Module 5 — NBDP / radiotelex

Written MF/HF communication: ARQ vs FEC modes, how NBDP fits GMDSS today, and its limitations.

NBDP (Narrow Band Direct Printing) is the maritime radiotelex system operating over MF/HF. Its operational use has declined with Inmarsat and other satellite services, but it remains part of the official GOC syllabus.

What it is and how it works

NBDP transmits characters in ITA-2 code at 100 baud with FSK modulation on the assigned telex frequencies in MF and HF. Unlike voice, it provides a printed or on-screen record of the message, removing ambiguity and leaving a written copy of the communication.

Operating modes

ARQ (Automatic Repeat reQuest) — point-to-point with acknowledgement of each 3-character block. The two stations identify each other with a selcall (4 digits) and negotiate the link. If a block arrives corrupted, repetition is requested automatically. This is the mode used for ship-to-coast correspondence.

FEC (Forward Error Correction) — broadcast without return channel. The sender transmits each character twice with a time offset; the receiver compares and corrects. Used to broadcast to all listening ships (collective FEC) or to a selected group (selective FEC).

Use in GMDSS

NBDP is part of the distress chain as a complementary medium to DSC and voice:

Reception of MSI (weather bulletins, navigation warnings) where NAVTEX coverage is absent.

Sending written distress, urgency or safety messages on MF/HF after the DSC alert.

Public and commercial correspondence with licensed coast stations.

NBDP distress frequencies

Reserved distress frequencies: 2174.5 kHz (MF), 4177.5, 6268, 8376.5, 12520 and 16695 kHz (HF).

Limitations and current context

NBDP depends strictly on HF propagation, requires precise tuning and copes poorly with QRM. Fewer and fewer coast stations keep the service open 24/7; in areas with good satellite coverage the traffic has migrated to Inmarsat C.

Common errors

Using ARQ when a broadcast was required, sending in FEC a message intended for one station, failing to update position before a distress message, and confusing the local print of the outgoing message with remote-end reception confirmation.

What NBDP is technically

NBDP (Narrow-Band Direct-Printing) is narrow-band direct-printing telegraphy, F1B modulation, on MF/HF. It uses CCIR 476-4 (ARQ, automatic-repeat-request between two stations) and CCIR 625 / FEC (Forward Error Correction, broadcast without acknowledgement). Section 4.3 and Part 5 of the manual position it as a path for MSI and ship-to-shore distress/safety traffic, primarily on HF.

Typical frequencies

For NBDP distress and safety traffic the same HF bands are used as for voice, with specific channels assigned by administration and published in ITU List IV. 518 kHz is reserved for international NAVTEX (another NBDP service), not for direct ship-to-shore traffic.

ARQ vs FEC

In ARQ the ship establishes a bidirectional link with the coast station; each block is confirmed, giving integrity at the cost of time. In FEC the station broadcasts without acknowledgement; the receiver prints what it receives without asking for repeats. MSI over NBDP uses FEC; personal or coordination telegrams use ARQ.

Role in the modernized GMDSS

Although the 2024 package no longer explicitly mandates NBDP as a primary medium in every case, the manual keeps it as a valid route for MSI and SAR where NAVTEX and the RMSS do not cover. On ships where it is retained, the operator must be able to read an NBDP message, identify its category and brief the bridge as with any MSI.

Paper discipline

Messages printed by NBDP are filed with the radio log. A receiver that merely prints is not a useful receiver: what is useful is an operator who filters, escalates and files. If the printer has no paper or ink, the set is operationally blind even if it is technically working.

ARQ vs FEC: when to use which

ARQ (CCIR 476-4) sets up a bidirectional point-to-point link: each block is acknowledged and, on error, repeats are requested; integrity at the price of being limited to two stations. It is used for routine telegrams to owners, urgency messages to a specific coast station and bilateral SAR coordination. FEC (CCIR 625, 4B3Y / 7B3Y variants) is broadcast without acknowledgement: the same frame is transmitted twice with a time offset and the receiver recovers errors without requesting repeats; it is used for broadcast to many stations (MSI over NBDP, general warnings). Choosing wrong kills the service: FEC for a telegram to the owner leaves it unconfirmed; ARQ for a general broadcast makes it impossible for more than one ship to receive it.

Alphabet and relationship with NAVTEX/518 kHz

NBDP uses a 7-bit alphabet with 3 ones and 4 zeroes (or 4B3Y variants) that allows parity error detection. It is not Morse code: it is automatic telegraphy. NAVTEX on 518 kHz is a specific NBDP-FEC service (another NBDP service) but operates on MF, not HF; HF NBDP is a separate service covering what NAVTEX cannot reach. Mixing them up leads to looking for NAVTEX content in the HF NBDP receiver or vice versa.

ARQ procedure: exact sequence

1Select the coast station by its 4- or 5-digit selcall ID listed in ITU List IV.
2Call: the terminal transmits the selcall pattern and waits for the response.
3Handshake: if the coast station answers, a bidirectional link is set up and the terminal shows 'TRAFFIC' or 'OVER'.
4Send the message block by block; the receiver acknowledges each block and, on parity error, requests a repeat (ARQ).
5Close with '+?' or the end-of-call sequence the station accepts. FEC on the other hand needs no handshake: select FEC mode, broadcast on the open frequency, and the station transmits for a fixed time (typical: 10-min slots every 4 h for MSI, as on NAVTEX). Traffic priorities: distress > urgency > safety > routine; the first three get priority access to the channel even if commercial traffic is in progress, per RR Article 53.

STCW Bridge Watch Lens

Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.

A proper look-out means a person who can give full attention to the task and a bridge team using all appropriate means without over-reliance on one sensor.

After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.

Exam Focus

Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.

If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.

Watch for trick answers that replace look-out with AIS, VHF or one quick radar glance. The rule demands a full appraisal of the situation.

Key Takeaways

ARQ = point-to-point with automatic acknowledgement in 3-character blocks.

FEC = broadcast without return; the receiver corrects by comparing the double transmission.

Still an official GMDSS medium for written distress messages on MF/HF.

Local print does NOT mean successful reception at the other end.

NBDP is F1B telegraphy with ARQ (two-way, acknowledged) and FEC (broadcast, unacknowledged) modes.

Section 4.3 keeps HF NBDP as an MSI/SAR path where NAVTEX or RMSS do not cover.

Distress NBDP channels live in the HF bands and are published in ITU List IV.

An NBDP message only matters if the operator reads, categorizes and briefs it to the bridge.

ARQ: bidirectional, point-to-point, with acknowledgement; for addressed telegrams and bilateral SAR coordination.

FEC: broadcast, unacknowledged, time-redundant; for MSI and warnings to many stations.

NBDP uses CCIR 476/625 alphabets (4B3Y / 7B3Y patterns), not Morse.

NAVTEX 518 kHz is a specific NBDP-FEC service on MF; HF NBDP is a separate service.

ARQ: selcall ID → handshake → block-by-block with acknowledgement → close with '+?'.

FEC: no handshake, broadcast in a fixed slot (typical 10 min every 4 h for MSI).

Traffic priority: distress > urgency > safety > routine (RR Article 53).

Common Mistakes

Using ARQ to broadcast to all stations (FEC is the correct mode).

Forgetting that HF propagation may prevent an ARQ link from completing.

Confusing local print with remote acknowledgement.

Failing to log a message sent by NBDP.

Confusing NBDP with NAVTEX: NAVTEX is a specific NBDP service on 518 kHz, not the other way round.

Letting the printer run out of paper/ink and assuming the receiver is still operational.

Filing NBDP messages without escalating action-requiring ones (SAR warnings, piracy).

Sending a personal telegram in FEC and leaving it unconfirmed.

Trying to broadcast MSI to many ships using ARQ: only the linked station receives it.

Looking for NAVTEX content in the HF NBDP receiver because 'both are NBDP'.

Forgetting to close the ARQ link with '+?' and leaving the channel occupied for other users.

Handling routine traffic as if it outranked urgent, pushing real distress out of the channel.

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