Author Topic: Mistake in AIS six bit-ascii decryption?  (Read 1157 times)

AntoineB

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Mistake in AIS six bit-ascii decryption?
« on: 07 July, 2016, 05:51:12 pm »
Dear Sir,




I'm trying to create my own AIS ASCII frames decoder in VB.NET. Therefore I tried to understand how does the conversion from 6_bits ASCII to binary works.




I tried on this decoder my own AIS frame:




!AIVDM,1,1,,B,18tHtHhP00Op00dKMK=N4?vT2@:q,0*61




Which is, in binary: 000001 001000 111100 ...




The first 6-bits, giving the message type which is Position Report Class 1 (PRC1) according to the decoder, I'm okay with that.

The following two bits, which are 00, gives the message repeat count as far as we are in a position report class A frame and according to Eric S. Raymond, in its paper from May 2015 (I anchored the link to the very specific passage on PRC 1

The  30 following bits gives the MMSI, which is , according to the decoder mmsi: 600194147

Yet, I find that the first 6-bits are 100011: s (?) and not 6!




I think I did the mistake when converting 6-bits ASCII to binary representation as far as the MMSI can't be composed by letters. Yet, I verified several times and 8 is actually coded as 001000. Therefore, I'm no more sure I did a mistake.




I would very glad to you to be taught my mistake if it were as such, can you help me?




Respectfully,