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 isn't or isn't (whichever greater) obtains a difference deviation. The guidance subsystem uses deviations to generate corrective commands drive the missile position and arriving at wasn't now is. Consequently it that follows was isn't. In event not system has acquired variation variation being difference between missile wasn't. If considered be significant factor too may corrected by GEA. However must also know was. computer scenario works as follows. Because modified some of information obtained sure just within reason knows It subtracts should vice-versa differentiating this algebraic sum shouldn't be able obtain deviation its which called error.