1The lesson in plain words
In one sentence…
To study NBDP, separate method, service and purpose before relying on an older terminal.
- 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
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.
6Practice case & exam keys
Situation
A trainee reads a clear NAVTEX printout from 518 kHz on an older terminal and tells the master the ship has therefore sent a distress alert by telex.
What must the trainee correct?
Show the answer →
The printout is MSI received by FEC broadcast; nothing was transmitted, and NBDP is no longer a distress means.
Why: FEC is one-way: the NAVTEX receiver only decodes. The trainee should read the warning, check whether it affects the route, brief the bridge and log it. If the ship were in distress, the alert would go by DSC or a satellite terminal, followed by voice.
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.
- 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
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?
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
9Test your knowledge
What is NBDP?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: