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You think you had it tough learning about Variation and Deviation? This is one of those Ďthings that arrive on your fax from time to timeí things. It came from a mate of mine, Milton Cain, who got it from a radio amateur (Lester Harris, ZL1APY), but there the trail ends. Enjoy, anyway. - Steve Punter.


 The aircraft knows where it is at all times. It knows this because it knows where it isnít. By subtracting where it is from where it isnít, or where it isnít from where it is (whichever is the greater), it obtains a difference, or deviation.

The Inertial Guidance System (IGS) uses deviations to generate error signal commands which instruct the aircraft to move from a position where it is to a position where it isnít, arriving at a position where it wasnít, but now is. Consequently, the position where it is, is now the position where it wasnít, thus it logically follows that the position where it was is the position where it isnít.

In the event that the position where the aircraft now is, is not the position where it wasnít, then the IGS has acquired a variation.

Variations are caused by external factors, the full explanations of which are beyond the scope of this report. A variation is the difference between where the aircraft is and where it wasnít. 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, ensuring it knows where it isnít.

Nevertheless, the aircraft is sure where it isnít (within reason), and it knows where it was. The IGS now subtracts where it should be from where it isnít, where it ought to be from where it wasnít (or vice versa) and integrates the difference with the product of where it shouldnít be and where it was, thus obtaining the difference between its deviation and its variation, which is a variable constant called error.


Note: If you have one of these little gems, email it to me steve@sta.co.nz or fax it to me on (09) 444 3704. Do your best to let us know where you got it so we can give credit to the authors.

Carpe Diem

Steve Punter ANZIM, Dip Bus (PMER), FHRINZ
Staff Training Associates Ltd, Auckland, New Zealand.
© Steve Punter 2001 All rights reserved by the author.

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