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Inmarsat-C System Operation: From IMN to SCADA

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To complement the Inmarsat-A: Equipment, Procedures, and Distress CommunicationInmarssat-A system, the Inmarsat-C system was introduced in 1991. This system provides low-cost global communication, using a small lightweight terminal. The system also provides a wide range of Terrestrial Urgency and Safety Communications on the Vesselscommunication services, including two-way store and forward messaging, distress alerting, EGC broadcasting and data reporting, polling and SCADA.

The Inmarsat-C system also complies with the GMDSS, and has become a mandatory requirement for passenger ships and Merchant Shipping Safetymerchant ships greater than 300 gross tonnes that makes international voyages.

System operation

The Inmarsat-C system lacks the ability of voice communication, but does provide a means of sending text messages and data communication to and from ships via the Inmarsat satellite system.

This system is based on digital technology, which means that anything that can be encoded into digital data, whether it be text typed in, numeric data read from instruments, or other information in digital form, can be Basic transmitters and receivers in shippingtransmitted and received by the system. (The Inmarsat-A system is based on analog technology).

Passenger ships
The Inmarsat-C system is mandatory for international passenger ships and merchant ships over 300 gross tons
Source: AI generated image

The basic technique used for sending a message via Inmarsat-C is called store-and-forward messaging. This means that the messages to or from ships are received and stored at the CES before being automatically transmitted to a ship or to a telex/datapak/facsimile subscriber. There is no direct connection between the ship and the called subscriber.

The message is encoded and sent in a series of data packets to an Inmarsat-C CES. The CES acts as an interface between the satellite link and the national/international telecommunication network. The CES stores the message briefly before forwarding it. The time a ship-to-shore or shore-to-ship message requires from being transmitted to being received at its final destination, is typically 3-6 minutes. If the CES receives any errors in the datapaks, it signals back to the SES to retransmit the packets in question, and the procedure repeats itself until the CES has received the complete message without the errors.

Inmarsat-C Equipment

The installations consist of an omnidirectional antenna placed above deck. The antenna must be mounted as high and free as possible to provide it with an unobstructed view from the sky to the horizon in all directions. It must also be able to maintain line-of-sight communication with the selected satellite. (This type of antenna has no moving parts, unlike the much larger Inmarsat-A antenna. The antenna’s view to a satellite is not affected by atmospheric conditions such as rain or clouds).

Read also: The Structure, Functions and Details of Professional Marine Radio Communication

The below-deck equipment consists of a transceiver, a Personal Computer (PC), a printer and a power supply. If the ship is fitted with a GPS navigator or other navigational equipment, it is recommended that the navigator is interfaced to the Inmarsat-C terminal. Thus the position of the ship is always updated.

Inmarsat-C
Inmarsat-C Ship Earth Station

Ships lacking the possibility of automatic position updating, should update the position manually at least once every four hours.

The Inmarsat-C system

Any of the following devices may be land-based subscribers connected via the national or Empowering Global Communication with INMARSAT Satellites in shippinginternational telecommunication network:

  • a telex connected to the international telex network.
  • a computer connected to the international data networks (PSDN) or the x.25 or x.400 networks.
  • a computer connected to the international telephone networks (PSTN – public switched telephone network).
  • a facsimile terminal connected to PSTN land lines. (The SES can send a message to a shore-based fax terminal, but the Inmarsat-C system does not allow a shore-based fax terminal to send messages directly to a SES).
  • dedicated equipment such as data processing systems.
The Inmarsat-C
The Inmarsat-C system

Inmarsat Mobile Number (IMN)

An Inmarsat Mobile Number is the number assigned by the National Routing Organization to an Inmarsat-C SES as its identity number.

The number is derived from the vessel’s MMSI number, and consists of article “Inmarsat: A Guide to Satellite CommunicationINMARSAT“. The format is 4MID XXX ZZ, as described in article “Inmarsat: A Guide to Satellite CommunicationInmarsat Mobile Number“.

The Inmarsat-C Communication services

Assuming that both the SES terminal and the selected CES support them, the following communication services are available:

Distress Alerting and safety Services

Inmarsat-C SESs make use of the Signalling Channel for distress alerting. By using the distress button, a short pre-formatted alert is transmitted directly to a CES, or an NCS as a back-up.

Distress priority ensures special processing at the CES for expeditious transmission to the associated Rescue Co-ordinating Centre (RCC).

The RCCs are able to organize search and rescue operations in response to a distress alert from a SES. Furthermore, many RCCs are equipped with Inmarsat-C terminals, which allow them to communicate with other RCCs and ships in vicinity of the distressed ship.

The SES terminal’s distress facility is a mandatory requirement for all sea-going vessels. A land based SES, however, must not issue a maritime distress call.

The distress alerting format of an Inmarsat-C SES may be updated manually from the terminal keyboard. Automatic position updating, however, may be provided directly via the interface of an integrated electronic navigation receiver (GPS or Loran-C Charts and Related InformationLoran-C).

New recommendations to SOLAS will probably amend that DSC – Digital Selective CallingDSC equipment and Inmarsat-C equipment are interconnected to a Use of Skywaves for Navigationnavigation receiver.

Sending a distress call

If the ship is threatened by grave and imminent danger, one can use Inmarsat-C SES to send a distress alert. The message will be routed automatically through a CES to an associated RCC.

You may send a distress alert to an RCC either using by the SES terminal menus, or by pressing the remote distress button(s), if fitted on your SES. Sending a distress alert using your terminal menus is recommended because it allows you to provide up-to-date information on your circum­stances.

Sending a distress alert using the remote distress button(s):

If the terminal is fitted with a remote distress button (or combination of buttons), you may send a distress alert simply by pressing the button(s) and keeping them depressed for the required number of seconds (typi­cally 5 seconds).

When employing this procedure, only the pre-programmed information, such as the ships identity and dating from when it was previously en­ tered, will be transmitted.

If the position is not updated automatically, the latest manually updated position will be transmitted.

This procedure functions even if the SES is not logged-in to an Ocean Region, or if the terminal not yet has been commissioned.

Sending a distress alert using the SES Distress terminal menus:
  • Access the distress alert menus on the SES terminal;
  • Fill inn the selections on the menus presented, entering the ship’s position if not automatically updated;
  • Select the nature of distress;
  • Select preferably the nearest CES to your ship within your Ocean Region;
  • Send the distress alert;
  • Wait for acknowledgement from the CES. If no reply within 5 minutes, repeat the producer.
During a distress call
 
When you make a distress call, the Inmarsat-C system uses the NCS Common Signalling Channel for the Ocean Region you are currently logged-in to, to establish a communication channel from an RCC to your SES.
 
To ensure that your SES remains tuned to this channel, set the automatic scan facility on your SES to scan only the Ocean Region you are logged-in to, by making it the preferred Ocean Region. So doing ensures that the RCC can continue to communicate with you (so long as you remains in the Ocean Region).

 

Two-digit telex code services

There are several two-digit telex codes services available for maritime safety services and general utility. As the system develops, these services will be available at a growing number of CESs.

Accessing a two-digit code service:

  1. Prepare the message or request you want to send on the SES text editor.
  2. Select type of message as “Special Access Code” or “Special Ac­cess Network” in the Transmit menu.
  3. Type the two-digit code required in the address field of the menu.
  4. Select the CES in your Ocean region that supports the required two-digit code service.
  5. Transmit the message or request.
  6. Wait for acknowledgement form the CES or positive delivery notification if you have requested it.
Note that using some two-digit codes is free of charge for ship operators.

Store-and-forward Messaging Services

Several different store-and-forward messaging services are available:

  • Telex, for sending and receiving telex messages to and from other telex terminals via CES.
  • Fax, for sending a message to a shore-based fax terminal. Note that the system does not allow a shore-based fax terminal to send a fax directly to a SES. Fax terminals must send the text message via a fax bureau service, where the fax message is re-typed and transmitted as a store-and-forward message to the SES.
  • Messages to and from a computer. Available if your SES, and the computer connected to PSDN or PSTN networks, are equipped with the necessary hardware and software.
  • Ship-to-ship message service, for transmitting and receiving messages to and from any other Inmarsat SES.
  • Electronic mail (E-mail). If the SES is registered with an E-mail service. E-mail is a global message handling system whereby subscribers to commercial E-mail services can interchange messages and electronic data files between computers.

Data Reporting, Polling and SCADA (Supervisory Control and Data Acquisition)

Data reporting allows the SES to send short data reports collected from instruments, or input manually, to an operational centre for analysis.

The number of Inmarsat-C CESs which support data reporting and the related application of SCADA, is limited. As the system develops, how­ever, more CESs are expected to support these services.

The SES must be registered in a data reporting closed network at the operational centre.

The centre arranges for the CES to download network identification via the Inmarsat-C system, for storage in the CES. This network information comprises of a unique Data reporting Network Identification (DNID) code, plus a Member Number for the SES within the network.

A typical ship report might be as follows:

  • Sailing plan.
  • Position report.
  • Deviation report.
  • Arrival report.
  • Departure report, and so on.

Operator Assisted and Value-added Services

An example of operator-assisted services might be a shore-based operator receiving a message from a SES terminal, and forwarding it as a voice message to a telephone, or as a letter to a specified address.

Other examples of operator-assisted services are the transmission of the delivery status of messages, directory services, translation services and access to databases.

It will be interesting: The Global Maritime Distress and Safety System – Principles & Practice

Value-added services are E-mail services, or routing arrangements provided to enable subscribers to send messages to SESs from countries which do not support direct dialling access to the Inmarsat-C system.

Other value-added services are fax bureau services and two-digit code services. The two-digit code service is undergoing continual development at several CESs, and is not yet fully available for public use.

Accessing different networks using an Inmarsat-C SES

Destination Type
Inmarsat system
Destination numberPresentation code (Format)
TelexTelex country code + Subscriber’s Telex Number5-bit or 7-bit
Mobile (telex message)

Inm-A Telex terminal
Inm-B Telex terminal
Inm-C Telex terminal

Telex Ocean Region Access Code + IMN5-bit or 7-bit
Mobile (fax message)
Inm-A Fax terminal
Inm-B Fax terminal
Inm-M Fax terminal
Telephone Ocean Region Access Code + IMN5-bit, 7-bit or 8-bit
Mobile (data message)
Inm-A Computer
Inm-B Computer
Inm-C Computer
DNIC + IMN

DNIC = Data Network Identification Code

8-bit
PSDN (X.25) (data message)DNIC + Subscriber’s X.25 (data) address5-bit, 7-bit or 8-bit
FaxTelephone Country Code + Subscriber’s Fax Number5-bit, 7-bit or 8-bit
PSTN PC + modem
PSTN modem + print.PSTN mailbox
Telephone Country Code + Subscriber’s
Modem address or number of the mailbox
5-bit, 7-bit or 8-bit
X.400Address information to be included in message7-bit or 8-bit
Two-digit code
Telex service
Special access Codes (SAC)5-bit or 7-bit
NOTE. All CESs support 7-bit (ASCII) code, but not all CESs supports 5-bit and/or 8-bit codes.

5-bit code is also known as Telex or ITA2 (International telegraph Alphabet ), or 5-bit packed. Files created using 5-bit code are smaller than using those 7 or 8 bit code, and are therefore about one third cheaper to send.

7- bit code is also known as ASCII or IA5 (International Alphabet 5)

8- bit code is also known as Data. Generally used to send binary coded data over the PSDN (data) landlines.

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Author
Author photo - Olga Nesvetailova
Freelancer
Literature
  1. Global Maritime Distress and Safety System: IMO 1987.
  2. Manual for use by The Maritime Mobile and Maritime Mobile- Satellite Services: ITU 1992.
  3. Radio Regulations 1-2-3: ITU 1990.
  4. Manual for Norwegian Mobile Radiotelephone Stations: The Norwegian Telecom 1992.
  5. Modem Electronic Communication: Gary M. Miller 1978.
  6. Brochures and data sheets from manufacturers of GMDSS Equipment.
  7. Inmarsat: Inmarsat-A User’s Manual, Inmarsat-C User’s Manual.
  8. Nodposisjonering: Bjomar Augdal, 1992.
  9. Skipsantenner: Bjomar Augdal 1991.
  10. COSPAS-SARSAT Secretariat: COSPAS-SARSAT System Data Documents.
  11. Admiralty List of Radio Signals.
Footnotes
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Январь, 28, 2025 101 0
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