About this page
NAVTEX reception problems and how to solve (some of ) them.
I have had a love-hate relationship with NAVTEX, tending heavily to the former. NAVTEX provides much information but, for most of us yachtsmen and women, weather is probably the most important. My experiences in installing a NAVTEX receiver and using it may help others.
All terrestrial radio systems have problems but those surrounding NAVTEX have attracted more than their fair share of criticism. This page is a commentary on some of these.
- A more detailed version of this page
- NAVTEX schedules
- HF Reception problems
- NAVTEX and why we need it
On this page -
- The NAVTEX signal
- Reception in harbour and near the coast
- Inter station effects
- Sky wave effects
- Equipment installation
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?
The NAVTEX signal
NAVTEX signals, of 424 t0 518 kHz and at low power (1 kW by day and 1/3 kW by night), can be absorbed by the ground but much less so over the sea, a good conductor of radio waves. For these reasons, NAVTEX transmitters are always sited for optimum results in their service area. See the diagram on the NAVTEX and why we need it page.
The wavelength, around 600 m, is at the long end of the medium wave band and reception is intended to be by ground wave rather than sky wave via the ionosphere as is the case for longer distance broadcasts eg the German Met service Radioteletype and Radiofacsimile broadcasts on 10 to 14 mHz.
The bottom of the ionosphere, the D-layer is ionized during the day and absorbs signals less than about 7 mHz... This prevents NAVTEX signals from reaching and being reflected by higher layers. These frequencies were chosen to limit range and enable many NAVTEX stations to transmit different messages at the same time with minimum mutual interference.
Reception in harbour and near the coast
NAVTEX is not intended for reception in harbour; the low transmission power, sources of electro-magnetic radiation from machines etc, screening by buildings and other vessels all contribute to block or distort signals. Military and commercial ports are particularly bad. You may well get satisfactory reception in harbour: but donít expect it.
Weakening where the signal has come over land is likely to give poor reception. Particularly poor reception will be when under steep cliffs in the direction of the transmitter. Go further offshore and the signals may well improve.
Day time inter-station interference
As far as possible station IDs, determining times of broadcast, are chosen to minimise interference between stations. Sometimes a station over-runs its slot and, if near enough, interferes with the next station in sequence, making the message unintelligible. In high pressure weather situations, there may be interference between stations in an adjacent METAREA.
Both effects should be rare occurrences but are difficult to avoid totally. NAVTEX transmissions are continuously monitored and failures to transmit are exceedingly rare.
Night time inter-station interference
This occurs when the sun goes down and ionization that has occurred during the day weakens. NAVTEX signals then penetrate the D-layer to be reflected by the E-layer and so may be received up to 2000 miles or more away. This can cause interference with a distant station with the same ID and, therefore, broadcasting at the same time. Being due to ionospheric weather conditions, the effect may occur on several nights in succession. It is impossible to avoid.
A schematic of NAVTEX transmissions. Ground wavs will give good signals over the sea to 250 miles, generally and up to 400 at most. Due to Ionospheric "weather", sky waves at night may overwrite even a good ground wave signal.
Night time signals penetrate the D- Layer to be reflected by the E-layer.
The path of the ground wave signal is in the dense troposphere while the sky wave path is mostly through the much less dense air higher up. Greater attenuation of the ground wave is, partly at least, why sky wave signals are the stronger and can completely overwrite a signal from a much nearer transmitter.
It goes without saying that a NAVTEX set should be installed as specified by the manufacturer. Location of the aerial is not critical and will normally work well from push-pit level. Budget sets can work as well as more expensive ones with differences being more in design features and longevity rather than performance.
One exception is that some older NASA sets seem to benefit by running an earth from the aerial screen, as near the aerial as possible, directly to a good solid earth, such as a sacrificial anode. According to a radio expert, this simulates the bottom half of a dipole Ė the optimum aerial configuration. The normal set earth via keel bolts or engine casing does not seem to work as well. Other sets may benefit likewise.
If having reception problems, first check whether others are also: if they are, try to use another system or move, probably only a short distance, elsewhere and try again. If your set has been installed some time ago, check the aerial, power and any signal cables, paying particular attention to the connections. Corrosion is the enemy of NAVTEX.
NAVTEX works pretty well by day in areas where it is intended to work. Night time performance can be determined by ionospheric conditions but is usually satisfactory. It is rare for your set to fail but always worth checking. It is rare for the service itself to break down.
If you are experiencing problems and are sure that it is not your set and that you are not in harbour or too near to land then please contact the UK Hydrographic Office where staff represent the UK on the IMO NAVTEX Co-coordinating Panel and will refer any reasonable comments or complaints to the relevant national authority or the Panel itself.
The IMO NAVTEX Manual at http://www.imo.org/includes/blastDataOnly.asp/data_id%3D9810/1122.pdf is a plain language, full description of what NAVTEX is all about.
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 SOLAS vessels and others bound by regulatory provision, it should also recognise the needs of all vessels in all user categories. IMO is likely to wish for a system that relies upon broadcasting all safety information. Those in the leisure sector are already moving to web based systems. These are clearly very powerful and flexible. Costs of equipment such as mobile and satellite telephones are likely to come down in time. Similarly operating costs. There are several technical possibilities which I will not go into here.
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.
Some thoughts on these issues are on my Future of the GMDSS page.
This page was initially written based on a page for the Cruising Association.