"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