Satellite systems operated by the Inmarsat, under contract to the International Maritime Satellite Organization (IMSO), are also important elements of the GMDSS.  Three types of Inmarsat ship earth station terminals are recognized by the GMDSS: the Inmarsat B, C and F77. The Inmarsat B and F77, an updated version of the A, provide ship/shore, ship/ship and shore/ship telephone, telex and high-speed data services, including a distress priority telephone and telex service to and from rescue coordination centers. The F77 is meant to be be used with the Inmarsat C, since it's data capability does not meet GMDSS requirements.  The Inmarsat C provides ship/shore, shore/ship and ship/ship store-and-forward data and email messaging, the capability for sending preformatted distress messages to a rescue coordination center, and the Inmarsat C SafetyNET service. The Inmarsat C SafetyNET service is a satellite-based worldwide maritime safety information broadcast service of high seas weather warnings, NAVAREA navigational warnings, radionavigation warnings, ice reports and warnings generated by the USCG-conducted International Ice Patrol, and other similar information not provided by NAVTEX. SafetyNET works similarly to NAVTEX in areas outside NAVTEX coverage.

Inmarsat C equipment is relatively small and lightweight, and costs much less than an Inmarsat B or F77. Inmarsat B and F77 ship earth stations require relatively large gyro-stabilized antennas; the antenna size of the Inmarsat C is much smaller. .

In July 2002 IMSO notified IMO of the decision by Inmarsat to withdraw provision of Inmarsat A services as from 31 December 2007.  On that date, Inmarsat A can no longer be used for any purpose.  The last type approval by Inmarsat for a new model of maritime Inmarsat A mobile earth station was granted in 1991, since when no new Inmarsat A models have been approved. 

Under a cooperative agreement with the National Oceanic and Atmospheric Administration (NOAA), combined meteorological observations and AMVER reports can now be sent to both the USCG AMVER Center, and NOAA, using an Inmarsat C ship earth station, at no charge. There is also no charge to register for this service and to receive the necessary Inmarsat C software. For more information, see the NOAA Shipboard Environmental (data) Acquisition System, or SEAS.

SOLAS now requires that Inmarsat C equipment have an integral satellite navigation receiver, or be externally connected to a satellite navigation receiver. That connection will ensure accurate location information to be sent to a rescue coordination center if a distress alert is ever transmitted.

High Frequency

The GMDSS includes HF radiotelephone and radiotelex (narrow-band direct printing) equipment, with calls initiated by digital selective calling. Worldwide broadcasts of maritime safety information are also made on HF narrow-band direct printing channels.

To meet these GMDSS requirements, the Coast Guard has improved high frequency (HF) ship-shore radio safety services from our Communication Stations to the maritime community, as well as narrow-band direct printing broadcasts.

Search and Rescue Radar Transponders (SARTs).

The GMDSS installation on ships include one or more search and rescue radar transponders, devices which are used to locate survival craft or distressed vessels by creating a series of dots on a rescuing ship's 3 cm 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 about eight nautical miles.  Note that a marine radar may not detect a SART even within this distance, if the radar settings are not optimized for SART detection.  For more information, see IMO SN/Circ.197, Operation of Marine Radar for SART Detection. (PDF)

Digital Selective Calling

The IMO also introduced digital selective calling (DSC) on VHF, MF and HF maritime radios as part of the GMDSS system. DSC is primarily intended to initiate ship/ship, ship/shore, and shore/ship radiotelephone and MF/HF radiotelex calls. DSC calls can also be made to individual ships or groups of ships. DSC distress alerts, which consist of a preformatted distress message, are used initiate emergency communications with ships and rescue coordination centers. DSC was intended to eliminate the need for persons on a ship's bridge or on shore to continuously guard radio receivers on voice radio channels, including VHF channel 16 (156.8 MHz) and 2182 kHz now used for distress, safety and calling. A listening watch aboard GMDSS-equipped ships on 2182 kHz ended on 1 February 1999.  In May 2002, IMO decided to postpone cessation of a listening watch on VHF CH.16 aboard ships.  That watchkeeping requirement had been scheduled to end on 1 February 2005.

IMO and ITU both require that the DSC-equipped VHF and MF/HF radios be externally connected to a satellite navigation receiver. That connection will ensure accurate location information is sent to a rescue coordination center if a distress alert is ever transmitted. FCC regulations actually require that ship's position be manually entered into the radio every four hours on ships required to carry GMDSS equipment, while that ship is underway (47 CFR 80.1073).The Coast Guard believes VHF, MF and HF radiotelephone equipment carried on ships should include a DSC capability as a matter of safety. To achieve this, the FCC requires that all new VHF and MF/HF maritime radiotelephones type accepted after June 1999 to have at least a basic DSC capability.

VHF digital selective calling also has other capabilities beyond those required for the GMDSS. The Coast Guard uses this system to track vessels in Prince William Sound, Alaska, Vessel Traffic Service. IMO and the USCG also plan to require ships carry a Universal Shipborne Automatic Identification System, which will be DSC-compatible. Countries having a GMDSS A1 Area should be able to identify and track AIS-equipped vessels in its waters without any additional radio equipment.

A DSC-equipped radio cannot be interrogated and tracked unless that option was included by the manufacturer, and unless the user configures it to allow tracking.

U.S. shore-based radio stations currently exist to support every element of the GMDSS, except for digital selective calling. The United States intends to declare an Sea Area A1 in 2006 and a Sea Area A2 for the contiguous U.S. will be declared once the 2 MHz infrastructure has been upgraded.

GMDSS telecommunications equipment should not be reserved for emergency use only.  The International Maritime Organization in COMSAR Circ.17 encourages mariners to use that equipment for routine as well as safety telecommunications.