A moments levity regarding INS...
Originally published I believe in a BA Staff magazine...
Inertial Navigation (INS 101)
The following explanation, in simplified terms, is typical of all IN systems:
1. An aircraft 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, it obtains a difference, termed deviation.
2. The IN system uses deviation to generate a corrective command to drive the system from
a position where it is, to a position where it isn't, arriving at a position where, if it wasn't,
it is now. Consequently, the position where it wasn't follows the position where it was to
the position where it isn't.
3. Futhermore, the position where it is now is not the position where it wasn't, and the system
has therefore acquired a variation. A variation is difference between where the aircraft is
and where it wasn't, and is considered to be a significant factor which needs correction.
The logic of the system is as follows:
Because variation has modified some of the information obtained by the aircraft, the
aircraft is not really sure where it is; however, it is sure where it isn't (within reason),
and it knows where it was. The IN system 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 variation, (which is called error), it computes the correct information to
compensate for all factors.
Mohit; hope this makes sense...from an avionics guy just up the road in Granada...