The fauna knows who misses it at all times. It knows this because it knows who doesn't miss it. By subtracting where it's missed 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 fauna from a position where it is missed 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 missed, is now the position that it wasn't, and it follows that the position that it was missed, is now the position that it isn't.
In the event that the status that it is in is not the status that it wasn't, the fauna has acquired a variation, the variation being the difference between where the fauna is missed, and where it wasn't. If variation is considered to be a significant factor, it too may be corrected by the GEA. However, the fauna must also know whether it was missed.
The fauna guidance computer scenario works as follows. Because a variation has modified some of the information the missile has obtained, it is not sure just whether it's missed. However, it is sure where it isn't missed, within reason, and it knows where it was missed. It now subtracts where it should be missed from where it wasn't missed, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn't be missed, and where it was missed, it is able to obtain the deviation and its variation, which is called error.
