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ALRS Volume 5

Admirality List of Radio Signals Volume 5 is a publication published by The United Kingdom Hydrographic Office. It contains a lot of up to date and well structured information about GMDSS:
  • Worldwide communication requirements for distress, search and rescue,
  • The only SOLAS compliant guide for GMDSS published in English,
  • Colour diagrams depicting worldwide operational DSC ranges,
  • Essential information for GMDSS students,
  • Easily updated from weekly Admirality Notices for Mariners,
  • Revised annually.


Coast Guard - A coast guard is a national organization responsible for various services at sea. However the term implies widely different responsibilities in different countries. In most countries is concerned with SAR (for example in the UK).


Space System for the Search of Vessels in Distress


Coast Radio Station - They are a maritime radio stations situated on shore which monitors radio distress frequencies, coordinate the radio traffic and relays ship-to-ship and ship-to-land communications. Some of CRSs are categorised as MRCCs or MRSCs or NAVTEX CRSs.


Digital Selective Calling - Broadcasting and receiving DSC Alerts is one of the major facilities on a VHF radio that is possible by VHF DSC controller. DSC alerts are used to “switch people on” to follow on with voice communication.


Duplex channels are normally only used for communication between vessels and CRS and for port operations and ship movement. With duplex channels our transmission can only be heard by the CRS. However, all ship stations listening to the same channel can hear the CRS transmission.


Emergency Position Indicating Radio Beacon - It is used as a distress alerting system, indicating to SAR authorities both the identity and position of a person or a vessel, which is in grave and imminent danger and requires immediate assistance.


Global Maritime Distress and Safety System - The GMDSS is an important part of the IMO (International Maritime Organisation) SOLAS (Safety Of Life At Sea) convention. It is an internationally agreed-upon set of safety procedures, types of equipment, and communication protocols used to increase safety and make it easier to rescue distressed ships.


Global Positioning System - Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time. GPS receivers may be found also on ships, for example as a part of navigational equipment and in some models of EPIRBs. They are usualy connected to DSC radio communication equipment, for example VHF radio.


Gross Register Tonnage - Represents the total internal volume of a vessel, with some exemptions for non-productive spaces such as crew quarters. 1 GRT is equal to a volume of 100 cubic feet (2.83 m³).


Hydrostatic Release Unit - It is a device that will automatically release the EPIRB once a depth of approximately 4-5 m is reached.


International Maritime Organisation - IMO is the source of approximately 60 legal instruments that guide the regulatory development of its member states to improve safety at sea, facilitate trade among seafaring states and protect the maritime environment. The most well known is the International Convention for the Safety of Life at Sea (SOLAS).


International Maritime Satellite Organisation - Inmarsat provides telephony and data services to users world-wide, via special terminals. An Inmarsat terminal contacts the satellite and communicates to a ground station through the satellite. It provides reliable communications services to a range of governments, aid agencies, media outlets and businesses needing to communicate in remote regions or where there is no reliable terrestrial network. Aside from its commercial services, Inmarsat provides GMDSS services to ships at no charge, as a public service.


Local Time - A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time. Most adjacent time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC.


Local User Terminal - LEOLUT and GEOLUT Earth based ground stations, receive and process Distress alerts from 406 MHz EPIRBs relaying Distress information, comprising casualty ID, Position and UTC time, to MCC.


Mission Control Centres - Their main purpose and function is to relay EPIRB Distress alert information to the appropriate MRCC who coordinates the deployment of SAR units in the SAR region in which the casualty is located.


Maritime Mobile Service Identity - MMSI is a unique 9 digit number and acts in the same way as a telephone number. The MMSI allows radio operator to make automatic calls through VHF DSC radio and is the identity that is automatically transmitted within DSC calls. These MMSI numbers are issued by the appropriate authorities in the country of registration of the vessel and can be referenced back to a database of information about the vessel and it’s owners and so forth.


Maritime Rescue and Coordination Centre - All SAR activities are organized by MRCCs and MRSCs within specified navigational sea areas normally bordering their coastlines.


Maritime Rescue Sub Centres - All SAR activities are organized by MRCCs and MRSCs within specified navigational sea areas normally bordering their coastlines.


Maritime Safety Information - Navigational and meteorological warnings, meteorological forecasts, and other urgent safety-related information broadcast to ships.


NAVigational AREA - Under the WWNWS the world's oceans are divided into 16 geographical sea areas, called NAVAREAs which are identified by Roman numerals and comprises NAVTEX CRS identified by a single letter of the alphabet from A to Z.


NAVigational TELex - It is a system for transmitting MSI in GMDSS Sea Area A1 and A2.


Nautical Mile - A nautical mile or sea mile is a unit of length. It corresponds approximately to one minute of latitude along any meridian. The international standard definition is: 1 NM = 1,852 m exactly.


Press To Transmit - PTT or “PRESSEL” switch is usually mounted on one side of the microphone or in the middle of the handgrip of a telephone style handset connected to the marine radio communication equipment. To operate correctly, you have to press it in order to switch the radio from receive mode to transmit mode and release it in order to revert to receive mode.


Radio Telephony - Radio communication known as RT uses a Tx (radio transmitter) to send out radio waves of a certain frequency and a Rx (radio receiver) to receive the radio waves at the same frequency. Radio waves consist of electro-magnetic energy in specific frequency bands within the radio frequency spectrum. Frequency of waves means the number of occurrences of a wave per second. For example, marine VHF RT works in the VHF part of the marine radio frequency spectrum, which extends between 156 MHz to 174 MHz.


radio receiver


Search And Rescue - SAR is an operation mounted by emergency services, often well-trained volunteers, to find someone believed to be in distress, lost, sick or injured at sea, whether close to shore or not.


Search and Rescue Satellite-Aided Tracking


Search and Rescue [Radar] Transponder - It is a portable device, which is used as a complimentary distress alerting system. It enables any ship/airplane/helicopter in the area to locate survivors easily by just the use of their proper radar system.


Simplex is the communication method where both transmitter and receiver are operating on a single (or the same) frequency. On simplex channels it isn't possible to transmit and receive simultaneously. Simplex channels are mainly used for Distress, Urgency, Safety and Routine Calling purposes, port, pilotage, harbour and inter-ship operations.


Safety Of Life At Sea - This International Convention is the most important treaty protecting the safety of merchant ships. It divides all vessels in 2 groups: SOLAS and non SOLAS vessels. SOLAS vessels need to comply with GMDSS sattelite and radio equipment carriage requirements. SOLAS vessels are all cargo ships of 300 GRT and upwards and all passenger ships with some exceptions. Detailed SOLAS vessels definition may be found in ALRS Volume 5. Non SOLAS vessels do not need to comply with GMDSS radio equipment carriage requirements, but will increasingly use it, because that causes an important increase of the safety at sea. Some countries have incorporated GMDSS radio equipment carriage requirements into their domestic marine legislation that is valid for non SOLAS vessels under their flag.


Short Range Certificate - Just as every driver needs a licence to drive a car, operators of marine radios also require a licence. The person operating VHF radio system sailing in GMDSS Sea Area A1 must be qualified to a minimum standard. This standard is the GMDSS SRC.


radio transmitter


Coordinated Universal Time - It is a high-precision atomic time standard. Time zones around the world are expressed as positive or negative offsets from UTC. Local time is UTC plus the time zone offset for that location, plus an offset (typically +1) for daylight saving time, if in effect.


VHF Channel - The marine frequency band for VHF radio communication, extending between 156 MHz and 174 MHz, contains 57 individual VHF CH (channels) numbered consecutively from VHF CH 1 to VHF CH 28 and from VHF CH 60 to VHF CH 88.

VHF radio

is a transmitting-receiving system, which allows the operator to either transmit or receive information on the marine VHF (Very High Frequency) band

VHF RT radio

It is a transmitting-receiving system, which allows the operator to either transmit or receive information on the marine VHF (Very High Frequency) band only by voice.


World Wide Navigation Warning Service - It is a co-ordinated global service for the promulgation of navigational warnings. In GMDSS Sea Area A1 and A2 they are broadcasted via NAVTEX system, and outside given areas via SafetyNET system.

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