# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Missile guidance system
From: Bill B
Date: 2007 Mar 20, 18:57 -0400

```I misquoted the guidance tutorial tape as "The missile knows where it is by
knowing where it isn't."

Following is the sound bite transcribed:

The missile 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 greater, it obtains a difference or
deviation.  The guidance subsystem uses deviations to generate corrective
commands to drive the missile from a position where it is to a position
where it isn�t, and arriving at a position where it wasn�t it now is.

Consequently the position where it is, is now the position that it wasn�t,
and it follows that the position that it was, is now the position that it
isn�t.

In the event the position that it is is not the position that it wasn�t, the
system has acquired a variation.  The variation being the difference between
where the missile is and where is wasn�t.  If variation is considered to be
a significant factor, it too may be corrected by the GEA.  However, the
missile must also know where it was.

The missile guidance computer scenario works as follows:  Because the
variation has modified some of the information the missile has obtained, it
is not sure just where it is.  However it is sure where it isn�t--within
reason.  And it knows where it was.  It now subtracts where it should by
from where it wasn�t, or vice versa.  And by differentiating this from the
algebraic sum of where it shouldn�t be and where it was, it is able to
obtain the deviation and its variation, which is called error

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