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#1
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
I have been seeing the following AIS message (my location is a bit north of
San Francisco), and I can't identify it in the specs I have access to: !AIVDM,1,1,,A,?03Ovk1GP0?`000,2*33 !AIVDM,1,1,,A,?03Ovk1GP`FP000,2*2A !AIVDM,1,1,,A,?03Ovk1a08oP000,2*1D (these were received in this order, with other messages interspersed) Note that the first character, which should be holding the message-type identifier, is '?', which is the 6-bit code for 63. Is there a "Type 63" message? I am using the SmartRadio SR-162 receiver, and it has been working well. I am receiving lots of other messgaes from ships and base stations, but this one has me stumped. I would appreciate any help! Thanks, Paul |
#2
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
"Paul" wrote in message
... I have been seeing the following AIS message (my location is a bit north of San Francisco), and I can't identify it in the specs I have access to: !AIVDM,1,1,,A,?03Ovk1GP0?`000,2*33 !AIVDM,1,1,,A,?03Ovk1GP`FP000,2*2A !AIVDM,1,1,,A,?03Ovk1a08oP000,2*1D (these were received in this order, with other messages interspersed) Note that the first character, which should be holding the message-type identifier, is '?', which is the 6-bit code for 63. Is there a "Type 63" message? I think you're mistaken. The '?' is 001111 binary, so it would be a type 15 message. Decoding an ASCII character to its binary value goes like this: ASCII value + 101000. If sum 1000000 then add 100000 else add 101000. Then use the resulting 6 LSB's. Meindert |
#3
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
"Meindert Sprang" wrote in message ... "Paul" wrote in message ... I have been seeing the following AIS message (my location is a bit north of San Francisco), and I can't identify it in the specs I have access to: !AIVDM,1,1,,A,?03Ovk1GP0?`000,2*33 !AIVDM,1,1,,A,?03Ovk1GP`FP000,2*2A !AIVDM,1,1,,A,?03Ovk1a08oP000,2*1D (these were received in this order, with other messages interspersed) Note that the first character, which should be holding the message-type identifier, is '?', which is the 6-bit code for 63. Is there a "Type 63" message? I think you're mistaken. The '?' is 001111 binary, so it would be a type 15 message. Decoding an ASCII character to its binary value goes like this: ASCII value + 101000. If sum 1000000 then add 100000 else add 101000. Then use the resulting 6 LSB's. Meindert Meindert, Thanks! You are right, of course. I was looking at the conversion table for ASCII-to-6bit when I should have been looking at the 6bit-to-ASCII conversion (a mistake I have made before). My program does this correctly, but I was just eyeballing the raw NMEA string. So now I get to figure out how to parse Message Type 16: "Assigned Mode Command, Assignment of a specific report behaviour by competent authority using a base station" Should be interesting! Regards, Paul |
#4
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
Meindert Sprang wrote:
Decoding an ASCII character to its binary value goes like this: ASCII value + 101000. If sum 1000000 then add 100000 else add 101000. Then use the resulting 6 LSB's. Meindert I think an easier way to decode binary numbers is positional: position weight 1 = 1 1 = 2 1 = 4 1 = 8 0 = 16 0 = 32 0 = 64 0 = 128 so the value of 001111 = 1 + 2 + 4 + 8 + 0 + 0 = 16 FYI, Download a table of ASCII values he http://www.cs.mun.ca/~michael/c/ascii-table.html Just my two cents Vic |
#5
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
Paul wrote:
So now I get to figure out how to parse Message Type 16: "Assigned Mode Command, Assignment of a specific report behaviour by competent authority using a base station" Where are these NMEA sentences documented? I would like to add them to my NMEA sentence parser S/W Vic |
#6
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
"Victor Fraenckel" skrev i en meddelelse ... Meindert Sprang wrote: Decoding an ASCII character to its binary value goes like this: ASCII value + 101000. If sum 1000000 then add 100000 else add 101000. Then use the resulting 6 LSB's. Meindert I think an easier way to decode binary numbers is positional: position weight 1 = 1 1 = 2 1 = 4 1 = 8 0 = 16 0 = 32 0 = 64 0 = 128 so the value of 001111 = 1 + 2 + 4 + 8 + 0 + 0 = 16 FYI, Download a table of ASCII values he http://www.cs.mun.ca/~michael/c/ascii-table.html Just my two cents You may deduct one ... as I would do in the above calc ...;o) ... Vic Flemming Torp |
#7
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
"Victor Fraenckel" wrote in message ... Meindert Sprang wrote: Decoding an ASCII character to its binary value goes like this: ASCII value + 101000. If sum 1000000 then add 100000 else add 101000. Then use the resulting 6 LSB's. Meindert I think an easier way to decode binary numbers is positional: position weight 1 = 1 1 = 2 1 = 4 1 = 8 0 = 16 0 = 32 0 = 64 0 = 128 so the value of 001111 = 1 + 2 + 4 + 8 + 0 + 0 = 16 FYI, Download a table of ASCII values he http://www.cs.mun.ca/~michael/c/ascii-table.html Vic- The decoding that Meindert shows is specific to the 6-bit coding used in the AIS NMEA messages, so there's more (and less) to it than the normal ASCII conversion. You probably know this, but in any case, the 6-bit coding uses selected ASCII characters to represent 6-bits of data. There are gaps in the mapping, thus the "if-then-else" in the decoding. -Paul |
#8
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
"Victor Fraenckel" wrote in message ... Paul wrote: So now I get to figure out how to parse Message Type 16: "Assigned Mode Command, Assignment of a specific report behaviour by competent authority using a base station" Where are these NMEA sentences documented? I would like to add them to my NMEA sentence parser S/W Here is a spec that includes the AIS NMEA sentences, and the coding used: http://www.navcenter.org/marcomms/IEC/80_269e.pdf - this has a description of the messages, the NMEA encapsulation, and of the 6-bit encoding. This is a big document, so here is where to find the highlights: * The message descriptions start in section 7.6.8 * There is an ascii-to-6bit mapping, shown in table 23, that is used to encode test strings. This is not the same thing as the 6bit-to-ASCII mapping, so don't get confused (as I did at the start of this string) * The 6bit-to-ASCII is described starting at section G.3.6 * Examples of AIS NMEA sentences are shown in section G.3.2 -Paul |
#9
posted to rec.boats.electronics
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Help me identify an AIS Message (type 63???)
Correction: "text strings", not "test strings" (see below)
This is a big document, so here is where to find the highlights: * The message descriptions start in section 7.6.8 * There is an ascii-to-6bit mapping, shown in table 23, that is used to encode test strings. This is not the same thing as the 6bit-to-ASCII mapping, so don't get confused (as I did at the start of this string) * The 6bit-to-ASCII is described starting at section G.3.6 * Examples of AIS NMEA sentences are shown in section G.3.2 -Paul |
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