Local navtex service on 490 kHz

tony378

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Has anyone had difficulty with the local Navtex service? I find that if they transmit a strong wind warning - IBxx message - my receiver does not store the following inshore forecast IExx message. I first noticed this in 2003 and then again in 2004. I have had my receiver checked by the maker and no fault found.

What happens is that the IExx message is not stored, it just scrolls across the bottom of the screen. I was able to monitor the signals using an HF receiver and Navtex decoder software on a p.c. which is how I found out what was happening. If the sequence is IBxx, IEyy, IBzz then only the two IB messages are stored.

I think that this is a transmission problem, but would like confirmation from others.

Have you ever checked the Navtex and been surprised that there was no Inshore Forecast, just strong wind warnings?
 

ShipsWoofy

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Ok I will have a stab at this. It is quite difficult as you do not give a location nor what type of navtex you are using.

To me it sounds like you have programmed the set to disregard certain messages. If as it sounds you are using a nasa unit then yes the message will be received but not stored; This you have confirmed by saying you see it scrolling across the bottom.

Try setting your navtex to receive all messages. It is definitely your set or setup. There is no difference in transmission with respect to messages.

IB is gale warnings
IE is weather forecast

I is the station ident, in this case Niton

I have the nasa clipper and have no problems receiving from Portpatrick, I have all other stations switched off while not cruising.
 

steve123

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Tony

I had just this problem last year since fitting a dual frequency aerial to my NASA Navtex Pro. I have carefully checked that I have it set to record the appropriate messages. But when a warning is broadcast before the normal forecast, the warning is stored but the on many occasions the forecast is not stored (but it can be seen scrolling across the bottom of the screen).

I have been in touch with a guy at the UKHO and he liases with the Met Office on Navtex issues. It appears that there may be problems when one message follows another. It seems that the either header for the second message is not complete or the phasing signal between the messages is incorrect and the NASA doesn't recognise it as a message to be recorded. Last I heard they were still investigating. I haven't had chance to record any local messages yet this year. I receive the Cullercoates signal but the messages are put together in Aberdeen before distribution to the different transmitters, so I suspect the problem is common to them all.

Steve
 

BlueChip

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I have an ICS Navtex and it stores all messages in NVR - let me know when this next happens to you and I will look back through the message log and see if it happened to me. Must say I have never seen this - I suspect it may be a NASA problem in the way it decodes the messages
 

tony378

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Thanks for the info, to deal with some points. The receiver is a Nasa Target Navtex Pro. It is programmed to receive all messages. I usually sail on the East Coast but currently hsve the kit set up at home in Norwich. I can received the other UK stations, in fact sometimes I get a Portugese station.

I have not monitored the other stations as closely as Niton (yes I do know 'I' is Niton) but it may be that they perform better.

Latest occurence was yesterday 18/04/05 at 17.20 utc I received IB 77 and the following IE70 was not stored. Previous on 17/04/05 IB73 and IB74 ok IE68 lost.
 

ShipsWoofy

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Not trying to teach the art of egg sucking, but have you set all messages on both National and International?
 

tony378

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Difficult, if you know the Nasa hardware, you will know that it cannot distinguish between National and International signals as the input is selected at the r.f. level, the 490 Khz service being frequency multiplied up to 518 kHz.
 

tony378

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Progress report. Thanks for all the contributions. Steve ,in particular, seems to have the same set up as me and shows that I am not alone.

I suggested to Nasa some time ago that there was not enough run in between messages. They said that there should be a minimum of 5 seconds and that I should try to time the interval. I have been doing this and find that the gap between the NNNN of one message and the ZCZC of the next is never more than 2 seconds. Unfortunately for the theory, some messages are correctly decoded even with this interval.

Another thing is the way the Nasa software handles text. Messages are displayed word by word. The space character is used as a flag to detect the end of a word and then display it. The software on my p.c. displays the message character by character. I wonder if this difference may be part of the problem.

I found out about this difference a while ago when in Brittany. I only received the start of a forecast, whereas the other boat we were with had the whole forecast, but with no spaces between the words! Their receiver was also a Nasa, but an earlier version with different software.

When, back in the UK, I contacted Nasa they told me that there had been a fault at BT who, at the time, managed the service.

How does the ICS display text? Is it word by word as the Nasa or character by character as the software on my p.c.
 
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