It knows this because it knows where it isn't.
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.
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.
Consequently, the position where it is,
Is now the position that it wasn't,
And it follows that the position that
It was, is now the position that it isn't.
In the event that the position that it is in is not the position that
It wasn't, the system has acquired a variation,
The variation being the difference between
Where the missile is, and where it wasn't.