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The following article is from the Canadian "Airspace Newsletter", issue 1/94
printed by the Transport Canada.  I hope this article will be able to help you
as much as it helped me to understand IGS!!!

Articles printed in Airspace Newsletter is a collection of letters from pilots
and distribution of the articles from the newsletter is encouraged, as long as
references made to the newsletter.


morteza@innovus.com
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                    INERTIAL GUIDANCE SYSTEM SIMPLIFIED

We are not sure who the author of the following article is, however we feel
that the article is one of the best, clearly defined descriptions of the magic
that resides withing the aircraft's black boxes.

The 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, or where it
isn't from where it is (whichever is the greater), it obtains a difference, or deviation.

The Inertial Guidance System uses deviations to generate error signal commands
which instruct the aircraft to move from a position where it is to a position
where it isn't, arriving at a position where it wasn't, or now is.  
Consequently, the position where it is, is now the position where it wasn't;
thus, it follows logically that the position where it was is the position where it isn't.

In the event that the position where the aircraft now is, is not the position
where it wasn't, the Inertial Guidance System has acquired a variation.
Variations are caused by external factors, the discussions of which are beyond
the scope of this report.

A variation is the difference between where the aircraft is and where the
aircraft wasn't.  If the variation is considered to be a factor of significant
magnitude, a correction may be applied by the use of the autopilot system.
However, use of this correction requires that the aircraft now knows where it
was because the variation has modified some of the information which the
aircraft has, so it is sure where it isn't.

Nevertheless, the aircraft is sure where it isn't (within reason) and it knows
where it was.  It now subtracts where it should be from where it isn't, where
it ought to be from where it wasn't (or vice versa) and intergrates the
difference with the product of where it shouldn't be and where it was; thus
obtaining the difference between its deviation and its variation, which is
variable constant called "error".

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