Inmarsat M & B: Understanding the System and its Services

In the realm of maritime communication, Inmarsat M and B systems have solidified their position as indispensable tools for ensuring safety at sea and facilitating efficient operations. This article delves into operational principles of these systems, providing a comprehensive overview of their single and multi-channel capabilities.

Additionally, we will explore the intricacies of Inmarsat M/B numbering schemes and outline the procedures for sending distress messages, making telephone calls, sending faxes, and initiating telex calls.

Inmarsat-B

Inmarsat-B was introduced in 1994 and is intended to be the successor of Inmarsat-A. The system uses digital technology to provide high quality telephone, fax, telex and data communications at generally lower charges than Inmarsat-A. The antenna size and weight are approximately the same as those of Inmarsat-A: Equipment, Procedures, and Distress CommunicationInmarsat-A. Single-channel and multi-channel Inmarsat-B models are available.

Inmarsat-B is also capable of transmitting high-speed data (up to 64 kilobits/second).

Inmarsat-M

Inmarsat-M was introduced in 1992 to complement the existing services.

This system uses digital technology and provides The Global Maritime Distress and Safety System – Principles & Practiceglobal telephone communications on an SES, which is inexpensive and compact in size.

The services possible on an Inmarsat-M SES includes two-way telephone, fax and (computer) data communications. Inmarsat-M SESs can be delivered as either single-channel or multi-channel models.

A multi-channel SES requires greater transmission power than a single channel SES, so that the antenna and power supply for a multi-channel SES would be larger and of higher gain than those of a single-channel SES.

Basic system architecture

The Inmarsat-M and Inmarsat-B SESs share the same basic system architecture. The purpose of this is cost effectiveness; the coast earth stations and the ship earth stations of both systems were developed simultaneously. Even if the operation of both SESs is similar, specific services such as fax and data are not identical.

System operation

The figure below shows typical equipment which may be connected at either end of the Inmarsat-M/B systems. At the SES end, several end terminals may be connected, depending on the options available on the SES.

These end terminals may include one or more telephone handsets, fax terminals, personal computers and TELEX (Narrow Band Direct Printing – NBDP) Procedurestelex equipment (for Inmarsat-B only) connected via an interface unit within the CES.

Telephone and fax terminal connections to an SES are not interchange able.

The Inmarsat-B and M systems transmit only digital data over the satellite link.

All communications, including telephone calls, must therefore be converted into digital form.

Read more: Informed for Safe Navigation at Sea with Critical Maritime Information

When converting telephone calls into digital form, the system uses a special voice coder/decoder at the SES and at the CES. At the transmitting end, a voice coder samples the human voice, analyses it and breaks it down into its component parts, representing them as digital information for transmission over the satellite link. At the receiving end, a voice decoder decodes the received digital information, and forms it back into a representation of the human voice.

Single and Multi-channel Inmarsat M and B SESs

If the ship needs to use more than one service at the same time, a Multi­-channel SES is required.

A Multi-channel SES is equivalent to several Single-channel SES sharing the same antenna, but operating independently. For example, Channel 1 can be receiving a fax at the same time as Channel 2 is making a telephone call.

To enable shore-based subscribers to transmit calls to any particular end terminal (telephone, fax, telex) connected to a Multi-channel SES, a unique IMN is assigned to each end terminal.

Inmarsat M/B numbering scheme

The Inmarsat Mobile Number (IMN) is derived from the ships MMSI number.

As part of the IMN numbering system, the two last digits are used to create a decimal number ranging from 10 to 99, which is used to discriminate, on the same ship, between:

• Different SESs.
• Channels of a multi-channel SES.
• Services such as telephony, fax, data and telex.

Sending a DISTRESS message by telex using an Inmarsat-B SES

By pressing the Distress Button you will request a distress priority and the connection of the telex circuit through a CES to a land-based RCC.

The procedure for sending a Distress Signals and Distress Trafficdistress message is outlined below:

1 Press and hold down the Distress push-button for at least 6 seconds.

2 Wait for automatic connection to the RCC.

3 Type the distress message using the following format:

• Mayday.
• Ship‘s name and callsign or other identification o f the vessel.
• Inmarsat Mobile Number and Ocean Region satellite.
• The position in latitude and longitude.
• The nature of distress.
• Type of assistance required.
• Other information which might help the rescue authorities.

4 Send the message to the RCC.

Sending a DISTRESS call by telephone using an Inmarsat-M or B SES

1 Lift the telephone handset and listen for the dial tone (or switch the handset to the TALK position, as appropriate).

2 Press and hold down the Distress push-button for at least 6 seconds.

3 Press the # key to initiate the call.

4 When the RCC operator answers, speak clearly and give the following message:

• Mayday.
• This is Ship’s name and callsign.
• Inmarsat Mobile Number and Ocean Region satellite.
• The positioning latitude and longitude.
• Nature of distress.
• Type of assistance required.
• Other information which might help the rescue authorities.

Making a telephone call

1. Enter the 3-digit access code for the CES through which you want to communicate.
2. Lift the handset and wait for the dialling tone.
3. When the dialling tone is heard, type in the following:

• 00 – is the two-digit code for automatic call request.
• # – is the key for ending the calling sequence.

Sending a fax

1. Select the 3-digit CES access code for a CES that supports the fax service.
2. Prepare the fax terminal to send a fax as instructed in the manu­facturers instructions, and key the following.

• 00 – is the two-digit code for automatic call request.
• # – is the key for ending the calling sequence.

Making a telex call (Inmarsat-B only)

1. Set up a communications channel to a CES in your Ocean region.
2. When the GA+ is received from the CES, key in the following:

• “+” – is the End of Number selection character.

Author

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.