Whats wrong with my Navtex printer ??

[clyst]

Hi all my Nav 4 printer has decided to mis-spell words !! It replaces ramdom letters with * s. Could this be caused by a bad signal or is it caused by something more sinister ??

Cheers

Terry

 

[PilotWolf]

Poor data reception. It replaces the characters it doesn't 'receive' with the *.

Fairly normal to miss some depending on where you are in relation to the station, weather, etc. - usual things that affect radio signal reception.

W.

 

[Bilgediver]

Hi all my Nav 4 printer has decided to mis-spell words !! It replaces ramdom letters with * s. Could this be caused by a bad signal or is it caused by something more sinister ??

Cheers

Terry

In one word.....Atmospherics As someone else has said it is usual for this type of receiver to replace missing letters with a symbol. The reason for missing letters is data corruption due either to an antenna or some other fault or weak signals as affected by conditions.

 

[shmoo]

The error checking algorithm in the Navtex protocol is quite easy to fool. This means that while you are seeing "*"s coming though you should also take other data with a pinch of salt. The "*" represent the octets the set knows it got wrong. Odds are there will be others that, although mangled, escaped detection. It matters especially for numeric stuff such as positions and met data.

If I recall Navtex uses one bit of parity and sends the whole octet twice (once back to front?) and if they don't compare at the receive end the character gets starred. However the law of sod says that it is not too unlikely the wrong two bits get hosed in a noise burst so as to sneak the damaged character through error detection.

It is pretty remarkable it works at all...