NAVTEX problems in more detail

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A more complete version of my NAVTEX reception problems page.

My brief page on NAVTEX problems probably leaves more questions unanswered than answered. This may fill the gaps.

This page assumes that the reader has studied :-

Choice of set

When we bought our first NAVTEX set, it came down, at the time, to the NASA NAVTEX Pro and the ICS NAV 4. I had heard good reports of both. Not being at all sure how useful NAVTEX would be, we chose the cheap and cheerful NASA set.

As ever, the technology has moved on and there are, now, better sets to choose from. Each will have pluses and minuses. SOLAS standard sets do not allow de-selection of message types A, B, C, or D (Navigation warnings, Gale warnings, Ice information and SAR & Distress). 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).

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..

A second NAVTEX frequency of 490 kHz is for "National" use, such as local language rather than English. The UK uses 490 kHz for Inshore waters forecasts that are not international SOLAS requirements.

The more expensive dual frequency sets can receive either the International frequency (518 kHz) or the National frequency completely automatically. With other sets, it is necessary to switch over manually from one to the other.

We upgraded to a Furuno SZ 300D. The 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 have not been aware of significantly better reception and had 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. With the Furuno, you know that you are receiving a message but do not know what it is or, even, if it is being saved or not

Reception Problems

Our early experience with NAVTEX was much as we had expected. We got nothing in the River Dart. At the time we thought this not surprising being in a marina surrounded by masts and in a steep sided valley. We began to receive immediately we were out at sea.

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.

The big let down was on our Summer cruise down to the Spanish Basque country. Crossing the Channel, from Plymouth to the Chenal du Four, we lost Niton's signal as we approached France and never received CROSS Corsen (the aerial is at le Stiff, Ushant). From then on we received nothing at all.

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

Aerials and Earths

I discussed my NAVTEX with others, much more knowledgeable on radio reception, What I learned is summarised here.

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 sea is a good conductor. The land is a poor conductor. Passage over land inevitably leads to signal losses. Range depends upon frequency, polarisation, location of transmitter and receiver, and ground conductivity. During the day time, there are not normally any problems due to 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 is 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 antenna at 2 metres or so 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.

The IMO requirement for the range of NAVTEX stations is, very roughly, 250 nm. A ground wave signal will have a very approximate range in km of 200/F where F= frequency in MHz; at 518 kHz, this is about 400 lm. Because of the conductivity effect, the range for a path completely over the sea may be well beyond the required 250 nm.

Some reviews and comments by sailors make great play of reception of some NAVTEX sets 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 should 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. 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

Even after earthing the aerial screen we still did not think that reception was as good as it might be although the north coast of Spain is over 300 miles from the Corsen transmitter at le Stiff and over 400 miles from Nitpn from which much of the track was over land.

We then looked at the aerial position on the push-pit and realised that it was near the vertical pole for the wind generator. 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'Ol�ron and Santander, 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 Coru�a 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 Na�). Near Barcelona, Valencia was getting erratic.

I still thought that my reception was not so good as it might but could not decide whether this was due to the budget priced set or the aerial. Then NASA brought out an active aerial. This is a short stub rather than their earlier piece of wire. The results have been excellent. For example, we were receiving Toulon down the east side of Sardinia. In 2002, in Tarragona marina, we were receiving both Toulon as well as Valentia.

Any yachtsman not getting good NAVTEX reception should inspect his on board installation carefully and experiment as we did. 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 from 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.

Reception of NAVTEX - What we have experienced

NAVTEX uses multiple stations on a single frequency. Inter station interference is minimised by careful management of time slots. However, the success of the system has resulted in more stations transmitting and time slots are fully utilized. Generally this is not a problem, but unusual propagation events result in either garbled signals or occasionally getting the data from a more distant and less relevant station. This long distance reception is the cause of much annoyance to users.

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.

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 at night with Valentia (W) in SW Ireland over-writing la Garde (also W). Along the south coast of Spain we and others have experienced Cullercoats over-writing Tarifa, both stations with indicator G. I have experienced 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 Valencia 960 nm to the west. Such distances are too great, and we have experienced them too often, for this to be explicable always as ground waves under a high pressure area. We thought that they were too near to be sky wave.

The distant stations generally came in as a strong signal giving clear text with few errors. There was often good reception from Crete,Iraklio,n and the Greek station, Lemnos, about 210 nm away. Rather oddly, reception from the nearest Greek station at Kerkyra around 100 nm was erratic. We received Split (some 330 nm) and Augusta (240 nm) as well by day as by night.

NAVTEX reception explained

Sky wave for NAVTEX frequencies is usually a night time effect (or winter days in high latitudes) and gives greatly enhanced range. 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 earth�s magnetic field, solar activity, cosmic radiation and, to a limited extent, tropospheric weather. Between them, these cause very pronounced changes in range. The reality is much more complex and less predictable than the standard text book descriptions would suggest. 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.

However, NAVTEX transmitter antennae produce a fairly broad beam width rather than the pencil thin beam described in the text book. This provides a mechanism for the first signal to return to ground either closer or further away depending on the reflective characteristics of the Ionosphere on any given day.

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.

When unusual propagation events occur improving 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?

Having got good signal, is NAVTEX useful? Navigation warnings, warnings of missing vessels, firing range messages are clearly all very interesting and useful from time to time. On one occasion when returning from France, there is no other way that we would have known about changes to the characteristics of the Eddystone light. Messages about large floating objects such as nets and containers can only be useful. However, in practice, only a small fraction of the vast wealth of information is likely to affect the yachtsman. It must be quite rare that a leisure sailor would come to harm without NAVTEX - but it is a form of extra insurance and an aid to peace of mind.

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.

Sensible decision making for yachts on passages of upwards of a day or so, across Biscay or the North Sea for example, need more frequent weather updates and longer period information. An additional reason for more frequent weather information is to guard against loss of signal or message garbling. Navigation and other warnings are repeated and will always be received sooner or later. If you miss a weather broadcast there is no repeat.

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. See the NAVTEX schedules page 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 At times, it has been our sole reliable source of water forecasts.

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 signals, I doubt that much can be done about this problem There are areas where there is no NAVTEX transmitter within 300 miles, Deep into the Southeast of Biscay is clearly one such. Here and elsewhere around coastal waters of western Europe, the BBC LW Shipping forecasts is the ultimate back-up system. Long However, Radio 4 has limited life expectancy as LW/MF public service radio is replaced by DAB..

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. In 2008, that situationsimproved but 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.

Summary of my experiences

My findings, experiences and general thoughts are that-

A good earth for the aerial cable screen to sacrificial anode or grounding plate may improve reception.
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.
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..
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 time problem cause by sky wave or abnormal ground waves. There is no obvious solution.
From time to time, NAVTEX may be your sole source of weather forecasts.

The Future?

Following the original version of this page there has been much progress on improving the content of the UK NAVTEX service. See the NAVTEX Weather Broadcasts page which I will try to keep up to date on developments of this important and potentially valuable service.

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.

In principle a NAVTEX Mark 2 is required but I have see no sign of any planning to that end. 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 likely de facto solution

I have discussed the future of the GMDSS on another page. My personal conclusions are that the Internet will become, it largely has already, the main source of MSI. The sooner that this is recognised officially, the better. Just a thought.

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