NAVTEX Safety Information

NAVTEX is an acronym for NAVigational TEXt messages. It implements an international automated service for dissemination of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships.

NAVTEX is one of the basic components of the Global Marine and Distress Safety System (GMDSS). The information system was developed to provide a low-cost, simple, and automated means of receiving this information aboard ships at sea within approximately 200 nautical miles of shore. NAVTEX stations are operated by the Coast Guard authorities all around the world. There are no special user fees associated with receiving NAVTEX broadcasts. The reports are in English language no matter which part of the world the information is being received.

System Overview

The international NAVTEX system operates world-wide on a single fixed frequency of 518 kHz (MF). This enables simple and cheap receiver hardware. NAVTEX reports are classified in different categories such as distress, search and rescue, navigational and meteorological reports. The NAVTEX receiver can be programmed to receive only specific stations and only certain message subjects. This way only the relevant information for a certain location and situation is displayed.

NAVTEX services in English on 518 kHz are now (2013) available in all coastal waters of Europe and in many other parts of the world so that world-wide access to marine safety information through NAVTEX may be realised in coastal waters soon.

Since all NAVTEX coast stations transmit on the same frequency (518 KHz) world wide, each NAVTEX Station is allocated a specific 10-minute time slot in a four-hour schedule. This way it is guaranteed that in each time slot only one station is transmitting thus preventing transmission interference of adjacent NAVTEX stations which would corrupt all messages sent simultaneously.
Each station can repeat and update it's NAVTEX messages in a four-hour cycle. However, urgent information, distress information, warnings of gales etc. can be inserted into the system at any time although the NAVTEX operator will ensure that the non-routine transmission is not made at the same time as a neighbouring NAVTEX routine transmission is broadcast. Any warning issued at a non routine time can be repeated in the following scheduled 10-minute slot.

The Coast Guard authorities responsible for the NAVTEX services, may on occasion have to defer or shorten the broadcast of a scheduled weather forecast via NAVTEX to ensure delivery of more urgent navigational and safety warnings. A lower priority is given to weather forecasts which have been broadcast previously. Commercial receivers are available for installation into vessels. These are designed to operate unattended and either record messages on to paper or to the receiver's internal memory for later review on the text display. There are also a number of software packages available allowing messages to be decoded by a PC with a suitable receiver connected to the computer's soundcard. Any general communications receiver capable of audio reception at 518 kHz or 490 kHz single sideband can be used for this purpose.

It is important understand that NAVTEX is a medium-wave transmission system for which an appropriate aerial is required and the reception is also very susceptible to the RF noise generated on the vessel. The 500kHz frequency band uses groundwave propagation, but can follow the curvature of the Earth, so it is not restricted to line of sight use. However, transmission ranges are notably greater over the sea than over land and signal strength in harbours and in some near coastal areas such as in bays and inlets may be very poor.
Off-shore reception may become poor if the desired NAVTEX station is more than 200 miles away. Essentially, NAVTEX has considerably greater range at night than during the day, but most stations adjust their transmission power to give a consistent range at all times - typically 1kW by day and 0.3kW at night.

At night under certain atmospherical conditions, skywave propagation can occur. In this case, the local station's transmission may be interfered with by another, more distant, station whose signal is received simultaneously by a skywave. However, the most common source of interference is when a station overruns its allocated time slot. It will then transmit a signal at the same time the next station starts its message bulletin. The adoption of standard abbreviations in NAVTEX messages has helped to avoid this by shortening messages. Also, although stations were initially allocated letters sequentially round the NAVAREA, the trend now is to geographically separate adjacent letters as far as possible. So if an overrun does occur, the interfering signal will be much weaker for the vessels sailing in the area covered by the next letter code.

Notice: In some areas, NAVTEX messages are also broadcast on MF 490 KHz and HF 4209.5 kHz. This service is used for the transmission of messages in the local language and is not part of GMDSS.

A summary of the GMDSS NAVTEX stations around the world and the transmission schedules may be found in the following list of Navtex Stations.

On the chart below, the light blue areas roughly indicate the regions that are within the range of at least one Navtex Station.

Format of NAVTEX messages

All broadcasts on 518 kHz are in English (International requirement). Broadcasts on 490 kHz may be in English, but more likely in the national language of the country providing the broadcast. For example, all NAVTEX stations in France broadcasting on 490 kHz are in the French language.

ZCZC

B1B2nn

xxXXXX

...    

NNNN

Message Start and End Characters

Message Header B1B2nn

• The B1 character denotes the broadcast station and indirectly the time of the (routine) transmission.
• The B2 character denotes the subject matter of message which follows.
• The index nn is the sequential number of the message to allow for a correct individual identification of each message.

For example, "SE45" in NAV-AREA I, indicates that the message is transmitted from Niton (B1 = S), and the message is a meteorological forecast (B2 = E). 4209.5 kHz using FEC mode Thus the heading indicates which NAVTEX station broadcasts it (B1), the type of information (B2) and a sequential number of the message (nn).
This header information enables the receiver to be programmed to print or store a message just once on its receipt the first time into the NAVTEX receiver. Thereafter, a message with the same B1B2nn heading will not be printed or held in store again in the receiver as long as it is not switched off. The format of the heading makes it possible to program one's receiver to accept or reject various classes of messages and also prevents the printing of the same message over and over again.

On a standard NAVTEX receiver compatible for use within the GMDSS regulations it should not be possible to reject messages with a B₂ character A (Navigational warnings), B (meteorological warnings) and D (Search and rescue information). But note that although category L can be programmed out, the information broadcast in this class may also be of interest to small vessels.

When programming the receiver it is advisable in some areas to ensure that only those stations which are required are programmed for reception otherwise many irrelevant messages will be received making it harder to keep an overview of the significant information.

Here is a record of received raw data with messages of different stations and different message categories. A good site to see examples of NAVTEX messages in almost real time is the http://www.frisnit.com/navtex/ site, which has messages broadcast by stations around the UK and Europe received at different locations.

Technical Information

System Details

NAVTEX broadcasts are primary made on the international medium frequencies of 518 kHz USB using narrow-band direct printing (NBDP). The characters are encoded using the 7-bit CCIR 476 character set and basic error detection is enabled by employing forward error correction (FEC). NAVTEX is broadcast using a data mode called SITOR-B (known as AMTOR in the amateur radio world). The modulation format used for SITOR-B is a binary frequency-shift keying (BFSK) modulation, with a frequency shift of 170 Hz on a centre frequency of 1700 Hz (mark=1615Hz,space=1785Hz). The receiver 6 dB bandwidth should be between 270-340 Hz. The data rate is 100 baud, which allows for a net character rate of about 5 characters per second.

Every NAVTEX message is preceded by a phasing signal of around ten seconds then a start of message group, "ZCZC", followed by four characters B1, B2, B3 and B4. B1 is an alpha character identifying the station, and B2 is an alpha character used to identify the subject of the message. Receivers can use these characters to reject messages from certain stations or if the message contains subjects of no interest to the user. B3 and B4 are two-digit numerics identifying individual messages, used by receivers to keep already received messages from being repeated.

The IMO recommendation for transmission power is 1kW at day time and 300W at night. The service range of NAVTEX stations is generally 100 to 500 nautical miles. With good receiving aerials, and suitable propagation conditions, stations can be received from much further afield.

SITOR-B

The data format used for broadcasting NAVTEX messages is known as SITOR-B, which is based on the more general RTTY format.

RTTY uses the ITA2 character code (Baudot code). ITA2 is a five-bit code with 32 possible code symbols. Four code symbols are used for null (BLANK), space (SPACE), carriage return (CR), and line feed (LF). Two code symbols are used for a letter shift (LTRS) or a figure shift (FIGS). The remaining 26 code symbols are used for characters in the letters and figures sets. Consequently, ITA2 can represent 52 (2*26) additional characters.

SITOR re-casts ITA2 into a 7-bit code called CCIR 476. Each 7-bit character in CCIR 476 has 4 marks (ones) and 3 spaces (zeros). Each valid character code has a Hamming distance of at least 2 from every other character. A one bit error will disrupt the balance of marks and spaces; a second bit error may (or may not) bring the count back to 4 marks and 3 spaces. Consequently, the CCIR 476 alphabet is guaranteed to detect all single-bit errors within a character.

The number of valid characters in CCIR 476 is the number of ways to choose 4 marks for 7 bit positions, and the number can be calculated using the binomial coefficient: C{7}{3} = C{7}{4} = 35. Thus CCIR 476 has 3 additional code symbols over ITA2. SITOR uses the additional code symbols for idle, phasing, and repeat requests. In addition, some of the ordinary characters are re-used as control signals (CCIR Rec. 625 Service Information Signals).

SITOR-B transmits each character in a message twice to gain reliability. If the receiver detects an error in the first character, it uses the copy. If both characters are garbled, the receiver cannot detect the transmitted character and will indicate this with an error character (usually "*"). Each character is transmitted twice but in an interleaved mode with four-characters delay. The first transmission (DX) of a specific character is followed by the transmission of four other characters, after which the retransmission (RX) of the first character takes place, allowing for time-diversity reception of 280 ms: four characters with 7 bit per character and 10 ms per bit (100 Baud).

DX      A   B   C   D   E   F   G ...      first transmission of characters
RX        x   x   A   B   C   D   ...      retransmitted characters (4 char. delay)
---   ----------------------------------
TX      A x B x C A D B E C F D G ...      characters in interleaved transmission

Development of the NAVTEX service in the United Kingdom and beyond

Notice: This text is based on information that appeared in The Marine Observer, - a journal produced by the "Meteorological Office (Marine Division)"

NAVTEX is an acronym for NAVigational TEXt messages. The technology for the dissemination of these text messages via a simple radio-telex system has been about for some considerable time, certainly since the late 1970s when the Post Office Coastal Radio Station at Cullercoats in the North-East of England commenced broadcasting weather forecasts and gale warnings for the North Sea and most of the English Channel shipping forecast areas from Fair Isle to Plymouth on what was then referred to as a temporary radio teletype broadcast. Apart from a change of name from the Post Office Coastal Radio Station Cullercoats, to British Telecom International Coastal Radio Station Cullercoats in the early 1980s the temporary service continued with reports coming in of its success. In April 1983 the service was declared as an operational service alongside those provided via the conventional means of the MF Morse broadcasts and those via the marine radio-telephony service.

On 1st October 1983 the service was extended to the BT coastal radio station at Portpatrick. The weather information broadcast from Portpatrick included forecasts and a gale-warning service for the western sea areas of the UK, including Fair Isle in the north and all the western coastal sea areas from Lundy in the South to South-East Iceland in the North.

During 1985, NAVTEX broadcasts were started via Land End Radio. This broadcast included warnings and forecasts for the English Channel, the Irish Sea and all the sea areas in the South-West approaches included in the main shipping forecast. However, reports suggested that the site at Lands End was not ideal for the intended area of coverage, and so the NAVTEX facility at Lands End was moved about a year later, in 1986, to the BT International coastal radio station at Niton on the Isle of Wight.

By 1987, interest in NAVTEX was growing and NAVTEX services were being successively introduced in other countries. Gradually the information was extended from just weather to cover all the Marine Safety Information (MSI) in the bulletins. Also, the service on 518kHz was formalised in the Radio Regulations and in the World Administrative Radio Conference for Mobile Services. In 1994, the 2nd edition of the NAVTEX Manual was produced. This has only just been superceded with a new edition coming into force in 2013.

NAVTEX was incorporated into the regulations for the Global Maritime Distress and Safety System (GMDSS), which was introduced in a transitional phase from 1992 to 1999, after which it became mandatory under Chapter V of the SOLAS regulations. From 1993, all SOLAS vessels were required to carry a NAVTEX receiver that always listened on 518kHz - if it was desired to receive on one of the other frequencies, a dual receiver system was required.

NAVTEX - SOLAS compliance

Vessels obliged to carry NAVTEX equipment within the GMDSS regulations must have the receiver switched on and tuned to the 518 kHz frequency continuously, and if access to the new national frequency is required then a second receiver is required.
On a vessel which is not required by the SOLAS regulations to carry NAVTEX equipment, it is feasible to have a receiver that can only receive one frequency at any one time. The user can switch to the national frequency of 490 kHz for reception of a bulletin and then switch back to the 518 kHz frequency when the bulletin has been received. This is possible since the NAVTEX station that one will be using will have different time slots for input into the 518 kHz and the 490 kHz service. Vessels that are required to carry NAVTEX equipment by the SOLAS regulations must carry a dual frequency receiver if reception on 490 kHz is required since availability to receive messages on 518 kHz must be maintained at all times.