Satellite GMDSS: the recognized services (Inmarsat C, Inmarsat Fleet Safety, Iridium) and legacy radiotelex

1The lesson in plain words

In one sentence

To study NBDP, separate method, service and purpose before relying on an older terminal.

What is NBDP?

It is a data method for sending characters over a maritime radio link: narrow-band direct printing, not a universal distress service.

ARQ or FEC?

ARQ uses a two-way link with acknowledgements and retries; FEC broadcasts one way, without a return acknowledgement.

When can it help?

NAVTEX uses NBDP-FEC; a retained HF terminal may receive MSI where that service is offered. Check the current service plan and manual.

What changed?

From 1 January 2024, SOLAS removed the NBDP distress-carriage requirement; that does not permit ignoring genuine distress received by an older system.

2Step by step

Safe reasoning with NBDP

1
Identify whether the message describes ARQ, FEC, NAVTEX or an old distress procedure.
The method and the message purpose are not the same.
2
Check the date and the named coast or broadcast service.
Published services can change.
3
Check the current plan and the retained equipment manual.
Do not assume a legacy path is available.
4
If distress exists, use the approved route available and follow the RCC or master.
A station in distress may use any available means to obtain assistance.
5
Record the message and the operational decision.
A printout alone does not show that it was assessed.

3Key data

NBDP: Narrow-band direct-printing telegraphy
ARQ: Two-way link; acknowledgements and retries
FEC: Broadcast without return acknowledgement
NAVTEX: Uses NBDP-FEC
SOLAS, 1 January 2024: Removed NBDP distress requirement
HF MSI: Where that service is provided

4Key points

1
Classify the message purpose first.
2
Separate carriage requirements from fitted equipment capability.
3
Check the service before relying on a legacy path.
4
Record the message and decision, not just the printout.

5Frequent mistakes

Wrong: "Every NBDP terminal is a current SOLAS distress route."
Right: NBDP is a method; check the available service and equipment.
Wrong: "Removing the NBDP carriage requirement removed NAVTEX."
Right: The modernization removed a carriage requirement, not NAVTEX.
Wrong: "A NAVTEX printout proves a distress alert was sent."
Right: It is MSI reception; identify the purpose and assess the message.

6Exam keys & interpretation

Exam keys

If you see "ARQ", look for a two-way link, acknowledgements and retries.
If you see "FEC" or "NAVTEX", separate broadcast reception from distress alerting.
If you see "since 2024", separate carriage, published service and legacy equipment.

How to read it

The same method can appear in different messages: classify the service first, then the equipment.
An older terminal may print a real message, but the printout does not prove its purpose or a sent alert.

7Go deeper: GOC exam level

The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.

Two-part diagram: above, a ship sends data blocks to a coast station that returns a tick, and one block is sent again; below, a coast station broadcasts doubled data blocks to three ships with no return path. — 1. Data sent in short blocks
2. The receiver confirms each block on a return path (ARQ)
3. A block received with errors is sent again
4. FEC broadcast: every character is sent twice
5. Any number of ships receive, but none can ask for a repeat
NAVTEX uses the FEC idea.

Why NBDP still appears in your syllabus

Many GOC manuals and national programmes (the Spanish 2002 syllabus, for example) still teach NBDP/radiotelex with ARQ, FEC, answerback and dedicated distress frequencies. They describe GMDSS as it was. The IMO modernization in force since 1 January 2024 and the WRC-23 decisions removed NBDP from MF/HF distress and safety communications.

ARQ and FEC side by side

Feature	ARQ	FEC
Direction	Two stations, a link	One to many, broadcast
Error handling	The receiver asks for blocks to be repeated	Redundancy is sent up front; the receiver corrects what it can
Confirmation	Yes, block by block	None
Typical historical use	Ship–shore telex correspondence	Broadcasts, MSI, messages to several ships

The same trade-off appears again in modern systems: a link with acknowledgement is reliable but needs a return path; a broadcast reaches everyone but cannot know who received it.

What replaced which function

Old NBDP role	Where it lives now
MF/HF distress and safety text	Withdrawn; DSC + voice remain on MF/HF, satellite services carry text
Setting up MF/HF connections	ACS (DSC-based automatic connection)
Text MSI on MF/HF	NAVTEX (itself narrow-band direct printing), satellite EGC, HF NBDP MSI broadcasts where an Administration still schedules them (4210, 6314, 8416.5, 12579, 16806.5, 19680.5, 22376, 26100.5 kHz), and in future NAVDAT

NAVDAT and e-navigation

NAVDAT is a digital shore-to-ship broadcast with much more capacity than NAVTEX. WRC-23 brought it into the GMDSS regulatory framework, with frequencies identified in MF and HF. It fits the e-navigation trend: structured data that can be updated and potentially fed into bridge systems. Operational roll-out depends on Administrations and coast networks.

Answering old exam questions safely

1Identify whether the question asks about history or current practice.
2If it asks for a "telex distress frequency", answer historically and add that NBDP is no longer part of MF/HF distress and safety.
3If it asks how to alert today on MF/HF, answer DSC followed by voice on the paired frequency.
4Never invent an NBDP step in a modern distress drill.

References: SOLAS IV as amended by MSC.496(105) · ITU-R M.541

GOC exam check

Which NBDP mode was used to broadcast to many ships at once with no return channel?
1. ARQ
2. ACS
3. FEC
4. DSC
Show the answer
C. FEC
Why: FEC sends redundancy up front so receivers correct what they can, with no confirmation. ARQ needs a two-way link with acknowledgements.
How does ARQ deal with transmission errors?
1. The receiver asks for blocks to be repeated
2. Redundancy is sent up front for correction
3. The sender repeats the whole message twice
4. The coast station re-broadcasts it on NAVTEX
Show the answer
A. The receiver asks for blocks to be repeated
Why: ARQ is a link with acknowledgement and repetition, block by block. Up-front redundancy describes FEC.
An old question bank asks for the MF/HF telex distress frequency. How should a GOC candidate answer?
1. Give the frequency as current distress practice
2. Refuse, since NBDP never had distress frequencies
3. Give the DSC frequency, as it replaced telex
4. Answer historically and note NBDP was withdrawn
Show the answer
D. Answer historically and note NBDP was withdrawn
Why: Answer the history, then state that NBDP left MF/HF distress and safety. NBDP did have dedicated distress frequencies, so denying it is wrong too.
In a modern drill, how do you alert and then communicate on MF/HF?
1. DSC, then NBDP telex on the paired frequency
2. DSC, then voice on the paired frequency
3. NBDP FEC broadcast, then voice on 2182 kHz
4. Voice MAYDAY first, then a telex follow-up
Show the answer
B. DSC, then voice on the paired frequency
Why: Today MF/HF distress means DSC followed by voice on the paired frequency. Never invent an NBDP step in a modern distress drill.
Which system uses DSC to set up MF/HF connections automatically?
1. NAVDAT
2. EGC
3. ACS
4. AIS
Show the answer
C. ACS
Why: ACS uses DSC to automate scanning, calling and choosing a working frequency. NAVDAT is a shore-to-ship broadcast, not a link set-up system.
After NBDP was removed from distress and safety communications, what happened to NAVTEX?
1. It was withdrawn together with NBDP
2. It became a voice broadcast on 518 kHz
3. It is kept only for national services
4. It remains a current MSI service
Show the answer
D. It remains a current MSI service
Why: NAVTEX uses the same narrow-band direct-printing family but is a current, widely used MSI service. Removing NBDP from distress did not remove NAVTEX.
Which description fits NAVDAT?
1. A digital shore-to-ship MSI broadcast on MF and HF
2. A VHF data link that shows ships on the chart display
3. A satellite EGC service run by Iridium
4. A DSC-based system for setting up MF/HF calls
Show the answer
A. A digital shore-to-ship MSI broadcast on MF and HF
Why: NAVDAT is a higher-capacity digital shore-to-ship broadcast with frequencies in MF and HF. Do not confuse it with AIS, SafetyCast or ACS.
A colleague proposes removing the NAVTEX receiver because NAVDAT now exists in the regulations. What is the right view?
1. Agree: NAVDAT replaced NAVTEX from 1 January 2024
2. Agree, provided the ship carries an AIS receiver
3. Disagree: roll-out depends on Administrations
4. Disagree: NAVDAT works only on the high seas
Show the answer
C. Disagree: roll-out depends on Administrations
Why: NAVDAT is an evolution, not an immediate universal replacement; deployment depends on Administrations and coast networks. AIS has nothing to do with it.
Your ship in A3 relies on MF/HF. At 1000 UTC an engine-room fire is out of control and the master orders a distress alert on HF. A cadet trained on an old manual starts setting up radiotelex on 8376.5 kHz. What do you tell him?
1. Send the telex first, as text is clearer than voice
2. Send a DSC alert on 8414.5 kHz, then go to voice
3. Send the telex on 8414.5 kHz instead of 8376.5 kHz
4. Send the telex on 2174.5 kHz, as MF is more reliable
Show the answer
B. Send a DSC alert on 8414.5 kHz, then go to voice
Why: NBDP was removed from MF/HF distress and safety, so today you alert by DSC and then work by voice on the paired frequency. Moving the telex to the DSC frequency is the tempting half-fix: it keeps the obsolete mode and puts it on a signalling channel.
Same scenario: a coast station acknowledges your 8 MHz DSC alert. On which frequency do you now transmit the MAYDAY call and message?
1. 8376.5 kHz, by radiotelex in FEC mode
2. 8414.5 kHz, the DSC frequency you used
3. 8291 kHz, the voice partner of the band
4. 2182 kHz, the MF voice distress frequency
Show the answer
C. 8291 kHz, the voice partner of the band
Why: After a DSC alert on 8414.5 kHz the distress traffic goes by voice on 8291 kHz, the partner on the same band where the acknowledgement came. 2182 kHz is a real distress frequency but on another band, and the station that answered you is listening on 8 MHz.

8Full lesson & sources

Open the full lesson (diagrams and official references)

NBDP is narrow-band direct-printing telegraphy. ARQ uses a two-way link with acknowledgements and retransmissions. FEC provides broadcast reception without a return acknowledgement. NAVTEX uses NBDP-FEC; HF NBDP broadcasts may also carry MSI where that service is provided.

The SOLAS modernization effective 1 January 2024 removed NBDP distress-communication carriage requirements. It did not abolish NAVTEX or permit operators to ignore a genuine distress message received by an older system. A station in distress may use any available means to obtain assistance. Verify current broadcast services and use the equipment instructions for retained legacy installations.

What NBDP actually does

Narrow-band direct-printing (NBDP), or radiotelex, sends text over MF/HF radio at 100 baud. Data at that speed gets damaged by fading and noise, so NBDP has two ways of protecting it, and the choice depends on whether the receiver can answer back.

ARQ is a two-way mode. The sender transmits text in small blocks of three characters, and the receiver answers each block: "got it" or "send again". A block hit by a burst of static is simply repeated. It works well for a call between one ship and one coast station, but needs a return path.

FEC is a one-way mode. The sender transmits every character twice, separated in time, so a short fade is unlikely to destroy both copies. The receiver never answers, so one transmission can serve a thousand ships at once. The price is that the sender never knows who received what.

Mode	Return path	Best for	Example
ARQ	Yes, block by block	One-to-one exchanges	A ship sending a telex to its company through a coast station
FEC	No	One-to-many broadcasts	A navigational warning to all ships in an area

Current status after the GMDSS modernization

The 2024 SOLAS amendments removed the NBDP distress-communication carriage requirement from 1 January 2024. Since then MF/HF distress and safety means DSC to alert, then voice on the paired frequency. The UK, for example, stopped its HF NBDP distress and safety services at the end of 2023, as recorded in the MCA bulletin on HF services.

That changes what you must carry and what you plan on. It does not make old terminals explode. A ship may still have a working NBDP set, and some Administrations still schedule HF NBDP MSI broadcasts. If you read a distress message on any equipment, it is still a distress message and you act on it.

NAVTEX still uses NBDP-FEC

NAVTEX still uses NBDP-FEC. The receiver on your bridge decodes 100-baud FEC on 518 kHz, which is why it can print a warning to every ship in range without a single reply. NAVTEX is not "old NBDP" left behind by mistake: it is a current MSI service that happens to use that technique. What left GMDSS in 2024 was the use of NBDP for distress and safety traffic, not the technique itself.

So keep two questions apart. What technique carries the text? (FEC.) What is the service for? (MSI to ships near the coast.) A NAVTEX receiver cannot transmit anything, so receiving a printout never means your ship has sent an alert. The ITU-R M.540 background explains the direct-printing MSI technique; the SOLAS text tells you what is required today.

Answering exam questions about NBDP

Old question banks still ask about NBDP. Use this routine:

1Does the question ask about history or present practice? Look for words like "today", "currently" or a date.
2For "the MF/HF telex distress frequencies", give the old answer (2174.5 kHz on MF, for example) and add that NBDP left distress and safety on 1 January 2024.
3For "how do you alert on HF today", answer DSC first, then voice on the paired frequency: never telex.
4For "which mode broadcasts to all ships", answer FEC; for "which mode confirms delivery", answer ARQ.

Official references · 2026-09-09

SOLAS IV (2024) · ITU RR 4.9 · NAVTEX