GMDSS GOC Suport Course 2010ok

7/26/2019 GMDSS GOC Suport Course 2010ok

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MARITIME UNIVERSITY OF CONSTANTZA

GMDSS-GOC CourseBased by

IMO MODEL COURSE

(GMDSS GOC)

1.25+Compendium

Support Course2010


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CONTENT

GLOSSARY 7

S 1 GMDSS INTRODUCTION 19

S 2 BASIC CONCEPTS OF THE GMDSS 29

S 3 RADIO WAVE PROPAGATION 43

S 4 BASIC TRANSMITTERS AND RECEIVERS57

S 5 GMDSS DISTRESS AND SAFETY

COMMUNICATIONS

71

S 6DIGITAL SELECTIVE CALLING - DSC

81

S 7 TELEX (NBDP) PROCEDURES 109

S 8DSC TERRESTRIAL DISTRESS

COMMUNICATIONS

127

S 9 TERRESTRIAL URGENCY AND SAFETYCOMUNICATIONS

149

S 10 RADIOTELEPHONY PROCEDURES 157

S 11INMARSAT SATELLITES

171

S 12 MARITIME SAFETY INFORMATION (MSI) 223


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S 13 EPIRBs and SARTs 237

S 14 GENERAL REGULATIONS 243

S 15 POWER SUPPLIES

251

S 16 ANTENNAS 259

S 17 TRAFFIC CHARGING 267

ANNEXES 275

ANEXX 1 GMDSS FREQUENCIES 277

ANNEX2.1 TABLE OF TRANSMITTING FREQUENCIES IN

THE VHF MARITIME MOBILE BAND

156-174 MHZ

279

ANNEX 2.2 VHF USA CHANNELS 281

ANNEX 3.1 INMARSAT A LAND EARTH STATION

OPERATORS AND ACCESS CODES

284

ANNEX 3.2 INMARSAT B/M LAND EARTH STATION


286

ANNEX 3.3 INMARSAT C LAND EARTH STATION


287

ANNEX 3.4 INMARSAT FLEET 77 LAND EARTH STATION


288

ANNEX 3.5 INMARSAT FLEET 33 LAND EARTH STATION


289

ANNEX 3.6 INMARSAT SWIFT 64 LAND EARTH STATION


290

ANNEX 3.7 INMARSAT FLEET 77 2 kbps DATA LAND EARTH

STATION OPERATORS AND ACCESS CODES

291

ANNEX 3.8 INMARSAT FLEET 55 LAND EARTH STATION 292


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ANNEX 3.9 INMARSAT MPDS HOME LAND EARTH STATION


293

ANEXA3.10 INMARSAT MPDS REGIONAL LAND EARTH

STATION OPERATORS AND ACCESS CODES

294

ANNEX 4 INMARSAT OCEAN REGIONS (AZIMUTH AND

ELEVATION)

295

Fig. A.4.1 Atlantic Ocean Region East Azimuth and

Elevation

295

Fig. A.4.2 Atlantic Ocean Region West Azimuth and

Elevation

296

Fig. A.4.3 Indian Ocean Region Azimuth and Elevation 297

Fig. A.4.4 Pacific Ocean Region Azimuth and Elevation 298

ANNEX 5 INMARSAT ANTENNA POSITIONING 299

Table A5.1 Antenna Positioning

Ship Located NORTH and EAST of selected

satellite

299


Ship Located NORTH and WEST of selected

satellite

300


Ship Located SOUTH and EAST of selected

satellite

301


Ship Located SOUTH and WEST of selected

satellite

302

ANNEX 6 INMARSAT INFORMATIONS 305


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Table A6.1 Telephone 2-digit Code Services 305

Table A6.2 Telex 2-digit Code Services 306

Table A6.3 INMARSAT A and INMARSAT B Telex

Fault Codes 309

Table A6.4 INMARSAT C Non-Delivery Notification

(NDN) failure codes 310

ANNEX 7 Table 7A. Telex command codes 314

Table 7B. Internaional Telex Service Codes and

Abbreviations

319

BIBLIOGRAPHY 323


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GLOSSARY GMDSS

AA (accounting authority): The organisation named on a commissioning applicationform to administer the billing and settlement of the communication charges incurred by

an MES.

AAIC (accounting authority identification code): An unique code assigned by the ITUto identify an accounting authority.

AMVER (Automated Mutual-assistance Vessel Rescue system): A vessel position-reporting system operated by the United States Coast Guard for any merchant vessel of1000grt or more on a voyage lasting longert han 24 hours, to and from anywhere on theworld.

Analogue: Any signal which represents a changing value over time.

Answerback: An identifier given to an Inmarsat MES and used in messagetransmissions. The format must befour letters (A-Z; no numbers) finishing with an x.

AOR-E: Atlantic Ocean Region (East).

AOR-W: Atlantic Ocean Region (West).Applicant: The person who completes and signs a maritime commissioning applicationform when applying to have an Inmarsat MES commissioned. The applicant must submitthe form to the national routing organisation for the country where the vessel isregistered.

ARQ (automatic request repeat): The error correction process used in store-and-forward messaging by which a receiver checks for errors in received data packets andrequests the sending end to re-transmit any packets which were received containing anerror.

ASCII (American Standard Code for Information Interchange): A standardalphanumeric character set based on 7-bit codes.

AUSREP: A vessel position-reporting system similar to AMVER, but operated by theAustralian Authorities.

BBER: Bulletin Board Error Rate.

Bit: The basic unit of digital communications; may be either 1 or 0.

Bit Error Rate (BER): used as a measure of the quality of reception by the MES of theBulletin Board of a TDM Channel.


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BPS (bits per second): A unit of measurement for speed of data transfer or throughput.

Bulletin Board (in a TDM channel): A data packet transmitted in each frame of a TDMchannel which contains information about the status of the Inmarsat-B/M, mini-M and Cnetwork configurations and the current frame number, used by the MES as a timing

reference.

Bulletin Board Service (BBS): A notice board on which information can be exchangedor posted for others to download.

Byte: One byte comprises eight bits and may represent either one alphanumeric characteror numeric information.

CAG: Customer Activation Group.

Case-approval: The official approval given by Inmarsat to an MES model which is

typically still undergoing development by a manufacturer so as to permit the model toaccess an Inmarsat communications system. See also type-approval.

CCITT (Comit Consultatif International Tlgraphique et Tlphonique): Anadvisory committee to the International Telecommunication Union (ITU). The CCITTpublishes standards and recommendations to enable telecommunications systems andequipment world-wide to communicate with each other.Examples of CCITT standardsare the X.25 and X.400 rotocols used on PSDN land-lines.

Channel number: The number representing the requency of an Inmarsatcommunications channel.

Character: One element of an alphanumeric character set. ne character is equivalent toone byte or 8 bits.

Class 1 Inmarsat-C MES: A Class 1 MES is capable of hip-to-shore and shore-to-shipmessage transfer and distress alerting, but is not capable of receiving EGC messages.

Class 2 Inmarsat-C MES: A Class 2 MES is capable of two modes of operation(selected by the operator): As Class 1, and also capable of receiving EGC messages when not engaged in Inmarsat-C traffic. Ready for EGC message reception exclusively (and not available in that modefor Inmarsat-C message transfer).

Class 3 Inmarsat-C MES: A Class 3 MES has two independent receivers, one forreceiving two-way Inmarsat-C messages, the other for receiving EGC messages.

Closed network: A private network, with access limited to registered users. TheInmarsat-C system allows two types of closed networks: data reporting networks,


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identified by a Data Reporting Network Identification (DNID) code, and EGCFleetNETTM networks, identified by an EGC Network Identification (ENID) code.

Commissioning: The process by which an MES is registered for use via the Inmarsatnetwork.

Companded: A method of transmission, meaning compressed/expanded, which is usedto improve signal-to-noise ratio. At the sending end, a compressor electronic circuitamplifies low-level signals and reduces high levels to a mean level according to analgorithm. At the receiving end, an expander circuit uses similar methods to return thesignal levels to their original values before passing them on to other circuits. See alsouncompanded.

COSPAS-SARSAT: A satellite-based distress beacon locating system.

Coverage area: See footprint.

CSS: Co-ordinator Surface Search.

Data report (programmed unreserved, P): A short collection of data (up to 32 bytes inthree packets) which is transmitted by an MES at random times in unreserved time slotsof a signalling channel after receipt of a polling command from an operational centre.

Data report (reserved, R): A small amount of data (up to 32 bytes in three packets)which is transmitted by an MES in reserved times slots in a signalling channel, inresponse to an earlier polling command from an operational centre.

Data report (unreserved, U): A small amount of data (up to 32 bytes in three packets)which is transmitted in unreserved time slots of a signalling channel by an MES to anoperational centre.

Data services: This is how a terminal may send and receive electronic messages such ase-mail.

DCE: Data circuit terminating equipment: a component part of an Inmarsat-C MES. AnMES contains a DCE receiver and a DCE transmitter which are used for communicationbetween the MES and an Inmarsat-C LES.

DECCA Navigator: A position-fixing system, based on chains of shore-based radiotransmissions.

DHSD: Duplex high-speed data (see HSD).

Differential GPS: A global positioning system used with Inmarsat terminals and basedon GPS satellites, with accuracy enhanced by the use of transmission ofdifferential corrections from suitably located shorebased radio beacons.


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Digital Signal: A signal which represents values in the form of binary numbers.

Distress alerting: A facility available on all maritime MESs, enabling the MES to senddistress priority messages through the Inmarsat system to a rescue coordination centre

(RCC). This is not available on the Inmarsat-mini-M network.

Distress priority message: This is a message prepared and sent with distress priorityusing the Inmarsat system to a rescue co-ordination centre (RCC).

DMG: Distress Message Generator.

DNIC: Data Network Identification Code.

DNID: Data reporting Network Identification code. See data report (unreserved), datareport (reserved) and data report (pre-assigned).

Downloading: The process by which an Inmarsat-C MES receives information from aservice provider. For data reporting purposes, an operational centre downloads a DNIDcode and Member Number to the MES. In the EGC FleetNETTM service, an informationprovider downloads an EGC Network Identification (ENID) code to an MES.

DTE (data terminal equipment): a component part of an Inmarsat-C MES, usedprimarily for storage and interfacing external devices (such as a keyboard or monitor).For other Inmarsat systems, this can be a computer connected to the MES for use for datacommunications.

Duplex: The ability of a communications channel to transmit data simultaneously in bothdirections. Also known as Full Duplex.

EGC: The EGC (Enhanced Group Call) services provided in the Inmarsat-C system areEGC SafetyNET EGC FleetNET and Inmarsat system messages.

EIRP: Effective Isotropically Radiated Power, a measure of transmitted power.

E-mail: Electronic mail: a global message-handling system whereby subscribers tocommercial e-mail services can exchange electronic messages and data files betweencomputers. E-mail services are provided by some service providers and privateorganisations. Access to e-mail services may be via PSTN, PSDN networks or theInternet.

ENID: EGC network identification (ENID) code.

EPIRB: Emergency position-indicating radio beacon.

ESAS: Electronic Service Activation System.


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Fax: Abbreviation for facsimile, a device used to transmit a copy of an originaldocument. The Inmarsat-A, B/M and mini-M systems support twoway fax transmissions.The Inmarsat-C system is able to send only text messages (no graphics) to a fax terminalin the ship-to-shore direction. It is only possible to send text messages (no graphics) in

the shore-to-ship direction by using a third party fax bureau.

Fax bureau service: A service offered by some private organisations and serviceproviders to send and receive fax messages.

FleetNETTM: A service provided by FleetNETTM information providers to distributecommercial information to MESs belonging to a FleetNETTM group, identified by anunique ENID code. Footprint (of a satellite): The area on the Earths surface (sea orland) covered by the satellite and where an antenna can obtain line-of-sightcommunications. In the Inmarsat systems, this area is also known as the ocean region orcoverage area.

Gateway: An interface between communications systems such as the Inmarsat-C systemand the national and international telecommunications networks.

Glonass: A global positioning system similar to GPS but using satellites of the formerSoviet Union.

GMDSS: The Global Maritime Distress and Safety System: the Inmarsat-A/B and Csystems are the only Inmarsat networks included in the GMDSS by the IMO InternationalMaritime Organisation.

Gold Franc (GF): A nominal currency used by LESs and accounting authorities tocalculate communication charges incurred by an MES. A fixed rate of exchange existsbetween the GF and the nominal currency the SDR: 1 SDR = 3.061 GF.

GPS (Global Positioning System): System which provides the geographic location of avessel. This service uses American military satellites which have been made available forcivilian use.

Ground segment: The network of LESs which provide a link between the space segmentand the terrestrial telecommunication networks.

HSD: High-speed data. This service allows for data to be transferred at data rates of up to64kbit/s.

IA5: International Alphabet 5 - a standard alphanumeric character set, also known asASCII, based on 7-bit codes. Supports both upper and lower case characters.

IHO: International Hydrographic Organisation.


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IMN (Inmarsat Mobile Number): The number assigned by the national routingorganisation to an Inmarsat MES as its identity number. An Inmarsat-A maritimeIMN hasthe format 1xxxxxx; an Inmarsat-B maritime IMN has the format 3xxxxxxxxx; anInmarsat-C maritime IMN has the format 4xxxxxxxxx; an Inmarsat-M maritime IMN hasthe format 6xxxxxxxxx; and an Inmarsat-mini-M maritime IMN has the format

76xxxxxxxx.

IMO: International Maritime Organisation.

Information provider: An organisation which provides MSI messages for broadcastingto MESs via the EGC SafetyNETTM service, which can be received by vessels fittedwith an EGC receiver.

Inmarsat: The operator of global mobile satellite communications, part of the InmarsatVentures Ltd group of companies.

Inmarsat-A: The original Inmarsat system, which has been operating since 1982, basedon analogue techniques and capable of global two-way telephony, facsimile, data andtelex communications.

Inmarsat-B: An Inmarsat system based on digital technology, and capable of highquality telephony, facsimile, data and telex services.

Inmarsat-C: A digital system based on a low-cost MES with low power consumption.This system provides global two-way store-and-forward messaging, distress alerting,EGC SafetyNETTM and FleetNETTM, data reporting and polling.

Inmarsat-E: A distress alerting system based on EPIRBs.

Inmarsat-M: Introduced in 1993, based on digital technology and capable of two-wayvoice telephony, distress alerting, fax and data services at lower data rates.

Inmarsat 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 reducedcoverage offered by the spot beams

Inter-station Signalling Links (ISLs): These signaling channels are used between anNCS and the LESs in its ocean region to pass system information around the system.

Internet: An international network of computers linked to enable information to beexchanged.

IOR: Indian Ocean Region.


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ISDN, Integrated Service Digital Network: A high capacity digital line which lets userssend voice and data at 64kbit/s over one telephone line from a common networkinterface.

ISP (Inmarsat Service provider): An entity which establishes a contract with one or

more of the SPs to bill, promote and retail the services of the contracted SPs to end users.It can be an alternative to an AA.

ITA2 (International Telegraph Alphabet 2): A standard alphanumeric character set,generally used for sending messages on the international telex networks. The characterset is based on 5-bit codes, also known as telex format, or 5-bit packed.

ITU: The International Telecommunication Union, which publishes a list of approvedaccounting authorities. See also CCITT.

JASREP: A vessel position-reporting system similar to AMVER, but operated by the

Japanese authorities.

Kbytes: 1024 bits or 128 characters.

LAN (Local Area Network): A network which allows computers and printers tocommunicate with each other, have access to and share expensive peripherals such as faxservers, modem servers and centralized databases.

Land earth station (LES): The name used in the Inmarsat network for a shore-basedreceiving and transmitting station which acts as an interface between MESs and theterrestrial communications networks. LESs are owned and operated by service providers.

LES TDM channel: A TDM channel used by an LES to transmit system information anddata addressed to an MES.

Log in: The action performed on an Inmarsat-C MES to inform the NCS in an oceanregion that the MES is available for communications.

Log out: The action performed on an Inmarsat-C MES to inform the NCS in an oceanregion that the MES is not available for communication.

LORAN-C: A position-fixing system, based on chains of shore-based, low-frequencyradio transmissions.

MEM: Macro-encoded message.

Member number: The number downloaded with a DNID to an MES, when the MES isregistered to a data reporting network.


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MES (mobile earth station): The generic name used to describe an Inmarsat-approvedterminal which is allowed to access the network, and applicable to both maritime andland mobile communications.

Message channel: A channel assigned by the NCS for an MES to send a message

through an LES to its required destination.

METAREA: Meteorological area corresponding to the NAVAREAs defined by theIMO.

MMSI (Maritime Mobile Service Identity): A nine-digit format assigned by themaritime authority to identify a vessel. The first three digits are the code of the countrywhere the vessel is registered as defined by the ITU.

Modem: MODulator/DEModulator, a device used to transmit digital data, by converting(modulating) a digital signal into an analogue form and re-converting (demodulating) the

analogue signal into digital form at the receiving end.

MSI (Maritime Safety Information): Information supplied by shore-based informationproviders and forwarded to an Inmarsat-C LES for broadcasting over the Inmarsat-Csystem to MESs fitted with an EGC receive capability.

Multi-channel MES: An MES which is capable of making more than one call at a time.Most MESs are only single channel.

NAVAREA: One of 16 areas of sea as defined by the IMO, into which the worldsoceans are divided for the dissemination of navigational and meteorological warnings.See also METAREA.

NAVTEX: The low-frequency system developed by the IMO for the broadcast andautomatic reception of coastal MSI by means of direct-printing telegraphy.

NCS: An Inmarsat network co-ordination station; a specially equipped LES appointed asthe NCS for each Inmarsat system and ocean region, which monitors and co-ordinates theoperation of all of the MESs and SPs within that ocean region.

NCS Common Signalling Channel: Also known as the NCS Common Channel. A TDMchannel used by the NCS to transmit system information and message announcements toMESs.

Network: A group of communication channels which enable the sharing of informationand resources between several users.

NOC: Network Operations Centre, located at Inmarsats headquarters in London, whichmonitors and controls the operation of the Inmarsat network.


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NUA:Network user address.

Ocean region: The coverage area of an Inmarsat satellite within which an MES may sendand receive messages.

Omega: A position-fixing system based on chains of shore-based, very-low frequencyradio transmissions.

Omni-directional antenna: An antenna which is capable of line-of-sight communicationwith a satellite without requiring any pointing. Generally used on an Inmarsat-C MES.

Operational centre: A shore-based centre for controlling a data-reporting network. Theoperational centre initially downloads a DNID code and member number to an MESwhich joins the group. The centre subsequently sends polling commands to instructselected MESs to return pre-assigned data reports or to perform a defined task such asSCADA. The centre also receives unreserved data reports from MESs belonging to the

closed network.

Operator-assisted services: Communications services provided by some serviceproviders, for example forwarding a text message from an MES as a voice message to ashore-based telephone.

Option 1 stand-alone EGC receiver: A type of standalone EGC receiver which canreceive only EGC messages and cannot engage in non-EGC message transfer.

Option 2 stand-alone EGC receiver: This type of standalone EGC receiver may beadded to the antenna of an Inmarsat-A or B MES so that the vessel may meet its GMDSSrequirements.

Packet: An envelope or block of data sent over a network; each packet containsaddressing information as well as the data being sent.

Polling: The facility whereby an operational centre sends an instruction (a pollingcommand) to selected MESs to perform a defined task, such as returning a preassigneddata report or performing a SCADA operation.

POR: Pacific Ocean Region.

Presentation code: A code included in a transmission (ship-to-shore or shore-to-ship),indicating to the recipient the presentation or formatting of the data contained in themessage.

Protocol: A defined set of communications standards which lay down the parameters towhich all users must abide. Protocols in general use are X.25 and X.400.

PSA: Point of Service Activation.


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PSDN: Packet Switched Data Network.

PSTN: Public Switched Telephone Network.

PVT: Performance Verification Test; used to test the performance of Inmarsat-C.

RCC: Rescue co-ordination centre.

SafetyNET This service is provided by SafetyNET information providers to distributeMSI to MESs fitted with an EGC receive capability.

SAR: Search-and-rescue.

SART: Search and Rescue Radar Transponder.

SCADA: Supervisory Control and Data Acquisition.

SCC: Satellite control centre.

SDR (Special Drawing Right): A nominal currency used by service providers andaccounting authorities to calculate communication charges incurred by an MES.A fixed rate of exchange exists between the SDR and the nominal currency of the GF: 1SDR = 3.061 GF.

Service provider (SP): A company or organisation which operates an LES.

Signalling channel (MES - LES): A random access TDMA channel, used by an MES totransmit signalling information and data to an LES.

Signalling channels (MES - NCS): A random access TDMA channel, used by an MESto transmit signalling information and data to an NCS.

SIM (Subscriber Identity Module) card: Used with Inmarsat mini-M, SIM cards areeasily installed and removed, allowing one terminal to be used by multiple users withouthaving complex billing arrangements.

Simplex: The ability of a communication channel to carry communication traffic in onedirection only.

SOLAS: Safety of Life at Sea.

Space segment: Consists of the communications satellites operated by Inmarsat.


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Special access code: A destination address code used in a ship-to-shore or shore-to-shipmessage to access a special service provided by a service provider. The two-digit codesare examples of special access codes.

Spot beam: A concentrated area offering coverage within the global footprint for

particular regions in the world.

Store-and-forward messaging: The protocol used by the Inmarsat-C system to transfertext or data messages in data packets to receiving equipment.

System message: A message originated by Inmarsat containing information relevant tothe Inmarsat system, broadcast on the NCS Common Channel and received by all MESs.

TCP / IP (Transmission control protocol / Internet protocol): The set of protocolsused to communicate via the Internet and between multiple networks.

TDM (Time division multiplex): The process by which multiple signals can share thesame communication channel, each using a different time slot.

TDM channel: The Inmarsat system uses different TDM channels, each transmitted onan unique frequency. The TDM channels are used for system control and messagetransfer to MESs. See LES TDM Channel and NCS Common Channel.

TDMA (Time Division Multiple Access): The process by which MESs communicatewith an LES or NCS.

TNID: Terrestrial Network Identity.

Terrestrial telecommunication networks: The national and international telephone,telex and data networks with which the service providers interface to route calls to andfrom MESs via the space segment.

Time slot: Basic unit into which one time frame of a TDM channel is divided.

Type-approval: The official approval given by Inmarsat to an MES model produced byan independent manufacturer when the MES meets the technical standards defined byInmarsat. Only models which have been granted type-approval (or case-approval) arepermitted to operate via the Inmarsat network.

Uncompanded: A method of transmission which does not use companding techniquesand is used for data and fax transmission on the Inmarsat-A network. See companded.

UTC (Universal Co-ordinated Time): A term which, for practical purposes, has thesame meaning as Greenwich Mean Time (GMT).


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Value-added service (VAS) provider: A private organisation which provides servicessuch as weather forecasting to vessels using Inmarsat and other networks.

Video conferencing: Video and audio communication between two or more people via avideocodec (coder/decoder) at either end and linked by digital circuits.

WAN (Wide Area Network): A network which connects users over large distances,often crossing geographical boundaries.

WMO: World Meteorological Organisation.

X.25: The communications protocol used on the national and international PSDNnetworks to exchange digital data between devices attached to the network.

X.400: A message-handling protocol used to exchange electronic mail (e-mail) messagesaround the world. Able to use the X.25 (PSDN) networks.

Two-digit codes: Special examples of Special Access Codes.

5-bit packed (also known as telex format or ITA2): A format based on 5-bit codes usedfor sending alphanumeric characters to and from telex terminals.

7-bit ASCII: A format based on 7-bit codes used for sending the alphanumeric charactersof the ASCII character set.

8-bit data: A format based on 8-bit codes used for encoding information such as text,national character sets and numerical information.


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SECTION 1

INTRODUCTION


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INTRODUCTION

Radio at Sea

Radio has been the foundation of the distress and safety systems used bye ships at seasince the first instance of the use of radio to save lives at sea in 1899. It was soon realized

that, to be effective, a radio-based distress and safety system has to be founded oninternationally agreed rules concerning the type of equipment, the radio frequencies usedand operational procedures. The first international agreements were established under theauspices of the predecessor to the International Telecommunication Union (ITU). Manyof the operational procedures for Morse telegraphy established at the turn of the centuryhave been maintained to the present day.

1974 SOLAS Convention

As more detailed regulations became necessary for the shipping industry, the most recent

of International Conventions for the Safety of Life at Sea (SOLAS 1974) was adopted in

1974. the 1974 SOLAS Convention has become one of the main instruments of theInternational Maritime Organization (IMO).

The distress and safety system used by most of the worlds shipping until 1992, asdefined by chapter IV of the 1974 SOLAS Convention and the ITU Radio Regulations,required a continuous Morse radiotelegraphy watch on 500 kHz for passenger ships,irrespective of size, and cargo ships of 1600 gross tonnage and upwards. The Conventionalso required a radiotelephone watch on 2182 kHz and 156.8 MHz (VHF channel) on allpassenger ships and cargo ships of 300 gross tonnage and upwards. Although the systemhas proven itself reliable over many years, its limitations of short range, manual alertingand aural watchkeeping have become a matter of increasing concern. Advances oftechnology led the IMO member governments to develop a new system based on moderntechnology and automation.

The GMDSS

The new system called the Global Maritime Distress and Safety System (GMDSS). Thissystem was adopted by IMO in 1988 and replaces the 500 kHx Morse code system. TheGMDSS provides a reliable ship-to-shore communications path in addition to ship-to-ship alerting communications. The new system is automated and uses ship-to-shorealerting bye means of terrestrial radio and satellite radio paths for alerting and subsequentcommunications. The GMDSS will apply to call cargo ships of 300 gross tonnage andabove, and to all passenger ships, regardless of size, on international voyages.

GDMSS Implementation

The GMDSS requirements for radiocommunications are contained in the new chapter IVof SOLAS 1974 adopted at the GMDSS Conference held in 1988. There is a transitionperiod from the old to the new system in order to allow industry time to overcome anyunforeseen problems in implementation of the new system. The transition period beganon 1 February 1992 continues to 1 February 1999.


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The phased implementation of the GMDSS started with a general requirement for thecarriage of NAVTEX receivers for the reception of maritime and satellite EPIRBs(Emergency Position-Indicating Radio Beacons) from 1 August 1993. during thetransition period, ships operating under the GMDSS will have to comply with the 1988amendments to chapter IV of SOLAS 1974. Until 1 February 1999, both systems will

require watchkeeping on 2182 kHz and VHF channel 16.

Governments have undertaken to ensure that the necessary shore installations will be inplace in order to provide the required communication services.

Digital Selective Calling DSC

DSC Technology provides a method of calling a station or stations using digitaltechniques, and as such forms the basis of GMDSS communications on VHF, MF andHF.

DSC provides automated access to coast stations and ships, in particular, for thetransmission and reception of both routine and distress calls, i.e., it is to be used as theinitial means of contact with other stations.

The DSC system allows for the name of the vessels in distress, the nature of the distressand the last recorded position to be displayed or printed out on receipt of a distress alert.DSC receivers sound an alarm when a distress call is received. Distress priority ship-toshore DSC calls receive priority handling by coast stations and are routed to the nearestRescue Co-ordination Centre (RCC).

Functional requirements

The GMDSS is a largely, but not fully, automated system which requires ships to have arange of equipment capable of performing the nine radiocommunication functions of theGMDSS, viz:

1. transmission of ship-to-shore distress alerts by at least two separate andindependent means, each using a different radiocommunication service;

2. reception of shore-to-ship distress alerts;

3. transmission and reception of ship-to-ship distress alerts;

4. transmission and reception of search and rescue co-ordinating communications;

5.

transmission and reception of on-scene communications;6. transmission and reception of signal for locating;

7. transmission and reception of maritime safety information;

8. transmission and reception of general radiocommunications to and from shore-based radio systems or networks; and

9.

transmission and reception of bridge-to-bridge communications.


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GMDSS GOC .23

Sea areas

The GMDSS is based on the concept of using four marine communications sea areas todetermine the operational, maintenance and personnel requirements for maritime

radiocommunications,:

A1. An area within the radiotelephone coverage of at least one VHF coast station inwhich continuous DSC alerting is available. Such an area could extend typically 30 to 50nautical miles from the coast station.

A2. An area, excluding sea area A1, within the radiotelephone coverage of at least oneMF coast station in which continuous DSC alerting is available. For planning purposesthis are typically extend to up to 150 nautical miles offshore, but would exclude any A1designated areas. In practice, satisfactory coverage may often be achieved out to around400 nautical miles offshore.

A3. An area, excluding sea areas A1 and A2, within the coverage of an Inmarsatgeostationary satellite in which continuous alerting is available. This area lies betweenabout latitudes 760of latitude, but excludes any other areas.

A4. An area outside sea areas A1, A2 and A3. this is essentially the polar regions, northand south of about 760of latitude, but excludes any other areas.

Carriage requirements

Equipment carriage requirements for ship at sea now depend upon the sea in which theship is sailing. (In the past it was only dependant upon the type/or size of the ship).Furthermore, ships operating in the GMDSS are required to carry a primary andsecondary means of distress alerting.

This means having VHF DSC as a primary system for a ship near coastal areas, backedup by a satellite Emergency Position-Indicating Radio Beacon (EPIRB). A ship operatingin an offshore ocean area could have Medium-Frequency DSC, High-Frequency DSC orInmarsat satellite communications as a primary system backed up by a satellite EPIRB.The type of equipment used in the primary system is determined by the sea area in whichthe ship will be navigating.

The carriage requirements are defined in SOLAS chapter IV for the four sea areas. TableS1-1 shows how the SOLAS Regulations would translate into the bare minimum carriagerequirements for the four sea areas. The majority of ships will, however, be fitted with amore comprehensive radio installation.


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GMDSS GOC .24

Table S1-1 Minimum GMDSS carriage requirements

Equipment Sea area

A1

Sea area

A2

Sea area

A3

Sea area

A4

VHF with DSC X X X X

SART (2) X X X XNAVTEX A A A A

EGC receiver B B B B

EPIRB X X X C

VHF portable (2 or 3) X X X X

2182 kHz watch receiver(until 1 February 1999)1

X X X X

2182 kHz 2-tone alarm signalgenerator(until 1 February 1999)1

X X X

MF R/T + DSC X X X

plusInmarsat-A, -B or -C X or

HF R/T with DSC and telex X X

Notes:

A. Required only in those areas where the NAVTEX service is available

B. Required only in those area where the NAVTEX service is NOT available;also, the EGC receive facility is included in the standard Inmarsat-C terminal.

C. 406 MHz COSPAS-SARSAT EPIRB

Maintenance requirements

The means of ensuring the availability of equipment are determined by the sea areas inwhich this ship sails (see chapter IV of SOLAS).

In sea areas A1 and A2, the availability of equipment shall be ensured by one of thefollowing strategies:

(a) duplication of equipment

(b)

shore-based maintenance

(c)

et-sea electronic maintenance(d) or a combination of the above, as may be approvedby the Administration.

In sea areas A3 and A4, the availability of equipment shall be ensured by using acombination of at least two of the above, as may be approved by the Administration.

1The Administration may exempt ships constructed on or after 1 February 1997 from these requirements


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GMDSS GOC .25

Radio Personnel

Regulation IV/16 of the SOLAS Convention requires that:

Every ship shall carry personnel qualified for distress and safety radiocommunication

purpose to the satisfaction of the Administration. The personnel shall be holders of

certificates specified in the Radio Regulations as appropriate, any one of whom shall bedesignated to have primary responsibility radiocommunications during distress incidents.

The provisions of the Radio Regulations require that the personnel of ship stations andship earth stations for which a radio installation is compulsory under internationalagreements 2and which use the frequencies and techniques of the GMDSS shall include atleast:

(a) for station on board ships which sail beyond the

range of VHF coast stations, taking into account the provisions of SOLAS: a holder of afirst-or second-class radio electronic certificate or a general operators certificate

(GOC)

(b) for station on board ships which sail within the

range of VHF coast stations, taking into account the provision SOLAS: a holder of first-or second- class radio electronic certificate or a general operators certificate or a

restricted operators certificate (ROC)3.

The combined effect of the requirements for maintenance and personnel in the four seaarea is that there must be at least one GOC holder on board ships sailing in A2, A3 or A4sea areas. The International Convention on Standards of Training, Certification anWatchkeeping foe Seafares, 1978, as amended in 1995, requires that all deck officers shalhold an appropriate qualification to operate VHF radiocommunication equipment; that is,ROC standard on GMDSS ships or whatever international/national requirementdetermine.

In those cases, particularly in sea area A1, where additional equipment, over and abovethe minimum carriage requirements, is fitted, a higher standard of operator certificationmay also be required in order to ensure that the operator knowledge requirements matchthe actual equipment comprising the radio installation.

EQUIPMENT INTRODUCTION

The exact equipment fitted will include a selection from the following list

2The SOLAS Convention.3An ROC covers only the information of GMDSS equipments required for GMDSS sea area A1, and doesnot cover the operation of GMDSS A2/A3/A4 equipment fitted on a ship over and above the basic A1requirements, even if the ship is in a sea area A1.


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GMDSS GOC .26

VHF Radiotelephone

Operated in the band 156-174 MHz. Duplex channels are available for Ship/Shoreworking and simplex channels for Ship/Ship and routine Ship/Shore calling. Maximumrange around 30-40 nautical miles, dependent upon heights of antennas.

VHF DSC

Operates on channel 70 and is used for both distress alerting and for routine calling.

VHF Portable Two-way

Radiotelephones

Required for emergency communications from survival craft.

SART

Search and rescue radar transpoder operating on the 3 cm radar X-band (9.3-9.5 GHz).Used to help search and rescue (SAR) units to locate survivors.

NAVTEX receiver

Used to receive maritime safety information (MSI) automatically by means of narrow-band direct printing from selected stations, using 518 kHz, 490 kHz and 4209.5 kHz.

EPIRBs

Satellite emergency position-indicating radiobeacons operate on 406 MHz (including121.5 MHz for homing by rescue aircraft) through the COSPAS-SARSAT network andon 1.6 GHz (L-band Inmarsat-E) through the Inmarsat network. DSC EPIRBs operating

on VHF channel 70 may be used in sea areas A1. EPIRB transmission serve to identifythe ship in distress, to inform the RCC of a distress incidents and to help to determine theposition of survivors.

NoteEPIRB transmissions are regarded as a distress alert

MF/HF DSC

Used to monitor the DSC distress frequencies in the 2, 4, 6, 8, 12 and 16 MHz bands.Also for routine calling or replying on the 2, 4, 6, 8, 12, 16, 18, 22 and 25 MHz bands.

MF/HF transceiverWith full R/T and telex facilities on all the Marine bands.

NoteThe DSC unit uses this equipment in order to transmit and to await a reply to aroutine call

Inmarsat-A/B

Used for voice, telex, data, video and facsimile communications.


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GMDSS GOC .27

Inmarsat-C

Provides telex, data, E-mail and polling on a store-and forward basis. Usuallyincorporates an EGC (Enhanced Group Call) receiver for the automatic reception ofmaritime safety information via the International SafetyNET service.

2182 kHz Watchkeeping Receiver

Receiver, with a muted loudspeaker, which is used to listen for the two-tone alarm, uponreception of which the mute is lifted to enable the distress call and message to be heard.

2182 kHz Radiotelephone

Alarm Signal Generator

Fitted into the MF R/T transceiver, it produces the two-tone alarm signal for 1 minute toalert others that a distress call and message is about to follow.


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GMDSS GOC .28


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GMDSS GOC .29

SECTION 2

BASIC CONCEPTS OF THE GMDSS


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GMDSS GOC .30


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GMDSS GOC .31

S2. BASIC CONCEPTS OF THE GMDSS

Functional requirements

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

transmitting ship-to-shore Distress Alerts by at least two separate andindependent means, each using a differentradio communication service;

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

ship Distress Alerts; transmitting and receiving search and rescue co-ordinating

communications;

transmitting and receiving on-scene 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.

Application

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 mandatoryfor such vessels as at February 1, 1999.

Commercial vessels under 300 GRT, or those above 300 GRT engaged on domesticvoyages only are subject to the requirements of their Flag State. Some Flag States haveincorporated GMDSS requirements into their domestic marine radio legislation - howevermany have not.

Equipment and Operational requirements GMDSS zones

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

Because the various radio systems used in the GMDSS have different limitations withregards to range and services provided, the new system divides the world's oceans into 4areas:


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GMDSS GOC .32

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

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 latitude70 degrees south); and Area A4 comprises the remaining sea areas outside areas A1, A2 and A3 (the

polar regions).

GMDSS communication systems

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

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

Sea Area A2 requires medium range services - Medium Frequencies (MF - 2 MHz) areused 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 - 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 bothoperating in the same area....this is a marked change from the pre-GMDSS systems.

This is illustrated in the diagram below:


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GMDSS GOC .33

Fig1.1 .The GMDSS Concept

GMDSS operational requirements

General

The GMDSS enables a ship in distress to send an alert using various radio systems. Thesesystems are designed such that the alert has a very high probability of being received byeither shore rescue authorities and/or other vessels in the area.

Equipment performing GMDSS functions must be simple to operate and (whereverappropriate) be designed for unattended operation.

Distress Alerts must be able to be initiated from the position from which the ship isnormally navigated (ie; the bridge).

EPIRBs are required to be installed close to, or capable of remote activation from theposition from which the ship is normally navigated.

Equipment to be carried

The SOLAS GMDSS regulations are structured such that all GMDSS ships are requiredto carry a minimum set of equipment, with (basically) more equipment being required thefurther the ship travels from land.

The SOLAS GMDSS regulations do not make particularly easy reading - a simplifiedversion of the equipment required to be carried for each sea area is detailed below.

Minimum requirements


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GMDSS GOC .34

GMDSS ships are required to carry the following minimum equipment:

A VHF radio installation capable of transmitting DSC on channel 70, and

radiotelephony on channels 16, 13 and 6. One SART if under 500 GRT, 2 SARTs if over 500 GRT. Two portable VHF transceivers for use in survival craft if under 500 GRT,

three if over 500 GRT. A NAVTEX receiver, if the ship is engaged on voyages in any area where

a NAVTEX service is provided. An Inmarsat EGC receiver, if the ship is engaged on voyages in any area

of Inmarsat coverage where MSI services are not provided by NAVTEXor HF NBDP (see note 1).

A 406 MHz or 1.6 GHz EPIRB

Note 1 - in practice, this means that all GMDSS A3 and A4 vessels are required to carryat least one Inmarsat C system.

Radio equipment - Sea area Al

Every ship engaged on voyages exclusively in sea area A1 shall be provided with theminimum equipment specified previously, with the option to replace the 406 EPIRB witha VHF DSC EPIRB.

Fig 1.2 VHF Radiotelephone

Radio equipment - Sea areas A1 and A2

Every ship engaged on voyages beyond sea area A1, but remaining within sea areaA2, shall be provided with the minimum equipment specified previously, plus:

An MF radio installationcapable of transmitting and receiving on thefrequencies 2187.5 kHz using DSC and 2182 kHz using radiotelephony;

a DSC watchkeeping receiveroperating on 2187.5 kHz

A 406 MHz EPIRB


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GMDSS GOC .35

Fig. 1.3. Typical GMDSS A2 station

The ship shall, in addition, be capable of transmitting and receiving generalradiocommunications using radiotelephony or direct-printing telegraphy by:

A HF radio installationoperating on working frequencies in the (marine)

bands between 1,605 kHz and 27,500 kHz. (This requirement is normallyfulfilled by the addition of this capability in the MF equipment referred to

earlier).

Radio equipment - Sea areas A1, A2 and A3

These vessels have two options to satisfy their GMDSS requirements. The options allow a

vessel to choose from theprimary method to be used for ship-shore alerting;

Every ship engaged on voyages beyond sea areas A1 and A2, but remaining withinsea area A3shall be provided with the minimum equipment specified previously, pluseither:

An Inmarsat C ship earth station : An MF radio installation and 2187.5 kHz DSC watchkeeping receiver; A 406 MHz EPIRB

or

An MF/HF radio installationcapable of transmitting and receiving on all

distress and safety frequencies in the (marine) bands between 1,605 kHzand 27,500 kHz: using DSC, radiotelephony; and NBDP

An MF/HF DSC watchkeeping receivercapable of maintaining DSC

watch on 2,187.5 kHz, 8,414.5 kHz and on at least one of the distress and

safety DSC frequencies 4,207.5 kHz, 6,312 kHz, 12,577 kHz or 16,804.5kHz; at any time, it shall be possible to select any of these DSC distressand safety frequencies

A 406 MHz EPIRB

An Inmarsat ship earth station


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GMDSS GOC .36

Fig 1.4 Typical GMDSS A3 station

In addition, ships shall be capable of transmitting and receiving generalradiocommunications using radiotelephony or direct-printing telegraphy by an MF/HFradio installation operating on working frequencies in the (marine) bands between 1,605kHz and 27,500 kHz. This requirement is normally fulfilled by the addition of this

capability in the MF/HF equipment referred to earlier.

In practice, MF only transceivers are not produced - all marine MF radio equipment isalso fitted with HF facilities.

Radio equipment - Sea areas Al, A2, A3 and A4

In addition to carrying the equipment listed previously, every ship engaged onvoyages in all sea areasshall be provided with:

An MF/HF radio installationas described earlier

An MF/HF DSC watchkeeping receiveras described earlier A 406 MHz EPIRB

In addition, ships shall be capable of transmitting and receiving generalradiocommunications using radiotelephony or direct-printing telegraphy by an MF/HFradio installation as described earlier

Means of ensuring availability of ship station equipment

Regulation 15 of the SOLAS GMDSS regulations defines 3 methods to ensureavailability of GMDSS equipment at sea;

At sea electronic maintenance, requiring the carriage of a qualifiedradio/electronic officer (holding a GMDSS First or Second class Radio-Electronics Certificate) and adequate spares and manuals;

Duplication of certain equipment; or Shore based maintenance


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GMDSS GOC .37

Ships engaged on voyages in sea areas A1 and A2 are required to use at least one of thethree maintenance methods outlined above, or a combination as may be approved by theiradministration. Ships engaged on voyages in sea areas A3 and A4 are required to use atleast two of the methods outlined above.

And of course what all that means is that 99% of A3 GMDSS ships, along with probably100% of A1 and A2 GMDSS ships do not opt for at sea maintenance - they eitherduplicate the equipment and use shore based maintenance (for A3 ships), or use shorebased maintenance only (A1 and A2 ships).

Equipment to be duplicated for area A3 vessels

GMDSS ships operating in A3 areas are required to provide the following duplicatedequipment;

Two complete VHF installations (including DSC), and either;

Two complete Inmarsat C systems and one MF radio system, or; One complete Inmarsat C system and one complete MF/HF radio system(including a scanning DSC receiver and NBDP equipment).

Many GMDSS ships opt for the latter option (1 Inmarsat C and one MF/HF DSCsystem), on cost grounds. Unfortunately, this has proven to be one of the underlyingcauses of the present extremely high false alerting rate on some GMDSS systems.

Power supply requirements

GMDSS equipment is required to be powered from three sources of supply:

ship's normal alternators/generators; ship's emergency alternator/generator (if fitted); and a dedicated radio battery supply.

The batteries are required to have a capacity to power the equipment for 1 hour on shipswith an emergency generator, and 6 hours on ships not fitted with an emergencygenerator.

The batteries must be charged by an automatic charger, which is also required to bepowered from the main and emergency generators.

Changeover from AC to battery supply must be automatic, and effected in such a waythat any any data held by the equipment is not corrupted (ie: "no break").


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GMDSS GOC .38

Operator qualifications

There are a number of different types of GMDSS qualifications, as follows:

First Class Radio-Electronic Certificate;

Second Class Radio-Electronic Certificate; and GMDSS General Operator's Certificate

The First and Second Radio-Electronic Certificates are diploma and associate diplomalevel technical qualifications. They are designed for Ship's Radio-Electronic Officers,who sail on GMDSS ships which use the option of at-sea electronic maintenance.

The GMDSS General Operator's Certificate is a operator qualification, designed forNavigating Officers.

Survival Craft Radio Equipment

Search And Rescue (Radar) Transponders (SARTs)

SART is a self contained, portable and buoyant Radar Transponder (receiver andtransmitter).

SARTs operate in the 9 GHz marine radar band, and when interrogated by a searching

ship's radar, respond with a signal which is displayed as a series of dots on a radar screen.

Fig. 1.5. SART

Although SARTs are primarily designed to be used in lifeboats or liferafts, they can bedeployed on board a ship, or even in the water.


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GMDSS GOC .39

SARTs are powered by integral batteries which are designed to provide up to 96 hours ofoperation.

Operation

When activated, a SART responds to a searching radar interrogation by generating aswept frequency signal which is displayed on a radar screen as a line of 12 dots extendingoutward from the SARTs position along its line of bearing.

The spacing between each dot is 0.6 nautical miles.

As the searching vessel approaches the SART, the radar display will change to wide arcs.These may eventually change to complete circles as the SART becomes continuallytriggered by the searching ship's radar.

Fig. 1.6 SART signal on radar display

Although not an actual SART response, this radar picture gives an impression of how aSART signal would be displayed

Some slight position error will also be caused by the SART switching from receive totransmit mode.

SARTs will also provide a visual and audible indication to users when interrogated by asearching radar.

Range

The range achievable from a SART is directly proportional to its height above the water.

A SART mounted at 1m (ie: in a liferaft) should be able to be detected at 5 nautical milesby a ship's radar mounted at 15m.


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GMDSS GOC .40

The same SART should be able to be detected at 30 nautical miles by an aircraft flying at8000 feet.

GMDSS carriage requirements

GMDSS vessels from 300 to 500 GRT are required to carry 1 SART, and vessels over500 GRT are required to carry 2.

Portable VHF transceivers

These units are designed to allow communications between searching vessels andsurvivors in liferafts. They operate on the VHF marine band in voice mode. DSCcapability is not fitted.

Performance standards

The IMO performance standard requires that the equipment:

provide operation on VHF channel 16 (the radiotelephone distress and

calling channel) and one other channel be capable of operation by unskilled personnel be capable of operation by personnel wearing gloves

be capable of single handed operation, except for channel changing withstand drops on to a hard surface from a height of 1 metre be watertight to a depth of 1 metre for at least 5 minutes, and maintain

watertightness when subjected to a thermal shock of 45 degrees Celsius.

not be unduly effected by seawater or oil

have no sharp projections which could damage survival craft be of small size and weight be capable of operating in the ambient noise level likely to be encountered on

board survival craft have provisions for attachment to the clothing of the user be either a highly visible yellow/orange colour or marked with a surrounding

yellow/orange marking strip be resistant to deterioration by prolonged exposure to sunlight


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GMDSS GOC .41

Fig. 1.7. Typical GMDSS VHF portable transceivers

GMDSS carriage requirements

GMDSS vessels from 300 to 500 GRT are required to carry 2 VHF portables, and vesselsover 500 GRT are required to carry 3.


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GMDSS GOC .42

REVISION QUESTIONS

1. How would you define:

(a)Sea area A1

(b)

Sea area A2

(c)

Sea area A3

(d)

Sea area A4?

2. State three EPIRB frequencies.

3. What is the 2 MHz band DSC distress/safety frequency?

4. Which channel is used for DSC distress and calling on VHF?

5. On which HF DSC frequency must a watch always be maintained?

6. On which frequency is NAVTEX transmitted?


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GMDSS GOC .43

SECTION 3

RADIO WAVE PROPAGATION


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GMDSS GOC .44


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GMDSS GOC .45

S3. RADIO WAVE PROPAGATION

Radio PathsThis topic area deals with the path taken by a radio wave when it leaves the transmitting

antenna. The main factor which determines the path taken is the frequency or wavelengthof the transmission.

Radio waves travel at the velocity of light, 300 x 106metres per second. The relationship

between the velocity of light (c), frequency (f), and wavelength () is :

=

cf

i.e., longer wavelength corresponds to lower frequency, shorter wavelength to higherfrequency.

Need for Radio

The radio waves is needed to carry the signal information efficiently and withoutdistortion. In the case of audio frequencies, which may range from about 50 Hz to 15kHz, it would not be technically feasible to radiate the information directly from apractical transmitter and antenna.

Higher frequencies can be radiated efficiently from antennas having dimensions typicallybetween a quarter and one wavelength. Thus, practical communication systems use aradio wave to carry the audio or other (e.g., vision or data) information between thetransmitting and receiving sites.

The Radio Spectrum

Practical transmitter and radiating systems can be realized for radio waves withfrequencies above 15 kHz. The radio frequency spectrum is divided into several majorband :

- Very Low Frequency 15 kHz to 30 kHz VLF- Low Frequencies 30 kHz to 300 kHz LF- Medium Frequencies 300 kHz to 3 MHz MF- High Frequencies 3 MHz to 30 MHz HF- Very High Frequencies 30 MHz to 300 MHz VHF

-

Ultra High Frequencies 300 MHz to 3 Ghz UHF- Super High Frequencies 3 GHz to 30 GHz SHF- Extra High Frequencies 30 GHz to 300 GHz EHF

Propagation

Mechanisms

Three main physical mechanism govern the propagation of radio waves from transmitterto receiver in maritime radio communications :


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GMDSS GOC .46

- Line of sight- Ground wave- Sky wave

The relative importance of each mechanism in establishing and maintaining reliable

communication by radio depends on the frequency and the distances involved.

Line of SightPropagation

Above about 50 MHz, propagation is essentially by line of sight. This isaccomplished, in the case of terrestrial radio, via the lower part of the atmosphere termed the troposphere and in the case of space communication via earth orbitingsatellites.

Figure S3 1 shows a stylised terrestrial radio link. In general, the received signal is the

sum of a direct signal along path a, clear of the ground, and several reflected signalsalong path such as b and c. Because a radio signal undergoes a phase reversal at thereflection point, the theoretical situation is that the direct and reflected signals shouldcancel out if the receiver antenna is at ground level. Since land has a poor groundconductivity, total cancellation does not occur in practice, as a simple experiment with aportable VHF FM receiver will show. However, the sea is a very good conductor, whichmeans that maritime VHF antennas should be mounted well above the sea in order toavoid severe cancellation effects.

Ground Wave Propagation

In principle, a transmitting antenna sited at the earths surface will set up a surface wavewhich follows the curvature of the earth. The distance over which reliablecommunication can be achieved by the surface, or ground wave, depends on thefrequency and the physical properties (i.e., ground conductivity and dielectric constant)be established with useful efficiency where the wavelength is greater than several tens ofmetres.

Seawater has highest conductivity and will support the propagation of a ground wavewith very little attenuation, in much the same manner as a metal plate. At the other endof the scale, an arid desert provides very lossy ground conditions and will not support theefficient propagation of a ground wave signal.

The significance of this form maritime communications is that long distance working ispossible at medium to low frequencies using only modest transmitter powers compared tothose for broadcasting at similar frequencies over land.


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GMDSS GOC .47

Sky Wave Propagation

Within the frequency range of 1 30 MHz, ionospheric reflection is the controlling factorin achieving long distance communications by radio waves.

Because the ionization processes in the upper atmosphere that is responsible for thiseffect is caused by the sun, it will be evident that the density of ionization will vary withthe time of day and the season of the year. The sunspot cycle, which takes approximately11 years, also has an effect. Ionospheric storms and other disturbances occur from timeto time and in extreme cases can cause a communication black-out lasting for somedays.

In general, the net result is that, to communicate over a given distance, a higher frequencyis necessary when the density of ionization is high and a lower frequency when thedensity of ionization falls.

The Ionosphere

Long distance propagation of radio waver at HF is mainly the result of single ormultiple reflections from ionized regions in the upper atmosphere known collectively asthe ionosphere. These ionized regions are generated at heights of 100 400 km (55 220 nautical miles) as a result of partial ionization of the molecules making up therarefied upper regions by ultraviolet and soft (long wavelength) x-ray solar radiation.The ionization process converts the molecules into a plasma of ions and free electrons.

There is a complex variation in the degree of ionization with height such that distinctlayers of more intense ionization are formed. The different layers result from differentparts of the ultraviolet spectrum. The heights of these layers vary from day to night andwith the seasons.

The most important layers for long distance propagation of radio waves are :-

the E-layer at 120 km- the F1 layer at 200 km- the F2 layer at 300 400 km.

At night and at mid winter the F1 and F2 layers combine to form a single F-layer at 250km. This is a result of a gradual recombination of the ions and electrons back into theatmospheric gas molecules during the night.

Below the E-layer is the D-layer, at a height of 50 90 km, which also has an influenceon propagation, but more as an absorber of radio waves than as a reflecting layer.However, at VLF and LF frequencies the D-layer is sufficiently reflective to guidesignals between the ground and the bottom of the D-layer for several thousand kilometreswith little attenuation.


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GMDSS GOC .48

Ionospheric reflection may be simply described as the phenomenon whereby a waveappears to undergo reflection on reaching a suitably ionized region. Free electrons are setin motion so as to re-radiate the wave in a changed direction. At it passes through theionized layers, the wave may eventually be reflected back to the earth. On a simplifiedview the effect may be viewed as reflection from an area at what is termed the mirror

height.

The effect is frequency dependent, with a greater degree of ionization being necessaryto cause reflection as the frequency is increased. Usually the higher layers have thegreater degree of ionization and therefore reflect the highest frequencies. Because of thegreater mirror height, the communication range achieved by a single reflection will alsobe greatest under these circumstances.

The solar radiation responsible for ionizing the atmosphere varies continuously from daya night and between the seasons. Sunspot activity also has a strong underlying effect onthe degree of ionization. The level of sunspot activity varies over a cycle of around 11

years, with periods of maximum ionization occurring when the number of sunspot is at amaximum.

Normally, the variation is predictable enough for the best frequency bands to be selectedfor the intended communication path without difficulty.

Ionospheric Disruptions

HF communications can, however disrupted by ionospheric stormsfor several days at atime when eruptions on the suns surface emit a stream of high energy charged particleswhich then obliterate the ionized layers the F-region in particular. Auroral displays inthe polar regions often accompany these events.

Ionospheric storms are often preceded by sudden ionospheric disturbances (sids)whenintensely strong bursts of ultraviolet radiation from the sun produce intense ionization ofthe low D-layer. When sids occur, waves are absorbed in the D-layer before reaching thehigher layers or are reflected over much shorter distances than usual, with the result thatlong distance communications will be blocked for hours at a time.

Circuit Reliability

In normal circumstances the selection of the optimum frequency for establishing andmaintaining communications is governed by the following considerations.

Maximum Usable Frequency

The maximum frequency which is reflected by the ionosphere over any particular path isknown as the maximum usable frequency (MUF). The MUF depends on:


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GMDSS GOC .49

(a) time of day ;(b) season ;(c) latitude; and(d) period of sunspot cycle.

The MUF varies according to which layer is responsible for reflection back to the earth.For each layer, the highest MUF is obtained when the ray path leaves the earthtangentially, so that the ray approaches the appropriate layer at as oblique an angle aspossible. As shown in Figure S3-2, this corresponds to an overall ground-to-grounddistance of about 4000 km (2200 nautical miles) for F2-layer propagation (path A); or2500 km (1300 nautical miles) for E-layer (path B). Any rays leaving the earth at ahigher angle of elevation (path C) will penetrate the layer and not be reflected. To usesuch ray angles, with consequently shorter path, it is necessary to reduce the operatingfrequency (path D).

In general, the strongest signals (i.e., those with least attenuation) will occur using

frequencies just below the MUF, for the particular path distance and layer involved.

When a wave is sent vertically upwards (see Figure S3-3), the highest frequency forwhich reflection by any particular layer will occur is termed the critical frequency, f0.This frequency is much lower than the MUF =f0/ cos A is the angle of incidence of theray to the layer. At frequencies higher than f0, the waves will penetrate the layer and belost, but as the angle of radiation is progressively lowered an angle be reached wherereflection occurs (termed the critical wave angle). Signals can then be received at a greatdistance (receiver Rx2 in Figure S3-3), and radiation at lower angles will be reflected toeven greater distances (e.g., receiver Rx3).

At points nearer to the transmitter no signals will be received by ionospheric reflection,but when sufficiently close to the transmitter (receiver Rx1 in Figure S3-3) to be withinrange of the ground wave the signals will again be heard. In between there is an area ofvery poor reception, termed the skip zone. The distance from the transmitter to thenearest point at which a wave at a particular operating frequency returns, after reflection,back to the earth (receiver Rx2) is known as theskip distance.

When the frequency is less than the critical frequencyf0there will, of course, be no skipat all. This situation is often found for frequencies below 8 MHz.

The critical wave angle for a particular layer depends on the operating frequency anddecreases as the frequency increases. In consequence, the skip distance increases withfrequency. The MUF therefore represents a limit which must not be exceeded for thereceiver to remain in the area of reception just beyond the skip zone. The result is thatthe skip distance extends towards the receiver as the operating frequency approaches theMUF. The reflecting layer also absorbs HF radiation, and this effect decreases markedlyas the operating frequency approaches the MUF.

The combined effect is that, for any particular radio circuit, the optimum working


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frequency lies just below the MUF for the particular path. Any rise in operatingfrequency or fall in MUF will result in a sudden drop-out of received signals as the skipzone extends to include the reception point.

Lowest Usable Frequency

As the operating frequency is reduced, the reflection will occur in the lower layers of theionosphere. However, at lower altitudes, and in the D-layer especially, the energy in thewave is subject to increased absorption caused by collisions between air molecules andelectrons which are set in motion by the radio wave. The effect increases at lowerfrequencies, and the limit for any particular path is reached at the lowest usable frequency(LUF).

While the MUF is determined solely by the physical properties of the ionosphere, theLUF also has dependence on the radiated power and the receiver sensitivity over the

circuit, and can be controlled to an extent by attention to optimizing equipment andantenna performance hence the need to keep both equipment and antennas in goodcondition.

Single Hop Condition

An HF radio circuit can also be set up by multiple reflections between the ionosphere andthe ground. Variability and absorption increase with each reflection (or hoop), so single reflection (hop) path, as described above, is to be preferred for maximum circuitreliability.

To avoid multiple hop conditions it is advisable to aim for the MUF for the highestionospheric layer, in the expectation that this will normally exceed the MUF for the lowerlevels and thereby avoid multiple reflections involving the lower layers.

Optimum Traffic Frequency (OTF)

Ionospheric absorption is much less at night than during the day and therefore theattenuation of the lower HF frequencies is very little different from that of higherfrequencies during the day. Since the MUF at night over a particular path will generallybe less than half the daytime figure, this means that for night-time long-distancecommunications it is possible to maintain considerably lower frequencies and stillachieve good reliability.The MUF for a particular path is higher during the summer months than in the wintermonths, but during ionospheric storms the MUF may become much lower fortransmissions in some directions but higher in other directions.

In planning the optimum traffic (or working) frequency for any particular time, season,distance and direction, it is therefore necessary to take all of these variations into account.


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At any particular time, a sky-wave path is available on channels in a window below theMUF and above the LUF. The MUF is defined by the prevailing ionospheric conditions,but the LUF is set by a combination of path loss and equipment parameters such astransmitter power, noise and receiver antenna performance. In practice, the first choiceof working frequency for sustained circuit reliability would be around 85% of the MUF.

The MUF can be predicted on a long term average basis. The variations in MUF can beup to a third higher or lower on a normal day-to-day basis and, in disturbed conditions,the MUF can be less than half the predicted value.

The LUF is typically about half the MUF for maritime HF equipment, but this can varyconsiderably.

Under normal conditions, the window of available frequencies varies predictably asfollows :

- daytime MUF is higher than night-time MUF;

-

winter MUFs are both lower than and vary more than summer MUFs;-

radio circuits less than 1000 km (600 nautical miles) normally use frequencies

bellow 15 MHz;- radio circuits greater than 1000 km (600 nautical miles) normally use

frequencies above 15 MHz ; and

- MUFsare higher when the sunspot number is high.

FREQUENCY BANDS AND PROPAGATION

The relationship of the different propagation mechanisms to the different frequency bandsis outlined below.

VLF

The radio wave follows the curvature of the earths surface and is known as a groundwave. The range of a ground wave signal is governed by the rate of loss of energy intothe ground, which in turn is governed by the value of ground conductivity. Theattenuation of the ground waves is least over seawater and greatest over dry rocky groundor deserts.

VLF signals are reflected well by the D-layer of the ionosphere and, because the height ofthe D-layer is of the same order of wavelengths at VLF, the net effect is of a waveguidefor VLF signals between the ground and the D-layer. The signal attenuation is very lowunder these conditions and transmission paths up to 12000 nautical miles are possible.

Large antenna arrays are normally used at VLF with very high output transmitter powers(750 kW) to give virtually world-wide coverage. VLF transmissions are therefore only


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used in the shore to ship direction. VLF signals penetrate the sea to a depth of a few tensof metres, making them very effective for maintaining communications with submergedsubmarines.

LF

At LF, ground-wave propagation predominates, as with VLF, but, due to the higherfrequency, the range is reduced, particularly over land, due to the relatively greaterattenuation effect of poor ground conductivity as the wavelength is reduced, particularlyover land, due to the relatively greater attenuation effect of poor ground conductivity asthe wavelength is reduced. The wave-guide effect between the ground and the D-layerstill applies at LF, and conditions are, in fact, more stable than at VLF. There is also animprovement as regards lower background noise levels at LF. However, the pathattenuation is higher.

Ranges of one two thousand nautical miles are possible at LF but, again, large antennastransmitter output powers are required.

MF

MF communications also depend mainly on ground-wave propagation but with a futherreduction in range because of the increased effect of attenuation by the earth. However,sky-wave propagation starts to become significant at MF, particularly at night, greatlyextending the range. This can be a negative effect, however, owing to mutualinterferencebetween stations on the same frequency, and interference fading caused bysignals arriving at the receiver by different paths from the transmitting station.

A coast station can achieve good ground-wave coverage for voice communications up to300 nautical miles. Ship stations, with less powerful transmitters and less elaborateantenna systems, can usually expect reliable ground-wave communications up to 150nautical miles for voice communications and 300 nautical miles for DSC/telex.

HF

In practice, a good guide to establishing reliable communication at HF is to monitor thetelex4(NBDP) channels of the wanted coast station on the more likely bands for the timeof day and season and then to call the station on whichever band provides a strong stablesignal. If this in not successful, the other bands should be tried. The ionosphere canbehave erratically at times and, on occasion, reception is better in the ship-to-shoredirection than in the shore-to-ship direction or vice versa. Communication is frequentlyunreliable around sunrise and sunset.

The considerable variability of radio communication at HF is a consequence of signalpropagation being predominately by sky wave, both day and night. A ground wave

4Most coast stations emit an idling signal on their HF telex channels when no traffic is present.


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signal is still present but attenuates too rapidly to be value for reliable commercialcommunications.

The D-layer of the ionosphere has little effect above 4 MHz and long-distancepropagation is by reflection from the E- or F-layers. In general terms, the higher the HF

band used, the greater the range. This is because the higher the frequency, the further thewave has to pass into the ionosphere before it undergoes sufficient bending to be returnedto earth. To a first approximation, therefore, the situation is that the higher the frequency,the greater will be reflection (mirror) height and so the greater will be the potential range.

Long-range propagation is also possible as a result of multiple reflection between theground, the ionosphere and even between the layers of the ionosphere itself. However,these modes of transmission are very variable and would not be used intentionally fornormal commercial communications.

The best policy for reliable HF communications is to use the highest frequency consistent

with the length of the radio circuit using a single reflection. The angle at which a radiowave enters the ionosphere is also an important factor, with reflection occurring at alower height for oblique incidence compared to vertical incidence (see Figure S3-3).

The highest frequency which can be used to communicate between two fixed points bysky-wave propagation is known as the maximum usable frequency, MUF. Since thisfrequency puts the receiver on the edge of the skip distance, it is better to use the lowerfrequency of 0,85 x MUF, termed the optimum traffic frequency, in order to improvereliability. Note, however, that theh preferred choice of channel may already be in use.

For example, to establish communications with Portishead Radio (United Kingdom)during the daytime, the following would apply :

3 MHz = N. France6 MHz = N. Spain8 MHz = N. Africa

12 MHz = Ghana16 MHz = Angola

22/25 MHz = South Africa

At night, due to changes in the ionosphere, the situation changes as the F1 and F2 layersmerge and the heights of the E and F layers fall. The general result is tahta, to cover thesame range at night it is necessary to halve the operating frequency; e.g., a link fromPortishead to Capetown during daytime is possible on 22/25 MHz, but during the nightthe 12 MHz bands would be the first choice.

When transmitting east west, the signal may pass from daytime to night-timeconditions, and it may be very difficult to establish effective communications. Onestrategy is to estimate the optimum transmission band according to the day/nightconditions at the midpoint of the radio circuit. The best course of action may be to waituntil the entire path between the two stations is in daylight or darkness.


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Above 50 MHz the predominant propagation mechanism is by straight-line paths, i.e.line-of-sight.

For satellite communications an unobstructured view of the satellite is required, and the

Ship Earth Station antenna must mounted to achieve the best view possible.

For terrestrial communication the range depend upon the heights of both the transmittingand receiving antennae.

Because of a slight bending effect on radio waves in the troposphere, caused mainly bywater vapour, the radio horizon is in fact greater than the optical horizon by a factor of4/3.Taking this factor into account, the maximum range at sea is given by the formulae:

( ) ( )

( ) ( )

( ) ( )

4

2,22

4,12

x x

x x

x x

Range in NM T ft R ft

Range in NM T m R m

Range in km T m R

= +

= +

= +

where Tx and Rx are the heights of the transmitting and receiving antennae above sealevel, measured in feet or metres as indicated.

TRANSMITTER ANTENNA RECEIVER

ANTENNA

a

b c

S~

radio paths

Fig. 3.1 Surface wave


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Figure 3.1 also shows a surface wave S propagating over a terrestrial radio link. Inprinciple, the received signal will be the sum of the line of sight signals and thesurface wave. In practice, however, one or other of the two components will predominatedepending on the transmission frequency and length of the radio link. Ground wavepropagation predominates at MF, LF and VLF.

Figure S3.2 Sky wave radio paths; Figure S3.3 HF communication paths


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SECTION 4

BASIC TRANSMITTERS AND RECEIVERS


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S4. BASIC TRANSMITTERS AND RECEIVERS

Basic Transmitter

The radio frequency generator produces the carrier, i.e., the frequency on which we wishto transmit.

The modulator is used to combine the information signals from the microphone or thetelex with the carrier. The type of modulation may be amplitude (AM), frequency (FM)or phase (PM). This modulated signal is then amplified within the transmitter and fed tothe antenna.

The antenna requires tuning to carrier frequency so that it will radiate efficiently.Antennas made from wire elements radiate most efficiently when they are one quarter of

a wavelength long.

It is not practicable on board ships to install an antenna which is physically the ideallength over all of the MF or HF bands. However, the electrical length of the antenna canbe lengthened or shortened with respect to its physical length by the introduction of extraradio frequency circuit elements, inductors and capacitors, in an Antenna Tuning Unit(ATU).

In most modern equipment, this is achieved automatically by pressing the buttonbefore actual transmission. A signal strength meter which measures antenna current

gives a visual indication of transmission. Most equipment allows forManualtuningmode on 2182 kHz in case the automatic tuning fails. Individual manufacturers manualsshould be consulted for further details. The default 2182 kHz setting need only be carriedout upon installation or if your antenna is moved or changed

Antenna

Radio FrequencyGenerator Modulator Amplifier

OscillatorSynthesiser

Mic Telex TelexMic Amp Modem Unit

Fig. 4-1 Basic transmitter block diagram


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Antenna A.G.C.

R/F Tuned Frequency I.F. Amps AudioAmplifier Changer Demodulator Amp

R/F Gain Tune Synthesiser Mode A/F Gain

Squelch

Fig. 4-2 Basic receiver block diagramBasic Receiver

The wanted signal is received by tuning the input to the receiver to the wanted frequency.Received signals vary greatly in strength due to a number or factors, e.g.,

(a) A local transmitter radiating high or low power.

(b) A distant station radiating high or medium power.

(c) Variations in the ionosphere which may affect signals on MF at night or on HF at anytime polarisation fading.

(d) Simultaneous reception by ground and sky waves on MF at night which mayconstantly vary in strength or phase and interact with each other interference fading.

(e) On the HF bands, signals can reach the receiver having taken different paths, againcausing interference fading.

The radio frequency or

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