Thank you for pointing out how to figure out the target's course!
I'm sure I will you this a lot!
Anyway about ID'ing the target to figure out speed as discussed above, You don't have to do this in a reported convoy, because the speed is given in the report. In any other case, you're right, except when you use Greyrider's method, which is of course a bit unrealistic.
You can also guess what it is by the sound type and frequency
Sorry for bringing up an old thread, but it's a good one isn't it?