Inertial Navigation Simplified"First, with INS the airplane 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 was, or where it isn't from where it is (whichever is first), it obtains a difference or deviation.
The inertial system uses deviations to generate corrective commands to drive the system from a position where it is, to a position where it isn't. Arriving at the position where it wasn't, it now is.
Consequently, the position where it is, is now the position where it isn't. In the event that the position where it now is, is not the position where it wasn't, the system has acquired a variation. The variation being the difference between where the airplane is and where it wasn't.
The +thought+ process of the system is as follows: Because a variation has modified some of the information which the airplane has obtained, it is not sure where it is. However, it is sure where it isn't (within reason), and it knows 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 difference between its deviation and its variation, which is called error, it computes the correct information to compensate for all factors, supplying accurate navigation information."
--- relayed by K0HBO via Packet Radio, Feb/08/1987.