Whether there is Weather on NAVTEX

My use of the system both in western European waters and in the Mediterranean has led me to think and question the operation.  Is there an alternative?

For a brief description of NAVTEX see another page on this site. For an easily accessible and very useful list of NAVTEX station that will

Commander Chris Pink, RN,  ex-UKHO and former Secretary of the IMO NAVTEX Coordination Panel gave me much good advice. He has been replaced by the equally helpful  Guy Beale. For contact details, see another page on this site. I have also been discussing (learning about, more correctly) radio reception problems with Ross Biddle. Ross is (by my standards) an expert on the general topic and has been very helpful with drafting the better sections of this page.

Choice of set

As ever, the technology has moved on and there are, now, better sets to choose from.  Each will have pluses and minuses.  Non-SOLAS standard sets will allow all messages to be de-selected except for category D (SAR and other emergency). Also there is no audible alarm, or it can be turned off when a D message is received..

SOLAS standard sets do not allow de-selection of A, B, C, or E  (Navigation warnings, Gale warnings, Ice information and Weather Forecasts). They will give an audible alarm when a D message is received. This can be annoying when a message is received at night time from a distant station (see below). 

The introduction of a second NAVTEX frequency of 490 kHz is relatively recent. This is for "National" use, such as local language rather than English.

Our choice was between this and the McMurdo/ICS NAV 6A Plus with the automatic dual frequency option. The Furuno won because of the large and very clear text although the dual frequency reception of the NAV 6A Plus was very attractive. We did not see any point in a set with a third frequency of 4209 kHz because so few stations use it, (only 2 worldwide at time of writing)..

After replacing the NASA set with the Furuno SZ 300D, we certainly have had better reception although with some loss of facilities. The NASA PRO displays what is being received (even if it is not being recorded) by means of a continuous scroll along the bottom of the screen. To save the message, then simply hit a key and it will record. This is very useful if the start of the message has been corrupted and the message not recognised for saying.

Problems

On our early season cruises to North Brittany and Normandy reception was adequate at sea but variable in harbour. At St Peter Port, for example, we received at high water but not at low. Clearly,  height of the aerial might be important, at least, in harbour.

Was NAVTEX over-hyped? Was the system really as useless as many claim?

Aerials and Earths

During the day, NAVTEX signals propagate as a ground wave. That is a wave that travels along the surface of the earth by virtue of inducing currents in the earth. The imperfectly conducting earth leads to some of its characteristics. Range depends upon  frequency, polarisation, location of transmitter and receiver, and ground conductivity. During the day time, there are not normally any sky wave effects.

Ground wav reception is not going to be improved by increasing aerial height  unless you are in a area screened by walls and boats such as a harbour. In fact, it likely that good counterpoise effects available at low heights will improve the signal when at sea. As NAVTEX wavelengths are  around 600 metres, the ratio of height to wavelength is so insignificant as to negate any gain normally associated with antenna height.

The only advantage in putting the aerial up high would be to get it away from man made sources of noise in ports and marinas. Placing the aerial anywhere near a big vertical and earthed structure like the mast could dramatically reduce signal strength compared to an antennae at 3 metres above sea level on the stern of the boat. Putting the antennae at the masthead may give an improved result but that is where I want my VHF antenna. At night, there can be enhancement of the signal. More of this later.

Some reviews of NAVTEX sets (and some manufacturers) make a great play of sets that receive NAVTEX at longer range than others. However, this is not necessarily a virtue. NAVTEX transmissions are deliberately kept at low power in order to restrict range, so avoiding inter-station interference. A good set could be defined as one that receives reliably within the normal ground wave foot print.

I found that with my NASA set, there was a very distinct gain from having a good short earth to the screen of the aerial cable. It is important that this should be to the sea via a sacrificial anode or a grounding plate. The effect of the earth is to simulate the lower arm of a vertical dipole; the NAVTEX signal is vertically polarised.

NOTE - the normal boat  earth is often to the keel bolts and, in many boats these are encapsulated and, so, not a good earth for NAVTEX purposes. The further, roundabout, connection to the sacrificial anode via the engine block seems not to work as well as a short, direct connection.

Finding solutions

We then looked at the aerial position on the pushpit and realised that it was near the vertical pole for the Aerogen.  We then moved the aerial to the other quarter and, after that, got generally acceptable results. Even in harbours as far south as St Dennis d'Oléron we were able to receive messages although with some garbling.

Coming up the coast of France reception improved steadily from both NITON and CORSEN. Interestingly and surprisingly, until we got as far north as South Brittany we found that NITON was often received more completely than CORSEN even though NITON was further away and the signal was coming over land..

On return to the Dart we found good reception there of both NITON and CORSEN. We later heard that CORSEN had been having problems that year. On our 2000 cruise to Barcelona from the Dart we found good reception  from la Coruña, once he came back on the air, as far south as Peniche. Lisboa  (Monsanto) was received from about Peniche round as far as Cape St Vincent but I lost him in Lagos. We received Tarifa from somewhere east of Faro until near Cartegena. After that we received Valencia (Cabo la Naõ). Near Barcelona, Valencia was getting erratic.

Any yachtsman not getting good NAVTEX reception should inspect his on board installation carefully and experiment as we have done. The results are well worth the effort.  It must be remembered that NAVTEX signals are only 1/3  kW at night and 1 kW by day.

Hoping to receive under the shelter of land, a long way from a transmitter is unlikely to be successful.  The proximity of other sources of radio emissions is also likely to affect, adversely, NAVTEX reception. On your own boat, there can be interference from other electrical devices such as inverters/voltage adaptors, battery chargers etc. Experiment is the only advice. We have a HF/SSB receiver and can usually hear interference on that either our own or external sources.

In short, I strongly suspect  that most reported NAVTEX problems are due to poor aerial siting, poor earthing or people simply trying to receive in poor locations. The latter can be in marinas surrounded by masts, in among buildings especially with electric equipment or in areas with a great deal of screening.

Reception of NAVTEX - What we have experienced

Text book discussion of long wave propagation (including those frequencies used by NAVTEX)  usually focus on ground wave and sky wave. Ground wave is primarily responsible for both day and night time propagation within the service range of a station. Depending on transmitter power, terrain and receiver quality, ranges out to a few 100 miles may be expected.

In due course, I realised that the actuality was rather different from received wisdom.  In the western Mediterranean, when in the Balearics and along the south coast of France we had considerable problems with  Valentia (W) in SW Ireland over-writing  la Garde (W). Along the south coast of Spain we and others have experienced. Cullercoats over-writing Tarifa, both stations with indicator G.  I have heard of Jeddah over-writing Iraklion, both H.

Whilst in the Ionians, I monitored All Stations for a number of nights.  We received generally good signals from a wide range of distances from less than 100 NM, Kerkyra (Corfu) up to as far afield as Karlsrona some 1000 or so NM to the north, Novosibirsk over 830 NM to the east and

The distant stations generally came in as a strong signal giving clear text with few errors. There was some improvement for reception from the station on Crete. Iraklion and another Greek station, Lemnos. about 210 NM away. Rather oddly, reception from the nearest Greek station at Kerkyra around 100 NM did not improve, sometimes it as worse.

We received Split (some 330 miles) and Augusta (240 miles) as well by day as by night. In fact, we received these two stations much more reliably than any of the nearer Greek stations. However, after making enquiries to friends, we think that this was a peculiarity of the Furuno SZ 300D, particularly for weather messages and from Kerkyra.. NASA sets are said to receive Greek stations with no problems. Three of us, all Furuno 300D owners, have problems receiving Kerkyra weather messages.

After contacting the IMO NAVTEX people, thence the Greek service the problem was sorted out.  I suspect that there were some errors in some Greek NAVTEX Weather messages (type E) that the Furuno could not accept but to which the cheaper sets are more tolerant. I asked Furuno for an explanation but did not receive a reply. 

NAVTEX reception explained

 However the Ionosphere is far from being static or predictable and, as a result, the range achieved can be affected by steeper or shallower angles of take off and return, the actual height of the Ionosphere,  bouncing off higher layers of the Ionosphere, high noise levels and multiple bounces between the earth and Ionosphere. (Halifax, Nova Scotia NAVTEX has been received in the English Channel.) 

The areas of the Ionosphere reflecting long waves are affected by the earths magnetic field, solar activity, cosmic radiation and, to a limited extent, tropospheric  weather. Between them, these cause very pronounced changes to range. The reality is much more complex and less predictable than the standard text book descriptions would suggest.

The reason that Sky wave is generally inactive during daylight hours is a low level layer in the Ionosphere, the D layer, that absorbs signals going up or back from the more reflective layers of the Ionosphere. The  D layer needs sunlight; it decays each sunset and reforms each dawn.

The text book model of propagation suggests that a gap exists between where ground wave runs out and before the first reflected sky wave returns to earth. In the real world this gap often does not seem to be present  for NAVTEX and, at night, stations at a large range of distances are often received.

In addition to the ground and sky waves, there is another mode of propagation called ducting or wave guide. The signal is trapped in an Ionospheric or atmospheric layer and 'piped' often 1000's of miles. This can occur when a  large and intense high pressure system creates a temperature inversion anywhere up to a 1000 metres above the earth's surface. Ducting takes place within this low layer of the atmosphere.

From a user's perspective I cannot determine which or which combination of  propagation modes my NAVTEX is receiving. The reason for this section is to try to help users understand what may be happening and so reduce frustration when a signal is not received.

Generally the time based separation system works well but users should expect to see some more unusual propagation during winter months, in higher latitudes and where the path between receiver and the transmitter are predominately over water.

When unusual propagation events occur the quality of the NAVTEX installation is unlikely to improve the situation. During these events, closer stations can be blanketed by stations hundreds or even thousands of miles further away.

Is NAVTEX useful?

My first impressions were that weather forecasts on NAVTEX were disappointing.  The limit of a 24 hour period with a very brief outlook for open sea areas does not really meet the needs of many cruising yachtsmen.

In time I have come to realize that it does provide a very useful service.  There have been great improvements around the UK with the introduction of longer outlooks and an inshore service on a secondary frequency of 490 kHz.  Click  here for further information.  Elsewhere, it is very useful to get, automatically, forecasts even though these need supplementing by information from other sources eg Radio Teleprinter, Radio Fax, MRCC/VHF etc

NAVTEX black spots

There are NAVTEX black spots such as in the deep Rias of Spain where other weather sources, such as VHF broadcasts, may also not be available. Without the introduction of far more stations or, perhaps more powerful

The area to the west of La Garde seemed to be another black spot, right up to and including the Languedoc-Roussillon coast.  To the east of Toulon, we got poor reception until near St Tropez.  I believe that the Gulf of Genoa is not good although we were getting good reception around much of Corsica, Sardinia and across to the Italian coast near Viareggio.

Other NAVTEX Problems

Time slots are tight and over runs caused by too long a message or a late start are not unknown. A regular offender used to be la Garde which often interfered with Cabo la Naõ. Because stations broadcast in alphabetical order, Cabo la Naõ (X) follows la Garde (W). This has now been resolved. Apparently, the reason was that the la Garde clock was wrong and had been wrong for a long time!

There were occasions when we were receiving a station at the time for a forecast but never received one. Whether this was an omission by the station or a reception problem. I was never quite sure but other yachtsmen

It is essential that those operating NAVTEX should exert more and better quality control over their broadcast material. Such [problems deter the leisure sailor from installing equipment that common sense says should be an essential to his safety.

The IMO NAVTEX Coordination Panel is very keen to reduce problems as far as possible. They welcome reports from all and those from leisure sailors are particularly valuable because we spend a long tome in an area and get to know the problems well. Problems should be made  known to Guy Beale,

Summary of my experiences

• That a good earth for the aerial cable screen may improve reception.
   
• That it pays to experiment with the position of the aerial.

• Most reception problems are due either to poor installation or lack of understanding of the realities of  radio signal propagation.
   
• That height of aerial may be important, especially in harbour. It may also be important in a rough sea to get extra height above large waves.  

• Weather forecasts apart, much of the broadcast content is likely to be irrelevant to the yachtsman but that there is always the chance that there will be some rather vital information concerned with safety and of interest to the coastal yachtsman..

• That to meet the needs of small craft in general, there needs to be more and more relevant weather information broadcasts.

• Anomalous propagation is, essentially a night tome problem cause by sky wave or abnormal ground waves.

• From time to time, NAVTEX may be your sole source of weather forecasts.

The Future?

If the MCA really want coastal vessels to use NAVTEX then they must ensure that the contents of the broadcasts meet the needs of those vessels. Much of the emphasis, currently, is on the needs of relatively few large ships rather than the vast majority of smaller craft. NAVTEX is old technology with data speeds that are akin to the tortoise compared to the hare of modern computer to computer links that sustain the Internet. 

As has been discussed above, there are a number of problems that  result from the technology,  the way in which it has developed and been implemented.  These militate against the installation and use of the system by those not bound to do so by regulations.  The safety of many seafarers, in small vessels, is jeopardized thus increasing the possibility of emergency services being called out. At some stage a NAVTEX Mark 2 will be required and planning should be already in hand.  While the system would have to meet, fully, the needs of

The danger is that a perceived poor performance of NAVTEX coupled with a long delay in implementing an adequate replacement, will simply encourage the yachtsman to rely solely on web based services and ignore the GMDSS.  This would be potentially disastrous. 

The challenge for IMO is to keep the very many small vessel skippers sufficiently interested and satisfied with NAVTEX to continue using it The UK is represented on the IMO NAVTEX Co-ordination Panel by  Guy Beale who has said that he is happy to hear about problems encountered by users of NAVTEX. His address is Guy Beale, The U K Hydrographic Office, Admiralty Way, Taunton, TA1, 2DN.  Contact addresses are on another page.