Re: 00540000 query

John Pumford-Green
 

Hi Peter,

I might have good news for you...

Looking at the symbols:

120 56 12 60 0 0 108 0 65 40 0 0 118 126 126 126 126 126 126 126 122 39

The final "39" is the checksum received in the message.

Calculating a checksum from the rest of the symbols:

120 56 12 60 0 0 108 0 65 40 0 0 118 126 126 126 126 126 126 126 122

gives a "calculated Checksum" of "7"

This doesn't match the 39 contained in the decoded message.... hence the "Check Sum: error"

The message consists of:

Format : 120 = correct (individual call)

To MMSI : 56 12 60 0 0 = 561260000 = An unknown Samoan vessel - this MMSI doesn't show up on a Marine Traffic.com search... is the possible error in one symbol of the ship MMSI?

Category : 108 = Correct (Safety)

From MMSI : 0 65 40 0 0 = 006540000 = We hope this is correct!

Telecommands : 118 126 = "Test" and "No Info" = Correct

Additional data : 126 126 126 126 126 126 = Correct (no info)

EOS : 122 = Correct (ACK)

ECC : 39 = Might have been received correctly, but might also be the symbol with the problem. If so, were stuck..

If we assume an error in the vessel MMSI, what symbols will give the correct Checksum (39)?

The first symbol might be wrong, and changing it to 24 would lead to the correct checksum of 39.

This would make the vessel MMSI : 241260000. This is the Greek tanker "Olympic Loyalty II" currently of Madagascar in the Indian Ocean. Likely to have tested successfully on 4MHz with Nouadibou RCC Nouakchot? Possibly not...

Instead of the first symbol of the ship MMSI, how about the second?

Changing the second symbol to 44 (instead of 12) giveS a checksum of "39"

the vessel MMSI becomes:

56446000

This is Singaporean container ship "Milan Trader".

Currently off the coast of Morocco and it has come north over the last 24 hours past Nouadibou, Mauritania.

That looks like a better possibility for a vessel testing with Nouadibou RCC Nouakchot on 4MHz in the middle of the night.


The message might have been:


120 120 56 44 60 0 0 108 0 65 40 0 0 118 126 126 126 126 126 126 126 122 39

Just by changing one symbol it becomes "Checksum : Ok" and the changed symbol leads to a vessel MMSI that is certainly a good candidate for testing with that coast station.

The symbol in the vessel MMSI that we think is in error:

12 or 44 ?

Looking at the 10-bit parity protected words for 12 and 44:

12 : 0011000101
44 : 0011010100

There are only two bits different between these two 10-bit words. That's a frequent problem with DSC symbols - it's possible to have a two-bit error that then gives a valid parity check.

I think it's as close as it comes to a positive ID for you Peter. Two bits flipped, wrong Ship MMSI. Checksum then fails. Flipping the two bits back gives a different Ship MMSI - the checkup passes. The "new" vessel is currently off the coast of West Africa and came past Mauritania at the time of the message... close enough?



Cheers,

John

On 18/08/18 17:50, peter via Groups.Io wrote:
Hi all,
This would have been a nice catch especially on 4 mhz.  but showing ....'error'
Any supporting evidence/comments out there?
73,    Peter   g3ufi
==============================================
(HF=4.2075 MHz) <Selective call to a particular individual station>
Called MMSI station address: 561260000 [Ship] (Samoa)
Category: Safety
MMSI self-identifier: 006540000 [Coast station: 18°07'N 015°59'W Nouadibou RCC Nouakchot] (Mauritania)
Telecommand 1: Test
Telecommand 2: No information
Neither RX/TX frequencies nor position supplied
Check Sum: error
Date and time of decoding: 17/08/2018 02:26:56
Codes: 120 120 56 12 60 0 0 108 0 65 40 0 0 118 126 126 126 126 126 126 126 122 39

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