception of on-scene communications.
   It involves the use of short to medium range communications during the course of the operation.
   Ships must be able to communicate with aircraft, other ships and shore authorities using dedicated GMDSS frequencies for voice and NBDP communications,
   Frequencies for RT (radio telephony) use are:
   • VHF Channel 16 and channel 6 (inter ship and ship-aircraft communications).
   • VHF 121.5 MHz and 123.1 MHz (ship-aircraft communications) It is compulsory for passenger vessels.
   • MF 2182 KHz (distress and safety voice communications).
   • HF 3023 KHz (ship-aircraft); 4125 KHz (ship-shore; ship-ship); 5680 KHz (ship-aircraft).
6. Transmission and reception of locating signals.
   Locating and homing signals are provided for in GMDSS by EPIRBs and SARTs.
   SARTs are intended for use in survival craft to provide a homing signal for ships and aircraft engaged in SAR operations.
   SARTs operate in the navigation radar frequency band (X band).
7. Transmission and reception of maritime safety information (MSI).
   GMDSS provides two independent systems for broadcasting MSI namely, NAVTEX and SAFETY NET.
   Navigation and meteorology warnings, meteorology forecasts and other urgent safety related messages for a given area (NAVAREA) are broadcast over NAVTEX and SAFETY NET.
8. Transmission and reception of general communications to and from shore-based radio systems.
   GMDSS provides facilities for all types of commercial and personal communications over commercial telecommunications networks.
9. Transmission of bridge to bridge communications.
   SOLAS (Safety Of Life At Sea) requires that access to VHF communication equipment must be available at the position the ship is normally navigated and controlled from. This includes the operation of channel 13 which is reserved for inter ship communications relating to the safety of navigation.