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 TM 11-5895-1847-12&P
OPERATOR AND UNIT MAINTENANCE
LCU 2000 GLOBAL MARITIME DISTRESS AND SAFETY SYSTEM
GMDSS CHARACTERISTICS, CAPABILITIES AND FEATURES
HISTORY OF THE GLOBAL MARITIME DISTRESS AND SAFETY SYSTEM (GMDSS)
In 1979, the International Maritime Organization (IMO) recognized the need for an updated maritime
communications system and helped create the International Maritime Satellite (INMARSAT) system employing
geostationary satellites positioned above the Atlantic, Indian and Pacific oceans. Shortly thereafter, a polar orbiting
satellite system was established to locate Emergency Position Indicating Radio Beacons (EPIRB). The IMO also
decided to commence a general upgrade of the distress and safety system to be known as GMDSS. This system would
provide rapid and automated distress reporting and improved telecommunications for the maritime community.
In 1988, the IMO amended its Safety Of Life At Sea (SOLAS) convention to complete this upgrade of the maritime
safety communications procedures and equipment for GMDSS. GMDSS applies system automation techniques to the
traditional maritime Medium Frequency (MF), High Frequency (HF) and Very High Frequency (VHF) bands, which
previously required a continuous listening watch. GMDSS incorporates the INMARSAT and the EPIRB satellite
systems to improve the reliability and effectiveness of the distress and safety system on a global basis. GMDSS also
provides for the timely dissemination of maritime safety information, including navigational and meteorological
warnings and weather forecasts.
On 1 February 1999, the voice watchkeeping requirement on 2182 kHz for GMDSS equipped vessels ceased. The
Coast Guard shore network now maintains a voice guard on channel 16 VHF and 2182 kHz MF. These networks are
being upgraded to include the GMDSS Digital Selective Calling (DSC) on channel 70 VHF and 2187.5 kHz MF.
While the Coast Guard plans to maintain the shore watch on channel VHF 16 for a number of years, there is no
assurance that the 2182 kHz MF and HF voice watches will be continued. Existing Coast Guard MF and HF watches
are being augmented with DSC to improve high seas telecommunications services to the maritime public.
OVERVIEW OF GMDSS
Distress Alerting
Distress alerting may be accomplished in three different ways: ship to shore, ship to ship and shore to ship. If
terrestial radio links, rather than satellite, are used, nearby ships will also hear the alert. The initial alert may be sent
in a number of ways. The alert may be sent via INMARSAT-C, VHF/FM DSC radio, MF/HF DSC radio or EPIRB.
All of these methods give the vessel's identity as well as its location. A DSC alert is the only type that can be picked
up by another vessel. It is normally the responsibility of the Rescue Coordination Center (RCC) to respond with an
acknowledgement. Vessels at sea should not normally acknowledge receipt of an initial distress alert.
Distress Relay
Once an RCC has heard and acknowledged a distress, it may wish to alert other vessels in the area by means of a
distress relay. The relay can be addressed to a precise geographic area so that vessels too far away to render help are
not involved. Vessels can be alerted using INMARSAT-C, VHF/FM DSC radio, MF/HF DSC radio or Navigational
TELEX (NAVTEX). Any vessel receiving a distress alert directly, or a distress relay, must contact the RCC to offer
assistance. Vessels at sea should not normally send a distress relay themselves.
Search and Rescue
When the Search and Rescue (SAR) phase is entered, all communication is two-way to coordinate the activities of
ships and aircraft using terrestial and satellite communication links available. Specific frequencies are allocated for
this purpose. Under all circumstances, a shore based RCC takes charge of the operation. The RCC may be located as
much as a hemisphere away from the actual casualty. Vessels and aircraft close to the casualty will communicate
between themselves using short range terrestial communications (VHF or MF). Specially designated Search and
Rescue (SAR) radio channels will be used. Precise location of the casualty will be aided by the use of a Search and
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