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 deviation 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 if follows that the
position that it was is now the position that it isn't.
In the event that the position 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 know 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.