== Forwarded Message Follows =========================================
 * Originally By: Dave Coble
 * Original Area: Funny
 * Forwarded by : Blue Wave/DOS v2.20
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".
                     Submitted by morteza@innovus.com (Morteza Ansari)
   The preceding 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!!!
   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.
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