AIS decoding

[AntoineC]

Good morning!

I am trying to create my own AIS decoder. Yet, I don't know where to start from after reading AIVDM. I only have AIVDM/AIVDO protocol decoding as reference and which seems to be quite indigest. For instance with the following frame:

!AIVDM,1,1,,A,18tHtHhP01Op01NKMKDrtOvp28@i,0*03

How can I know which message type it is?

[edmundsteele]

Hi Antoine,
I have never looked at this before but I glanced at the document in the protocol decoding link you included. Your example has 18tHtHhP01Op01NKMKDrtOvp28@I in the “Payload” data block. If I read this correctly, the “armoring” (where do they get these names??) uses a truncated form of ASCII using six bits per character and so the the first six bits are the message type - in your example simply a “1”. Table 4 in your decoding link gives this as a “Position Report Class A”. Table 6 gives the breakdown of this type with the next two bits being the repeat indicator (I make this as “0”) and the next 30 bits the MMSI number, a 9 digit decimal number encoded as an unsigned integer. I make this as 600194147 in your example. And so on.....
Ed

[AntoineC]

Thank you for your answer. I think I did a mistake when trying to decoding it because I got, with the help of the document to translate the ASCII into actual 6-bits frames.

000001 001000 111100...

Wich gives me 1, so the message type you gived,
0
But then S. Which is false as far as it isn't even a number.


The following one isn't even 0

The numbers you find actually being those of my mmsi's, can you tell me how did you did it? ?

[AntoineC]

How, I understand, I didn't have to take them number by number but as a whole number!

[edmundsteele]

You have it Antoine!
BTW, if you have a scientific calculator (does anyone use these anymore?) with a “hex” “oct” “dec” button, you can convert your binary sequences into something easier to manipulate – I used hexadecimal; enter this value into your calculator and hit the “dec” button.

For example the first 8 “truncated ASCII” characters in your example were 18tHtHhP These map in binary to 000001 001000 111100 011000 111100 011000 110000 100000. The 30 bits for the MMSI number are the 9th thru the 38th bits (fields 8 to 37) which is this part 1000 111100 011000 111100 011000 11. I made this an “unsigned 32 bit integer” by padding it back out to a 32 bits with two leading zeroes giving 001000 111100 011000 111100 011. Now convert each group of 4 bits to a hexadecimal character and it becomes 23C63C63. Stuff this into your calculator, hit the “dec” button and it converts to your MMSI number of 600194147.
Ed