[Snowman999]

But he is asking the question in relation to targeting with the TDC. It does not allow negative speed, so a backing ship would have legal heading minus 180º for TDC entry. Otherwise your torpedo isn't coming anywhere near that target.

I didn't refer back to the OP, but in the case he's talking about (TDC) he (and you) shouldn't be talking about heading at all. Course only. At an ahead bell course and heading are the same thing and conning officers sometimes use the terms interchangably. But as I correctly stated, they are not fundamentally linked and the two numbers can be quite different. (I left out the head-scratching question of what term is correct when a tug is made up midships and the hull is moving perpendicularly to the heading.)

And from a plotting point and TDC point of view, a stationary ship has no heading or angle on the bow, as heading and AOB in that frame of reference are always related to the motion of the target.

Completly wrong.

AOB has no input from speed or motion. It is an instantaneous measure of the angle between the line-of-sight and the target's hull in the direction toward the target's bow, with the addition of port or starboard to maintain the AOB at 180 degrees or less. It is only due to this forum's insistance on gaining AOB from time-series plot data that this misperception is held. AOB can be reliably gained if both own ship and target are motionless, both moving forward, moving in reverse, or one moving and the other not moving. I've done all four in RL.

The very term itself tells you what it is--the angle on the bow. Period.

Actually for a ship with zero speed, you an input any heading or angle on the bow you choose for a correct targeting solution. No measurements of any kind are necessary. Use your favorite number! I use 42, the answer to life, death, the universe and everything.

Your chances of a hit are significantly different with an AOB of 180 than 90, but your point is true so far as it goes. That doesn't mean heading and course are the same thing.

A target making 2 knots across a 2 knot current would have quite a different plotted heading than the compass course of the ship, which would be irrelevent to the submarine making the observation.

Still not heading. You have compared observed course-made-good and steered course. Observed course is the only concern of the Approach Officer. Again, "heading" is not a variable in torpedo fire control. Never has been.

I've served with two submarine COs and I can assure you that there were times they were concerned with "heading." Just not during FC party-time. That deck log has their name on it, however, and heading is a legal term important to accurate logs in certain circumstances.