Maintain and repair Bridge Navigation Equipment and ships communication systems

Course Topics

  • 9.1 Introduction to GMDSS.
  • 9.2 RADAR — Maintenance & brief theory.
  • 9.3 UMS operation — BNWAS, Dead Man’s Alarm & Watch keeper alarms systems.

9.1 Introduction to GMDSS.




Introduction to GMDSS


During the 18th century, the ships sailing in international and coastal waters were dependent on the Morse code to send any kind of distress signal to a coastal authority or ships in the nearby vicinity during emergency. Since it was a transmission of texture information using tones or lights, this kind of message was never very clear to understand what kind of emergency is there on board ships. Therefore, an internationally agreed safety procedure was adopted by IMO under SOLAS chapter IV which is known as GMDSS- Global Maritime Distress Safety System.


GMDSS and its Uses


On 1st Feb 1999, the fully implemented GMDSS came to picture. It was a set standard for usage of communication protocol, procedures and safety equipment to be used at the time of distress situation by the ship. Under GMDSS, all the passenger ships and Cargo ship above 300 GT involved in the voyages in international waters have to carry equipment as per GMDSS.

When a ship uses GMDSS, it basically sends a distress signal via a satellite or radio communication equipment. It’s also used as a medium for sending or receiving maritime safety information and general communication channel.

Since the time of the Titanic, Marine Radio has helped to save tens of thousands of lives, and become the key element in Marine Search and Rescue (SAR)

Pre-GMDSS Marine Radio equipment was required to provide operation over a minimum specified range of 150 nautical miles.

This was based on the assumptions that ships usually travelled well-used routes and that there were sufficient ships at sea and shore stations dispersed about the world to receive distress calls.

However, if a ship was outside of the normal shipping lanes or was rapidly overwhelmed by the forces of nature, her distress alert may go unheard.... many ships have gone to the bottom without any distress signal being.

The 1979 IMO Assembly decided that a new global distress and safety system should be established in conjunction with a coordinated SAR infrastructure to improve safety of life at sea.

The system would take advantage of the latest technological developments.

And so was born the Global Maritime Distress and Safety System (GMDSS).




The GMDSS is specifically designed to automate a ship's radio distress alerting function, and removes the requirement for manual (ie: human) watch keeping on distress channels.

The new system is quicker, simpler and, most importantly, more efficient and reliable than the old manual Morse Code and radiotelephone alerting systems.

The basic concept is that Search and Rescue (SAR) authorities ashore, as well as shipping in the immediate vicinity of the ship or persons in distress will be rapidly alerted so that they can assist in a coordinated SAR operation with minimum delay.

The new system moves the emphasis from ship-ship alerting to ship-shore alerting.

The system also provides for urgency and safety alerting and also for the broadcast of Maritime Safety Information (weather reports and navigation warnings).


Very High Frequencies (VHF)


For the purposes of maritime communication, the range of 156 MHz to 174 MHz is allocated. Channel 16, which is set at 156.800 MHz, is for Distress, Urgency and Safety communication. Channel 70, set at 156.525 MHz, if for routine VHF DSC (Digital Selective Calling) watch.

GUARD channels are set put above and below Channel 16 to avoid any interference on Channel 16. One cannot have seamless traffic on Channel 16 with interference with regard to other communication aside from distress, safety and urgency. So the Guard channel frequencies are 156.775 MHz and 156.825 MHz.

Among other things, the VHF set runs on a 24 Volt DC supply with J3E type of transmission for Radiotelephony and G2B type of transmission for VHF DSC.


High Frequency


A GMDSS system may include High Frequency (HF) radiotelephone and radiotelex (narrow-band direct printing) equipment, with calls initiated by digital selective calling (DSC). Worldwide broadcasts of maritime safety information can also made on HF narrow-band direct printing channels.


The different elements of GMDSS are as follows:


 INMARSAT:  It is a Satellite operated system that includes ship earth station terminals – Inmarsat B, C and F77. It provides telex, telephone and data transfer services between ship-to-ship, ship to shore, and shore to ship along with a priority telex and telephone service connected to shore rescue centres.

 NAVTEX: It is an internationally adopted automated system which is used to distribute MSI-maritime safety information, and includes weather forecasts and warnings, navigational warnings, search and rescue notices and other similar safety information.

Emergency Position Indicating Radio Beacon (EPIRB): EPIRB is an equipment to help determine the position of survivors during a SAR operation. It is a secondary means of distress alerting.

 Search and Rescue Locating Equipment: Primarily the Search and Rescue Radar Transponder. This is used to home Search and Rescue units to the position of distress which transmits upon interrogation.

 Digital Selective Calling (DSC): This is a calling service between ship to ship, ship to shore or vice versa for safety and distress information mainly on high or medium frequency and VHF maritime radio.


Documents to be carried onboard with regard to GMDSS:


Ship’s Radio License

Radio Operators Certificates

Safety Radio Certificate

GMDSS Radio Log Book

ITU List of Cell Signs and Numerical Identities of Stations used by Maritime Mobile and Maritime Mobile Satellite Services

ITU List of Coast Stations

ITU List of Ship Stations

ITU List of Radio determination and Special Service Stations

Antenna Rigging Plan

Valid Shore Based Maintenance Certificate




NAVTEX is an international, automated system for instantly distributing maritime safety information (MSI) which includes navigational warnings, weather forecasts and weather warnings, search and rescue notices and similar information to ships. A small, low-cost and self-contained "smart" printing radio receiver is installed on the bridge, or the place from where the ship is navigated, and checks each incoming message to see if it has been received during an earlier transmission, or if it is of a category of no interest to the ship's master. The frequency of transmission of these messages is 518 kHz in English, while 490 kHz is sometime used to broadcast in a local language. The messages are coded with a header code identified by the using single letters of the alphabet to represent broadcasting stations, type of messages, and followed by two figures indicating the serial number of the message. For example: FA56 where F is the ID of the transmitting station, A indicates the message category navigational warning, and 56 is the consecutive message number.


Emergency position-indicating radio beacon (EPIRB)


Cospas-Sarsat is an international satellite-based search and rescue system, established by Canada, France, the United States, and Russia. These four countries jointly helped develop the 406 MHz Emergency Position-Indicating Radio Beacon (EPIRB), an element of the GMDSS designed to operate with Cospas-Sarsat system. These automatic-activating EPIRBs, now required on SOLAS ships, commercial fishing vessels, and all passenger ships, are designed to transmit to alert rescue coordination centers via the satellite system from anywhere in the world. The original COSPAS/SARSAT system used polar orbiting satellites but in recent years the system has been expanded to also include 4 geostationary satellites. Newest designs incorporate GPS receivers to transmit highly accurate positions (within about 20 meters) of the distress position. The original COSPAS/SARSAT satellites could calculate EPIRB position to within about 3 nautical miles (5.6 km) by using Doppler techniques. By the end of 2010 EPIRB manufacturers may be offering AIS (Automatic Identification System) enabled beacons. The serviceability of these items are checked monthly and annually and have limited battery shelf life between 2 to 5 years using mostly Lithium type batteries. 406 MHz EPIRB's transmit a registration number which is linked to a database of information about the vessel.


High Frequency


A GMDSS system may include High Frequency (HF) radiotelephone and radio telex  (narrow-band direct printing) equipment, with calls initiated by digital selective calling (DSC). Worldwide broadcasts of maritime safety information can also made on HF narrow-band direct printing channels.




A Search and Rescue Transponder (SART) may be triggered by any X-Band (3 cm) radar within a range of approximately 8 nautical miles. Each radar pulse received causes it to transmit a response which is swept

repetitively across the complete radar frequency band. When interrogated, it first sweeps rapidly (0.4 microseconds) through the band before beginning a relatively slow sweep (7.5 microseconds) through the back band to the starting frequency. This process is repeated for a total of twelve complete cycles. At some point in each sweep, the SART frequency will match that of the interrogating radar and be within the pass band of the radar receiver. If the STRT is within range, the frequency match during each of the 12 slow

sweeps will produce a response in the radar display, thus a line of 12 dots equally spaced by about 0.64 nautical miles will be shown.


Digital Selective Calling


The IMO also introduced Digital Selective Calling (DSC) on MF, HF and VHF maritime radios as part of the GMDSS system. DSC is primarily intended to initiate ship-to-ship, ship-to-shore and shore-to-ship radiotelephone and MF/HF radio telex calls. DSC calls can also be made to individual stations, groups of stations, or "all stations" in one's radio range. Each DSC-equipped ship, shore station and group is assigned a unique 9-digit Maritime Mobile Service Identity.

DSC distress alerts, which consist of a preformatted distress message, are used to 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 February 1, 1999. In May 2002, IMO decided to postpone cessation of a VHF listening watch aboard ships. That watch keeping requirement had been scheduled to end on February 1, 2005.

IMO and ITU both require that the DSC-equipped MF/HF and VHF radios be externally connected to a satellite navigation receiver (GPS). That connection will ensure accurate location information is sent to a rescue coordination center if a distress alert is transmitted. The FCC requires that all new VHF and MF/HF maritime radiotelephones type accepted after June 1999 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 Ship borne 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.

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


Functional requirements of the GMDSS


The GMDSS regulations (chapter IV of the International SOLAS Convention), require that every GMDSS equipped ship shall be capable of;

transmitting ship-to-shore Distress Alerts by at least two separate and independent means, each using a different radio communication service;

receiving shore-to-ship Distress Alerts; transmitting and receiving ship-to-ship Distress Alerts;

transmitting and receiving search and rescue coordinating communications;

transmitting and receiving locating signals;

receiving maritime safety information;

transmitting and receiving general radio communications relating to the management and operation of the vessel;

Transmitting and receiving bridge-to-bridge communications.




The GMDSS applies to vessels subject to the SOLAS Convention - that is:

Commercial vessels of 300 Gross Registered Tons (GRT) and above, engaged on international voyages.

The GMDSS became mandatory for such vessels as at February 1, 1999.

Commercial vessels under 300 GRT, or those above 300 GRT engaged on domestic voyages only are subject to the requirements of their Flag State.

Some Flag States have incorporated GMDSS requirements into their domestic marine radio legislation - however many have not.

Equipment vs Operational requirements

The major difference between the GMDSS and its predecessor systems is that the radio communications equipment to be fitted to a GMDSS ship is determined by the ship's area of operation, rather than by its size.

The new system divides the world's oceans into 4 areas:

Area A1 lies within range of shore-based VHF coast stations (20 to 30 nautical miles);

Area A2 lies within range of shore based MF coast stations (excluding A1 areas) (approximately 100 - 150 nautical miles);

Area A3 lies within the coverage area of Inmarsat communications satellites (excluding A1 and A2 areas - approximately latitude 70 degrees north to latitude 70 degrees south); and

Area A4 comprises the remaining sea areas outside areas A1, A2 and A3 (the Polar Regions).

INDIA and its surrounding SAR area are declared as Sea Area A3. There are no A1 or A2 areas in INDIA


GMDSS communication systems


The GMDSS utilizes both satellite and terrestrial (ie: conventional) radio systems.

Sea Area A1 requires short range radio services - VHF is used to provide voice and automated distress alerting via Digital Selective Calling (DSC).

Sea Area A2 requires medium range services - Medium Frequencies (MF - 2 MHz) are used for voice and DSC.

Sea Areas A3 and A4 require long range alerting - High Frequencies (HF - 3 to 30 MHz) are used for voice, DSC and Narrow Band Direct Printing (NBDP - aka radio telex).

Equipment requirements vary according to the area the ship is trading to or through.

Accordingly, it is quite possible that a small 300 ton cargo vessel may carry the same amount of communications equipment as a 300,000 ton oil tanker, if they are both operating in the same area....this is a marked change from the pre-GMDSS systems.



The signals are monitored worldwide and the location of the distress is detected by non-geostationary satellites, and can be located by trilateration in combination with triangulation, respecting the varying quality of the signal received.









Very high frequency waves permit only the line of sight communication over a dependable range 30-50n.m

Radio  waves of still higher frequencies [say, UHF or SHF frequency bands], penetrates the ionosphere and are lost in space .

Should it be possible to reflect these waves  back to the earth by some sort of reflector, the problem of over crowding the frequency spectrum of the MF, HF, and VHF bands could be overcome.  




The idea of putting artificial satellite in the earths orbit for just as the planets revolve round suns orbit, the artificial satellites could be made revolve round the earths  orbit

Typical paths of an orbiting satellite could be equatorial, polar or inclined .

if a satellite made to orbit in a equatorial at the same rate as that of axial rotation, of the earth, it will appear to be geostationary  with respect to the earth, above a fixed point of an equator.

 This is possible when a satellite is placed 35,000 km distance from the earth, in an equatorial  plane 

The international Maritime (INMARSAT) thus placed in geo stationary orbit, remains in the same relative position to the earth, above the equator.

 The geo stationary satellite, however, tends to drift. This necessitates a certain amount of station keeping or position control which is provided by TT&C (tracking telemetry and command) ground control station

A communication satellite is basically an electronic package placed in a geo stationary orbit around the to aid communication from one point on the earth to another .

Solar panels provides the required power and hydrazine gas motors help provide minor positional correction in orbit Far from being a passive relay station the satellite receives signals from the earth (up link) with the help of a number of transponder carried on board.

The radiation pattern of the satellite antenna which determines the down link coverage on the (in accordance with the equivalent Isotropic Radiated Power –EIRP) is called the satellite footprint.


Four satellites are placed at


Longitudes -54 deg W(AORE-W)

Longitudes -15deg W(AORE-E)

Longitudes -64 deg E(IOR-E)

Longitudes-178 deg E(IOR-E)

To provide world wide coverage.








Inmarsat of date is the only provider of world wide communication for distress sand safety purpose for general/ commercial purpose.

 This system of communication stretches the office telephone and telex to a ship at sea sea.

Communication originating at the office is responded at the ship’s terminal after sequential routing through local exchange international telephone network, coast earth station and the satellites.

Aside from its commercial services, Inmarsat provides global maritime distress and safety services (GMDSS) to ships and aircraft at no charge, as a public service.

Satellite systems operated by the Inmarsat, overseen by the International Mobile Satellite Organization (IMSO) are also important elements of the GMDSS.

The types of INMARSAT ship earth station terminals recognized by the GMDSS are: Inmarsat B, C and F77.

INMARSAT B and F77, an updated version of the now redundant INMARSAT 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 centres.


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 provides


The INMARSAT C SafetyNET service is a satellite-based worldwide maritime safety information broadcast service of high seas weather warnings,

Inmarsat C equipment is relatively small and lightweight, and costs much less than an Inmarsat B or F77 station.

Inmarsat B and F77 ship earth stations require relatively large gyro-stabilized unidirectional antennas; the antenna size of the Inmarsat C is much smaller and is omnidirectional.

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 centre if a distress alert is ever transmitted.

A distressed call may be intercepted at distance of

150 n.miles (extending upto 400 n.miles)on 2185khz MF R/T

30 n. Miles (extending upto 50 n.miles) on 156.8 Mhz VHF R/T

A ship in distress may able to receive assistance only if where is another ship within range.




The Inmarsat system has three major components:

1. The Space Segment provided by Inmarsat

2. The Ground Segment provided by signatories.

3. The Ship Earth Station.



Four geo stationary satellites serve ocean regions designated as  AOR(E), POR AND IOR and AOR(W) Four spare satellites are also provided to serve as back-ups.


Ground segment


Telemetry, Tracking and Command station (TT&C) Control the position of the satellite They linked with the SCC located at London.

Satellite Control centre. (SCC) Tracks the exact position of the satellites and transmits appropriate signals to TT&C station

Network Control Centre. (NCC) ) Monitor and co-ordinate the activities of the network.  It is located close to the SCC located at London.

Operations control centre. (OCC) It is the nerve centre of the INMARSAT sys.  Control the whole network of fixed station, mobile stations and satellites and located at London. The OCC is responsible for the commissioning of  Ships Earth Station (SES)




To monitor distress priority request – should the CES not respond to distress priority requests, the NCS would itself take appropriate action

To transmit TDM carrier- to monitor, control, and co-ordinate the operation and allocation of the channels to each CES and SES.

To assign only telephone channels

To control and monitor both telex and telephone channels.

To transmit ALL Ship’s announcement Each NCS interacts with the CES in its Ocean Region and other NCS as well as the enabling mutual transfer of information throughout the system.




The Coast Earth Station provides the link between the satellites and terrestrial communication network

It has a 11-14 metre parabolic (Cassegrain) dish antenna.

It operate in  C(6Ghz and14Ghz) band for transmission of signal to from the satellite.

It also operates in the L (1.6Ghz and 1.5 Ghz) band transmission and reception of network control signals.

The Network Co-ordination Station (NCS)

The NCS is provided one for each service- in each Ocean region.




SES or MES consist of

The above deck equipment (ADE) consisting of the antenna unit

The below deck equipment (BDE) consisting of the transceiver unit, with VDU, keyboard, printer, and interfaces for telex, telephone and fax.




In order to receive and send channel assignment, the Mobile Earth Stations and Coast Earth Stations monitor the NCS on the common signalling channels termed Time Division Multiplex (TDM)

channels- TDM-0 or TDM-1.

Each  TDM in turn consist of 22 time slots to permit multiplexed or shared use of the same TDM channel.

The SES in time division multiplex access (TDMA) is assigned a corresponding time slot during which it has access to the satellite.




Since the satellite communication is line of a sight communication, the satellite in principle has to be acquired i.e the antenna of the user equipment must be aligned i.e pointing to the satellite at all times to maintain communication.

The acquisition is achieved when, the user equipment antenna locks on to the satellite signals i.e. Common Signalling Channel (CSC) and the signal strength is sufficient to track the satellite. Ship's heading is necessary for the antenna tracking system.

Based on the vessel's position, every ship could be in the foot print of one, two or three satellites, In order to acquire the satellite, azimuth and elevation charts or maps or information tables which are ready available.

The azimuth and elevation of the satellite A  antenna are adjusted to point the antenna to the satellite.

The antenna beam width is wide enough to enable coarse acquisition of the satellite.

The electronic step track circuitry in the equipment does the necessary finer alignment to obtain a strong enough signal to track and lock and look on to the satellite.

The entire acquisition program can be automatically achieved at the time of switching on although it may take several minutes to be fully acquired.

Sufficient care should be exercised to ensure acquisition of the desired satellite in over-lapping ocean areas.




It consist of

The ADE called External Mounted Equipment (EME)

It is made up of Data Circuitry Terminal Equipment (DCE)

The DCE interfaces the SES, with its transmitter/receiver/omni directional antenna having no moving parts, to the satellite system.

The below deck equipment called Internal Mounted Equipment (IME)

It is made up of the Data Terminal Equipment(DTE)

The DTE interfaces external inputs/outputs devices viz. Keyboard, screen and printer for message processing to the SES.

An external computer (Laptop or desktop) complete with its own peripherals serving as a multi-tasking computer ,may also be used.


Inmarsat A

The original Inmarsat system, which was withdrawn from service at the end of 2007.

Based on analogue techniques, it provided global two-way telephony, facsimile, data and telex communications to the maritime community for a quarter of a century.


Inmarsat B

The first digital successor to Inmarsat A, capable of high-quality telephony, facsimile, data and telex services, and compatible with the Global Maritime Distress and Safety System (GMDSS).


Inmarsat C and Mini C

A digital system based on a low-cost satellite terminal, providing two-way store-and-forward messaging, distress calling, EGC SafetyNET™ and FleetNET™, data reporting and polling.

The system is approved for use under the (GMDSS) and mandatory for Solas-compliant ships operating outside Navtex coverage areas.


Inmarsat M

The first briefcase satphone, introduced in 1993.

Also available in the maritime market, as a smaller, digital alternative to Inmarsat B, offering two-way voice telephony, distress alerting, fax and data services at lower data rates.


Mini M

Introduced in 1995, based on digital technology and capable of two-way voice telephony, alerting, fax and data services.

Operates only in the reduced coverage offered by the Inmarsat-3 spot beams, but its notebook size has made it one of the most popular Inmarsat services on land and at sea.

What are the advantages of the GMDSS over the former system?

provides worldwide ship to shore alerting, it is not dependent upon passing ships

simplifies radio operations, alerts may be sent by "two simple actions"

ensures redundancy of communications, it requires two separate systems for alerting

enhances search and rescue, operations are coordinated from shore centers

minimizes unanticipated emergencies at sea, Maritime Safety Information (MSI) broadcasts are included

eliminates reliance on a single person for communications, it requires at least two licensed GMDSS radio operators and typically two maintenance methods to ensure distress communications capability at all times.