Re: REU - DSC Logs from Ariano Irpino, South Italy

John Pumford-Green
 

On 24/07/2019 14:15, peter via Groups.Io wrote:

Hi Rosario & Jay,
I tend to agree with Jay though my knowledge on the *026* is poor - but the message was signed as an *ERR* I have nothing logged here at that time relevant.  Maybe John can explain more..
73, Peter

I agree that there's at least one error in the message - the ECC check fails - hence the "ECC: 84 ERR"

Looking at YaDDNet I see that there's a message from Novorossiysk Radio, 002734411 at the same time as Rosario's report of "Vladivistok", addressed to the same vessel, and received without ECC errors by Frikko Nuijen (NL8811):


Input : [NL8811];2187.5;SEL;273357650;SAF;002734411;TEST;NOINF;--;--;ACK;ECC 62 OK
Time : 2019-07-23 19:39:12
Monitor : NL8811
Frequency : 2187.5 kHz
Format : SEL
To : SHIP 273357650 : GELIUS 1, UGIA, Cargo : Russia
Category : SAF
From : COAST 002734411 : Novorossiysk Radio : Russia
Tele 1 : TEST
Tele 2 : NOINF
Freq : --
Pos : --
EOS : ACK
ECC : OK


Rosario's message is almost certainly the same one, from Novorossiysk.

The correct symbols for the "real" message would have been:

120 120 027 033 057 065 000 108 000 027 034 041 010 118 126 126 126
126 126 126 126 122 062 122 122

The "062" is the transmitted ECC character, obtained by XORing together the message symbols:

"120 027 033 057 065 000 108 000 027 034 041 010 118 126 126 126 126 126 126 126 122"


Compare this to Rosario's message:

120 120 027 033 057 065 000 108 000 027 034 041 *026* 118 126 ~~~ 126
~~~ 126 126 126 ~~~ ~~~ 040 ~~~

His message shows that YaDD calculated an ECC value of 84 (ECC: 84 ERR) which was obtained by XORing together the available symbols:

"120 027 033 057 065 000 108 000 027 034 041 026 118 126 126 126 126 126 040"

There are a few things wrong :

1) the message does not have ALL the symbols to work with (some have failed the earlier parity test and are displayed as ~~~ but are ignored in the overall checksum calculation)

2) the MMSI symbol "026" is wrong (in 99.9% of cases the final digit should be "0", but there are some rare exceptions, and AKAIK never used by coast stations)

3) the "040" symbol at the end is the "End of Sequence" symbol and cannot have a value of 040. The only acceptable values are
117 = REQ
122 = ACK
127 = EOS



Rosario, in general ONLY messages that have "ECC xx OK" are to be believed. If a message has "ECC xx ERR" this means the checksum has not calculated correctly and there are errors in the message - which could include the MMSI of the coast station.

Regards,

John

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