In the same vein:<p>&quot;The missile knows where it is at all times. It knows this because it knows where it isn&#x27;t. By subtracting where it is from where it isn&#x27;t, or where it isn&#x27;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&#x27;t, and arriving at a position where it wasn&#x27;t, it now is. Consequently, the position where it is, is now the position that it wasn&#x27;t, and it follows that the position that it was, is now the position that it isn&#x27;t. 
In the event that the position that it is in is not the position that it wasn&#x27;t, the system has acquired a variation, the variation being the difference between where the missile is, and where it wasn&#x27;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.<p>The missile guidance computer scenario works as follows. Because a 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&#x27;t, within reason, and it knows where it was. It now subtracts where it should be from where it wasn&#x27;t, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn&#x27;t be, and where it was, it is able to obtain the deviation and its variation, which is called error.&quot;<p>Audio on yt[1]<p>[1]<a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=bZe5J8SVCYQ" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=bZe5J8SVCYQ</a>