Old technology with global use
(This article is made only for my own desire to learn)
Those who have read my blog is aware of the fact that I have a major interest in digital modes in ham radio. Throughout my work life I’ve come in contact with a lot of different types of digital modes in mainly commercial systems. One of these systems, or services maybe, is NAVTEX. Not only is it a broadcasting service for maritime messages, it’s also an example of a mode using a simple form of error correction with a worldwide usage.
The technology is, with modern standards, maybe old and even out of date. There also are better and much effective ways to deliver small messages, but many of them is dependent of different types of infrastructure to function. NAVTEX is a robust and, for the case of it’s special use, effective way for delivering the information.
What is NAVTEX?
NAVTEX (Navigational Telex) is actually the name of the service aimed towards the maritime community. The service is used for transmitting navigational warnings, security and weather information to seafarers. It does this in broadcast mode and traditionally, the received messages is printed out directly from the receiving device. The service is also called NBDP, Narrow Band Direct Printing. NAVTEX is a part of the Global Maritime Distress and Safety System (GMDSS) which is an international marine safety and distress system agreed and managed by, among others, the IMO (International Maritime Organization), ITU (International Telecommunication Union) and other UN organisations. For ships complying under the SOLAS regulations (Safety Of Life At Sea), practically any merchant and/or passenger vessel, a NAVTEX receiver is mandatory to carry. For small recreational vessels however, this is an optional equipment to have.
The service is made up of a land based broadcasting network using a number of transmitters in the world’s coastal areas. The network itself is managed by the respective country it is located in. For Sweden it is the Swedish Maritime Administration, Sjöfartsverket, who owns and maintains the transmitters. The Swedish transmitters is located at three sites along the Swedish coast, Grimeton (next to the World heritage Alexanderson Generator site), Gislövhammar in the county of Scania and Bjuröklubb in the county of Österbotten. On the same sites, MF/HF transmitters and receivers are located for the Swedish coastal radio.
The broadcast range is typical 300 to 400 nautical miles of the coast but it is common, depending on the quality of the installation of the monitoring station ,to receive more distant transmitters.
To coordinate and structure the NAVTEX network the coastal regions of the world has been divided into NAVAREAS. The image below shows the worlds NAVAREAS and also the administrative responsibility for the region.
Figure 1. NAVAREAS
Each NAVAREA has a number of transmitters which are arranged into time slots which is identified by the alphabet from A to X. This means practically that within a NAVAREA, each transmitter has a identifier letter. The same identifier letter can however be reused within a another NAVAREA why the letter can´t be used as a “true” individual transmitter identifier.
To reduce the effect of overhearing two transmitters using the same timeslot the transmitters and their timeslots are coordinated so that assigned timeslots and transmitter range is considered. The transmitters follows a predefined transmission schedule where each identifier has six time slots during a 24h cycle. Each timeslot is ten minutes in duration.
The NAVTEX service uses a transmission mode called SITOR (SImplex Teleprinting Over Radio) which is based on RTTY. There is a major difference though as SITOR has the ability of error correction which RTTY has not. The mode uses FSK (Frequency Shift Keying) with a shift of 170 Hz and a typical baud rate of 100 bd. SITOR is also divided into two submodes, SITOR-A and SITOR-B.
- SITOR-A uses an error correction technology called ARQ (Automatic Repeat reQuest) and requires a answering station for managing the error correction capability. This mode is often used for point to point connections.
- SITOR-B on the other hand is a broadcasting mode which either can broadcast to a collective group of receiving stations (CBSS) or a selected receiving station (SBSS). The two ways of broadcast modes uses the same form of error correction, FEC (Forward Error Control). When using FEC, each letter or symbol in a transmitted message is transmitted twice in different time frames. The time diversity between the two frames is about 280 ms which is intended to reduce the effect of interference and deterioration due to propagation effects. However, If the information in both time frames are lost, there is no way of retrieving it.
The NAVTEX service uses the SITOR-B (CBSS) mode for its transmissions and each transmission is initiated with a number of phasing signals. These phasing signals is easy recognised by its aural sound. The purpose of the phasing signals is to serve as a “wake up” signal to the decoder and to allow the decoder to determine the location of the duplicated frames (FEC) in the transmission and start printing the information that follows. The phasing therefore serves as a form of synchronization between the transmitter and receiver.
The SITOR mode is a good way to send or broadcast plain text messages, however it is not capable to handle the complete ASCII character set why it is not useful for transfer of binary information or files.
The SITOR mode has an HAM radio equivalent, more or less, in AMTOR. The use of AMTOR on the HAM-bands is nowadays practically non existent. The introduction of PSK31 and digital mode softwares have, sadly, seemed to pass AMTOR to digimode history. Also, the introduction of PACTOR with its high speed modes in PACTOR-2 & 3, have contributed for a wide usage as it is able to transfer binary files.
The NAVTEX service is found on designated frequencies assigned especially for NAVTEX. These frequencies are 490 kHz and 519 kHz, where 490 kHz is intended for transmissions in local languages and 519 kHz is the standard frequency with messages in English. Apart from the MF bands, there is an assigned frequency in the 4 MHz band (4209,5 MHz) for national use. In europe, Turkey is using this frequency for it´s national transmissions.
The video below shows an example of an SITOR-B transmission received by an Kantronics KAM modem.
The messages that is transmitted from the NAVTEX network are mainly navigation, safety and weather information. A transmitted messages follows a certain format which contains not only the conveyed information, but also the identification of the transmitter (timeslot) that is sending the message.
A typical transmission is started with the phasing signals which last for about 10sec. After this the characters “ZCZC” is sent followed by a space which indicates the start of the message. Next, a four character code is transmitted which is called the B1, B2, B3 and B4 characters.
- The B1 character is called Transmission Identification Character and is a single letter which correspond to a transmitter within a certain NAVAREA. As I mentioned earlier, this also correspond to the transmitters timeslot.
- The B2 character is called the Subject Indicator Character which indicates if the messages is classed as Navigational warnings, Meteorological warnings, Ice reports, Meteorological forecasts and so on. Totally there are 26 types of subject groups were 12 are in use. Each subject is assigned a letter spanning from A to Z where A to L and Z is currently used. The subject indicator character Z indicates that there is no message on hand to transmit.
- The B3 and B4 characters are Message Numbering Characters which indicates the sequential serial number of the message within the subject group ( refer to the B2 character above). The messages can be assigned a serial number between 01-99.
After the B1 to B4 characters have been transmitted, a carriage return is sent followed by a line feed and the actual message information is transmitted. In the end of the message four characters “NNNN” indicates the end of the message and a carriage return plus two line feeds are sent. If there are no more messages to transmit, the transmitter will send idle characters for about 2 seconds before it will turn the transmitter of. However, if there are more messages to send the it will start sending phasing signals for about 5 seconds and then repeat the cycle from ZCZC to NNNN.
Figure 2, NAVTEX message transmit cycle.
The message below is an example of a NAVTEX transmission received by my own receiving equipment.
Figure 2, NAVTEX message
In the message header you can find the “ZCZC” followed by “JE85”. “J” is the identifier for the transmitter, which in this case is the Gislövshammar site in the county of Scania in Sweden. The subject character is “E” which indicates that the message is a Meteorological forecast and the numbering characters indicates that it is the 85:th message in sequence in the subject group. After the header, the actual message is shown and is ended with “NNNN”.
Decoding NAVTEX can be done either by software or hardware. Generally there are a lot of products supporting this and are available in both professional and hobby versions. Regarding softwares, there are a lot of different softwares available for the purpose and I have selected two of them:
- FLDigi is a multimode software which can handle a number of different digital modes. It is capable of decoding NAVTEX in pure text mode. FLDigi can be found here.
- YaNd (Yet another NAVTEX decoder) is a personal favorite. It´s a dedicated NAVTEX decoder software with a lot of built in functions and informations regarding NAVTEX. The software also keeps a NAVTEX log with all decoded message and automatically identifies the transmitting station. You can find the software here.
When it comes to hardware decoders, there is a number of dedicated NAVTEX receivers primarily aimed to the maritime market. The receivers can utilize the output information on a printer,LCD/TFT display och directly into an ships integrated bridge system and/or computers. The receivers ranges from 600 euros to 2000 euros in cost.
Figure 3. Commercial NAVTEX receiver with direct pritning.
For personal and HAM use, there are a number of hardware modems that can handle NAVTEX decoding. Two of these which I have come in contact with myself are:
- PTC-range controllers from SCS. These modems or controllers, at least from the PTC-2 controler, is able to handle NAVTEX decoding. The units is popular among private around the world sailors using e-mail services via HF. These units are expensive but from experience they are high quality products, even beeing use professionally by the Red Cross and UN- organisations.
- Kantronics KAM series modems (KAM, KAM+, KAM 98). The Kantronics modems was popular among HAM-radio operators as these could handle a lot of different digital modes. However they become somewhat obsolete as the soundcard base digimode softwares took of. Even if the modems are old, they are still useful as they have a built in NAVTEX decoder mode which presents the decoded message directly on the terminal screen after reception. The NAVTEX mode does however discard messages if it has too many errors during the reception why it is recommended to use the AMTOR listen mode as this displays what´s received even if there are errors.
How to and why?
Getting started monitoring NAVTEX transmissions is easy if you already have a HAM-radio transceiver covering the MF bands. The antenna could however be a issue but generally is you live near the coastal areas you can use your existing antenna for reception. The frequencies in the MF bands can be subject of interference in populated areas but if you are lucky you are able to monitor a lot of stations.
Regarding decoding I can personally recommend the YaNd software as this is easy to use and contains all you need for NAVTEX monitoring. The software uses the computer sound card and its microphone input. A simple 3,5mm audio connector cable and a gentle adjustment of the audio input levels is also required. The software runs under Windows and is free to use.
Then why should you monitor NAVTEX? Well, just for fun or what else? 🙂
For those of you interested in DX:ing, NAVTEX monitoring can be an exciting mode to monitor because of it’s location of the MF band. It also gives you an interesting experience of propagation on those frequencies. Also it has, at least for me, a small nostalgic reason as well as a desire of understanding existing HF-communication systems and its implementations.
73 de SM7VRZ