In a message dated 09/21/2000 11:25:37 AM Central Daylight Time,
rwright@... writes:

<< Airline Inertial Guidance Systems
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
integrates 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".
>>