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how to read AIS data from encapsulated NMEA VDO sentence
"tcdang714" wrote in message ... Wrote: Terry, you're making this sound alot more complicated than it really is. The 6-bit encoded part of a VDM sentence is simply a bitstream, packed using a special code. Each character represents 6 bits of data from 0x00 ot 0x3F. It starts with the message ID (6 bits), repeat (2 bits), MMSI (30 bits), etc. This is all defined in m.1371 and 61993-2, even if it is a bit confusing at first. The first 6 bits being a character and the message ID make it easy to tell that the example string above is a message 1. Hi guys, I'm actually working on a similar project right now except I have to do the ENCODE part of the VDM message. At first I thought every character represents 6-bit but that is not true. As you can see in the message there are lowered case alphabet characters (i.e. 'e') and the ascii value for 'e' is more than 6-bits. I'm a little stuck on my project right now too, can someone give me more info. Perhaps an example in C or C++. Thanks. -- tcdang714 I realize that you've asked about *encoding*, but here's an example of a decoder written in visual basic, from a program I wrote for my pocket PC. Perhaps you will be able to reverse-engineer something from this. ------------------ Function AsciiToHex6(AsciiIn) AsciiToHex6 = A2H6(Asc(AsciiIn)) End Function Dim A2H6(128) Sub InitA2H6 Dim i, h For i = 0 To 127 h = i If i 48 Then h = -1 ElseIf i 88 Then h = h - 48 ElseIf i 119 Then h = -1 ElseIf i 96 Then h = -1 Else h = h - 56 End If A2H6(i) = h Next End Sub InitA2H6 ------------------ The array A2H6() contains the hex6 value of the ascii input character, using the character as an index. I put "-1" in the illegal positions, and ought to do a little more defensive error-checking (but I probably won't ever get around to it). The function AsciiToHex6(AsciiIn) just looks up the code and returns it. The IEC documents that describe the messages and coding aren't free, but while I was googling around I did find this one: http://www.gicoms.go.kr/knowledge/download.asp?filename=IEC%20standard%2061993(class %20A%20AIS).pdf&filepath=D:%5CGICOMS_FILE%5Cupdown %5CIEC%20standard%2061993(class%20A%20AIS).pdf This contains a decent description of the 6-bit ascii encoding, as well as the message structure. Note that the AIS data elements are not necessarily six-bits long, so there will be an arbitrary alignment of the various parameters across the 6-bit-ascii character stream. Also, some of the AIS messages are longer than allowed in the NMEA sentence, so the messages have to be split into multiple NMEA sentence. I can post my code for decoding the arbitrary data if you think it would be instructive. Be aware that some of the earlier discussions of 6-bit ascii encoding for AIS show a different encoding (don't ask me how long I struggled with that issue!). The one in my code sample, and the document above seem to be the encoding actually used. Good Luck, Paul |
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