Only ships operating in areas A3 and A4 are obliged to carry satellite communications meaning radios (operating on VHF, HF and MF) are still considered the primary means of communication in emergency situations. In addition, search and rescue transponders (SARTs) and NAVTEX (Navigational Telex) are also required for GMDSS compliance.
SARTs are devices which are used to locate survival craft or distressed vessels by creating a series of dots on a rescuing ship’s X-band radar display. The detection range between these devices and ships, dependent upon the height of the ship’s radar mast and the height of the SART, is normally less than about ten miles. Initially only radar SARTS were allowed but since the advent of AIS, a hybrid AIS-SART has been permitted as an alternative. Most SARTs are mostly cylindrical and in safety orange colour.
NAVTEX is an international automated MF direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships. It was developed to provide a low-cost, simple, and automated means of receiving information aboard ships at sea within approximately 200 nautical miles off shore. A NAVTEX is usually a bracket mounted cabinet with a small LCD screen displaying broadcast messages with an optional printout. Inmarsat’s SafetyNET service is an alternative to NAVTEX for ships that are equipped with satellite GMDSS equipment and it provides similar information.