Model Profile:

The missile knows where it is at all times. It knows this because it knows where it isn't.

What turns me on:

By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is greater), it obtains a difference, or deviation.

What turns me off:

The guidance subsystem uses deviations to generate corrective commands to drive the missile from a position where it is to a position where it isn't, and arriving at a position where it wasn't, it now is.