Maritime Communication between ships or with the shore was carried with the help of on board systems through shore stations and even satellites. While ship-to-ship communication was brought about by VHF radio, Digital Selective Calling (DSC) came up with digitally remote control commands to transmit or receive distress alert, urgent or safety calls, or routine priority messages. DSC controllers can now be integrated with the VHF radio as per SOLAS (Safety Of Life at Sea) convention.
Satellite services, as opposed to terrestrial communication systems, need the help of geo-stationary satellites for transmitting and receiving signals, where the range of shore stations cannot reach. These marine communication services are provided by INMARSAT (a commercial company) and COSPAS – SARSAT (a multi-national government funded agency).
While INMARSAT gives the scope of two way communications, the Corpas Sarsat has a system that is limited to reception of signals from emergency position and places with no facilities of two way marine communications, indicating radio beacons (EPIRB).
For international operational requirements, the Global Maritime Distress Safety System (GMDSS) has divided the world in four sub areas. These are four geographical divisions named as A1, A2, A3 and A4.
Different radio communication systems are required by the vessel to be carried on board ships, depending on the area of operation of that particular vessel.