The missile knows where it is at all times
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, 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 that 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
If variation is considered to be a significant factor, it too may be corrected by the GEA
However, the missile must also know where it was
The missile guidance computer scenario works as follows
Because a variation has modified some of the information that 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 should be from where it wasn't, or vice-versa
And by differentiating this from the algebraic sum of where it shouldn't be and where it was, it is able to obtain the deviation and its variation, which is called error