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 word isn't from where it is whichever is greater it obtains a difference or deviation the guidance subsystems 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 that 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 isn't in the event that the position that it is and is not the position that it was not in the system has acquired A variation the variation being difference between where the missile it is and where it wasn't if the variation is considered to be a significant factor it may be corrected by the DEA however the missile must also know where it was the missile guidance computer scenario works as follows because of variation has been modified some of the motion that the missile has obtained it is not sure just where it is lover it is sure within reason and it knows where it was a now subtract where is it should be from where it wasn't or vice versa and by differentiating this from the algebraic sum a word shouldn't be and where it was it is now able to obtain the deviation and it's variation which is call e r r o r
