Wednesday, June 22, 2016

How many slots can an AIS message have?

Since up to 9 VDM messages can be chained together, there is some confusion as to how many bits can be in an AIS message.  Over the radio, you are allowed to have up to 5 slots.  See ITU-1371-5:

The first slot is short with just 128 bits available.  The other following 4 slots are 256 bits each.  That gives a total over the VHF radio payload of 1152 bits.  Once that is armored into NMEA VDM characters at 6 bits per character, that gives 192 characters.  Those 192 characters can be spread out across multiple NMEA lines that are grouped together.  Each NMEA sentence can be up to 80 characters long.  Those sentences are defined in the NMEA standard and IEC_61162, both of which are paywalled.  According to NMEA 4.0, there can be up to 62 characters per sentence.

While it is possible to chain 9 sentences (1 to 9) together of VDM armored NMEA with 558 characters of armored data at 6 bits per character for a total of 3348 bits, the VDL (VHF Data Link) will only let you send those 5 slots or 1152 bits.  A full length 1152 bit message could be packed in as little as 4 NMEA lines if fully using the 62 characters per sentence limit.

And to sounds like a broken recorded, paywalled specifications are evil.  In the case of specs for maritime systems, closed specs are detrimental to safety of life at sea and reduce the quality of the tools available to mariners.  I take strong issue with NMEA, IEC, ITU and ISO for paywalling so many specifications documents.

Here is what some multi-sentence TAG BLOCK encoded NMEA looks like:

Or as actual text:



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