21 June 2009

AIS class B update for packetlogger


Last week Ben Ellison at Panbo found that there is no AIS class B static data PGN agreed upon yet so when you connect a Navico NAIS-300 to a Garmin display there is no class B static data shown. As you can see in his NMEA 2000 PGN received list the NAIS-300 puts out a Navico specific PGN: 130842.

As I had a few days on the boat with the kids I dragged my NMEA 2000 network to the boat and rigged up a temporary network to verify some things. One reason was to see if I stumbled upon a Class B sender so I had some packets to analyze. Luckily I found one willing Class B sender half way down the IJsselmeer. This showed up nicely on the Lowrance HDS-8 that showed all this on the Info display shown up right. Side note: making pictures of displays out in the sunshine is harder than I thought. Weird how that dust shows up prominently on the photo, it was absolutely not irritating in the real world.


As a result I can verify that the Navico PGN 130842 does indeed contain the missing Class B static data: call sign, vessel type, dimensions. There are some other fields as well, but I can't yet place those. Interestingly, the PGN 130842 is transmitted every 6 minutes (on receipt of the over-the-air AIS message 24 Class B CS static data report) and then twice in succession, each with different length and somewhat different data.

The first format is 37 bytes long, and an example is (first in hex bytes, then ascii, then what I can make of it):
41 9F 81 FF 18 0F EA 8B 0E 25 54 52 55 45 48 44 47 50 45 34 32 35 38 00 6E 00 28 00 14 00 6E 00 FF FF FF FF C0
A . . . . . . . . . T R U E H D G P E 4 2 5 8 . n . . . . . n . . . . . .
A = 65 B = 159 C = 129 E = 24 MMSI = 244050447 Type of ship = Pleasure F = TRUEHDG Callsign = PE4258 Length = 11 m Beam = 4 m GPS from port = 2 m GPS from bow = 11 m L = 192

The second format is 29 bytes long, and an example in the same formats is:
41 9F 80 FF 18 0F EA 8B 0E 50 41 56 41 4E 45 00 00 00 00 00 00 00 00 00 00 00 00 00 00
A . . . . . . . . P A V A N E . . . . . . . . . . . . . .
A = 65 B = 159 C = 128 E = 24 MMSI = 244050447 Name= PAVANE

Note the peculiar string embedded in the first format: TRUEHDG. This is likely to indicate that this NAIS-300 is based on a design from True Heading. After opening the case I did indeed find one Navico and one SRT circuit board, linked with a single four wire interconnect. The Navico specific board is much simpler, and only contains a smallish microprocessor. The SRT board contains a massive ASIC and a Texas Instruments 320Cxx class CPU, memory etc.

This seems to indicate that the communication between the two boards is a simple serial protocol, and that the Navico board only serves as the power circuit and NMEA 2000 interface.

The packetlogger program now understands the above forms of PGN 130842 as well as PGN 129039 (Class B position report) and can be downloaded from here. The list of messages and fields understood can be downloaded as well.

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