the missile knows where it is at all times
it knows this because it knows where it isn't
by subtracting where it is to where it isn't or where is isn't to where it is (whichever is greater) it obtains a difference or a 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 where it wasn't and the position where it was is now the position where it isn't
in the event that the position that it is in is not the position where it wasnt the missile has acquired a variation
a variation between where the missile is and where it wasn't
if variation is considered to be a significant factor it too maybe corrected by the GEA however the missile must also know where it was
the missile guidance computer scenario works as follows
because of variation has modified some of the information the missile has obtained it is not sure just where it is however it is sure where it isn't within reason and it knows where it was
it now subtracts where it shouldn't be from where it wasn't (or vise versa) it is able to obtain the deviation and it's variation which is called error.