Navtex Problems

[JamesS]

Has anyone experienced problems over the past two weeks receiving 24 hour weather forecasts by Navtex from Niton - message prefix SE - on the 518kHz band?

I receive all the other messages, including the long range forecast, but not the 24 hr forecast.

Have tried Cullercoats - same problem.

Falmouth Coastguard who coordinate weather transmissions, confirm that the messages are being entered into the system.

Any help appreciated.

Cheers

 

[ParaHandy]

Might not be able to help, tho' might know why. Presume you mean the 0700 and 1900 utc transmissions on 518khz?

The time slot Falmouth (who actually control the Niton transmission) have is 10 minutes and if the transmission exceeds that length, it gets corrupted by the following transmissions (T&M code at 0710 & 1910) from Ostend. The unfortunate fact is that the weather transmission *follows* all the other clutter eg nav warnings, gps off etc and if the coastguard has not calculated his timing correctly, the weather will be missed or only partially transmitted. This happened to me twice last week but is not necessarily a regular occurrence. For example, yesterdays 1900 transmission took 8 minutes.

Question for you: did you receive no forecast or a partial one? If none. something wrong with your receiver programming?

 

[JamesS]

Thanks for your reply - in the meantimeI think I may have discovered at least a reason for the loss of messages.

According to the ICS web site FAQ (www.icselectronics.co.uk) it could be due to a weak signal. This could mean that most messages, which are of short duration, are accepted whereas the weather forecast, being of longer duration, is more suseptable to interference. A count of 16 consecutive error characters will result in the rejection of the message by the Nav4.

This coincides with the SE messages reading 'Navtex Message Rejected' which I am receiving.

The ICS suggestion by the way is to (1) Raise the aerial and (2) Run a ground wire from the Nav4 to the seawater ground point (3) Eliminate any electrical 'noise' from chargers etc.

Thanks for all your help.

Cheers

 

[ParaHandy]

Is the Nav 4 dual frequency and, if so, do you receive 490khz OK?

Might find that you are experiencing poor reception due to location eg in a harbour rather than anything to do with grounding or aerial. There is a lot of interference in a ferry port.

 

[alex_rogers]

You didn't mention which Navtex receiver you have. I've had a Furuno NX300 for the past year and it's reception of Niton is always very patchy. The last few months were fine but it didn't get anything from Niton this weekend.

The makers claim it is some problem with Niton as the set receives other stations perfectly.

 

[ParaHandy]

It seems that an awful lot of people are not receiving 518khz and as it's the only forecast most folk out of sight of land can use, it's quite important.

The navtex weather message string has a header "zczc seXX" where XX is a sequential number and "s" denotes the transmitting station (Niton in this case) & the letter "e" identifying that the transmission is a weather bulletin. The footer is the string "nnnn". I presume what happens is that the receiver reads the header and, if selected, transfers the data into an internal buffer until the footer string is received. Any error, eg another header or sequence/number of non ascii characters, and the transmission is dumped.

This weekend, I received all navtex correctly but I noticed that Falmouth inserted a gale warning (sb50) at 0318 on 1 April which cut off the Oostende ta89 0310 transmission. On further investigation, this goes on all the time where one station transmits without thinking about what they're doing. The bod at Falmouth might not realise just how easily the transmission from Niton can be picked up 100s of miles away from the intended area. Monsanto was picked up during that night and that's at the bottom of Portugal.

At 0746 Corsen might have accidentally switched their transmitter on to deliver a gale warning, ab51, which they then repeated at 0800. The French also tend to utilise their alloted slot fully which really isn't on as the transmitter injects leading and trailing characters which last for 10-20 seconds.

The stations are obliged to transmit these warnings as soon as possible after receipt but my argument is that they might apply some commonsense before they do.

For comparison, the 490khz local transmissions are faultless - as far as I can tell.

 

[JamesS]

We have a Nav4 - it seems a nightmare that Niton can be received hundreds of mile away but not in the Solent!!

Cheers

 

[JamesS]

No, we have a single frequency (518kHz) - see my comments below re Niton.

Cheers

 

[ParaHandy]

Hang on a mo! The point isn't that 518khz Niton can be received from 100s miles away. It is that 20 miles out to sea, *in front of Niton*, the signal just drops. With my ssb set-up I can measure signal strength etc and this happened not once, but three times over two days with the result that I missed 3 consecutive complete weather bulletins. Also, I am not at all sure that the signal dropped thro' interference - there was no audible signal for both latter occasions.

With my set, I can at least receive the transmission up to the point that it drops off. Your dedicated navtex sets won't have that ability because they reject the transmission as soon as an error is detected (eg not receiving the footer).

The MCA, whose responsibility this is, are at times a most condescending bunch who readily criticise others. They are, however, in their objectives giving themselves responsibility for improving safety at sea and, imho, they are not doing so if this continues.