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 (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 where it wasn’t, and it follows that the position that it was, is now the position that it isn’t.

In the event that this 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 the variation is considered to be a significant factor, it too may be corrected by the GEA. However the missile must also know where it is.

The missile guide computer scenario works as follows. Because a variation has modified some of the information the missile has obtained, it is not sure where it is. However, it is sure where it wasn’t, within reason, and it knows where it was. It 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 it’s variation, which is called “Error”