"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