Module 7 — NAVTEX and maritime safety information
NAVTEX receiver on 518 / 490 kHz, station identification, filtering of rejectable messages, and operational reading of navigation and weather warnings.
NAVTEX is an automatic receiver of MSI on 518 kHz (international English) and 490 kHz (local language).
Message format
Header `ZCZC B1 B2 B3 B4`:
Rejectable and non-rejectable messages
Types A, B, D and L are non-rejectable: the receiver does not allow filtering them out. The other types can be disabled in the configuration.
Operation
The operator selects stations relevant to the position and the required message types. New messages are printed or displayed automatically.
Interpretation
A real NAVTEX warning must be translated into a decision: does it affect my area? Does it alter my route? Should I inform the bridge? Is it to be logged?
What NAVTEX is (3.2.4 and 4.2)
International NAVTEX is the coordinated automatic broadcast on 518 kHz of MSI and SAR-related information, in English, via NBDP-FEC, for coastal waters out to about 400 nautical miles offshore. National NAVTEX on 490 kHz and 4209.5 kHz allows local-language broadcasts. The service is coordinated by the IMO NAVTEX Coordinating Panel; each administration runs stations within its NAVAREA.
Message format B1 B2 B3 B4
Every NAVTEX message starts with four characters: B1 identifies the transmitting station (a letter A–Z assigned inside a NAVAREA, with geographical separation > 400 NM between stations sharing the same B1); B2 is the subject indicator (A navigational warnings, B meteorological warnings, C ice, D SAR, E met forecasts, F pilot, G AIS, H LORAN, I reserved, J satnav, K other aids, L additional navigational warnings, etc.); B3 B4 are the 01–99 serial number. A message with the same B1 B2 B3 B4 already received is suppressed (repetition handling).
Timeslots
Each NAVTEX station is assigned a maximum of 10 minutes every 4 hours. Strict timeslot discipline is essential because NAVTEX operates on a single shared frequency within the NAVAREA.
Categories that cannot be disabled
The operator may reject categories not needed (for example F pilot if not applicable), but the manual stresses that A (navigational warnings), B (meteorological warnings), D (SAR), L (additional navigational warnings) and categories covering piracy, tsunamis and other natural phenomena cannot be deselected. Filtering wrongly 'to save paper' can hide mandatory MSI.
Receiver requirement
Receivers installed after July 2005 must include two receivers capable of operating on 518 kHz and at least two other recognized NAVTEX frequencies (490 and 4209.5 kHz).
NAVAREAs, METAREAs and MSI geography
The globe is divided into 21 NAVAREAs (navigational warnings) and matching METAREAs (meteorological warnings), coordinated by IMO and WMO. European waters span NAVAREA I (UK-coordinated, North Sea and NW approaches), NAVAREA II (Bay of Biscay and eastern North Atlantic, coordinated by France) and NAVAREA III (Mediterranean, coordinated by Spain). Each NAVAREA contains multiple NAVTEX stations with different B1 letters; one coastline can see several B1s that still belong to the same NAVAREA.
Mandatory categories and EGC
Categories A (navigational warnings), B (meteorological warnings), D (SAR), L (additional navigational warnings), and piracy and tsunami messages cannot be deselected; they are locked on in every receiver. FleetNET (over Inmarsat) is a commercial closed-group broadcast contracted by an operator or fleet; SafetyNET (Inmarsat) and SafetyCast (Iridium) are the EGC MSI services coordinated by states and RCCs. To receive MSI correctly over EGC the terminal must have the current NAVAREA / MetArea set and, above all, a live position: many messages filter by geographic rectangle and a terminal with a stale position silently drops them.
Header and detailed timing
Every NAVTEX message starts with ZCZC B1B2B3B4 and ends with NNNN. B1 = station letter (A-X assigned by the IMO NAVTEX Coordinating Panel, with more than 400 NM of geographical separation between stations sharing a B1 in the NAVAREA). B2 = subject: A nav warnings, B met warnings, C ice reports, D SAR info and piracy warnings, E met forecasts, F pilot service, G DECCA (historical), H LORAN (historical), I reserved, J SATNAV, K other electronic nav aids, L additional nav warnings, T/U/V/W/X reserved for national use, Y reserved, Z not used. B3B4 = 01-99 serial; serial 00 is reserved for urgent/vital messages which the receiver must always print even if the category is filtered. Schedule: each station within a NAVAREA is allotted a 10-min slot in a 4-h cycle; stations in the same NAVAREA are offset so that no two transmit simultaneously on 518 kHz. The receiver filters on B1 (wanted stations list) + B2 (enabled categories) + maximum error rate; if the operator mis-sets the filter the printout is garbage or silence.
NAVTEX interpretation exercise
Given the header 'ZCZC IA07', the student must decode and interpret: (a) station B1 = I (in the IMO NAVTEX Coordinating Panel tables, I is assigned to a specific coast station within its NAVAREA; for example in NAVAREA III the letter I has been assigned to Niteroi or another station published in ALRS Vol. 5); (b) category B2 = A (navigational warnings), a non-rejectable category; (c) serial number B3B4 = 07 (seventh warning of the current cycle for that station); (d) coverage: out to ~400 NM from the transmitting station; (e) route impact: if the warning affects the rectangle or grid of the passage plan, action is required (diversion, speed reduction, special attention); (f) action: read in full, assess impact, pass to the officer of the watch, record in the bridge log and, if applicable, update the route. Non-rejectable categories: A (nav warnings), B (met warnings), D (SAR), L (additional nav warnings), plus piracy and tsunami. Categories E/F/G/K (met forecasts, pilot, AIS, other aids) may be filtered by the operator if the ship does not need them. Reference: IMO NAVTEX Manual (MSC.1/Circ.1403 as revised).
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.
Build the traffic picture with sight, hearing, radar/ARPA and chart context. Do not let AIS or one isolated bearing replace systematic observation.
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.
Steady or nearly steady bearing is the classic trigger, but close range, a large vessel or a tow can still mean risk even when bearing change seems small.
Key Takeaways
518 kHz = international English; 490 kHz = local language.
Types A, B, D, L cannot be filtered out by configuration.
An unread NAVTEX warning is as dangerous as a message never received.
International NAVTEX = 518 kHz, English, NBDP-FEC, out to about 400 NM.
490 kHz and 4209.5 kHz carry national NAVTEX in local languages.
B1 B2 B3 B4 format: station, category, serial; duplicates are suppressed automatically.
10-minute timeslot every 4 hours per station; A, B, D, L, piracy and tsunamis cannot be disabled.
21 NAVAREAs and METAREAs cover the world; NAVAREA III is the Mediterranean (coordinated by Spain).
A, B, D, L, piracy and tsunami are mandatory NAVTEX categories; they cannot be disabled.
FleetNET = commercial group; SafetyNET/SafetyCast = EGC MSI services coordinated by states and RCCs.
Keeping the EGC terminal's NAVAREA and position current is the condition for receiving area-targeted MSI.
Header: ZCZC B1B2B3B4 ... text ... NNNN; B3B4 = 00 means vital and always prints.
Categories B2: A nav, B met warning, C ice, D SAR/piracy, E met forecast, F pilot, L additional nav.
Each station has a 10-min slot in a 4-h cycle within the NAVAREA, all sharing 518 kHz internationally.
Common Mistakes
Disabling all types because of paper-tail noise.
Selecting a station that does not cover the sailing area.
Failing to log warnings that affect the planned route.
Disabling categories 'to save paper' and losing mandatory MSI (navigational warnings or SAR).
Confusing B1 (station) with NAVAREA: several stations may operate inside the same NAVAREA.
Not reviewing the suppressed-duplicates queue and believing 'nothing is coming in'.
Ignoring that the receiver must watch 518 kHz plus at least one other recognized frequency.
Confusing NAVAREA with B1 station: several stations share the same NAVAREA.
Treating FleetNET as if it were official MSI: it is a private commercial broadcast.
Letting the EGC terminal position freeze and silently filtering out warnings for the current area.
Filtering on B1 but forgetting that serial 00 always prints as a vital message: assuming the receiver is broken.
Misconfiguring the B1 list and missing messages from the nearest station in favour of a remote one.
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