Where It's At

An excerpt from a report explaining the operation of the Ferranti
Bloodhound Inertial Guidance System.

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, depending on
which is the greater) it obtains a difference or deviation. The
inertial guidance system use deviations to generate corrective
commands to drive the missile from the position where it is to the
position where it isn't. The missile arrives at the position
where it wasn't, consequently the position where it was is now the
position where it isn't. In the event that the position where it
is now is not the same as the position where it originally wasn't
the system has acquired a variation, (variations are caused by
external factors and the discussion of these factors is not
considered to be within the scope of this report) the variation
being the difference between where the missile is and where the
missile wasn't. If the variation is considered to be a
significant factor it too may be corrected by the inertial
guidance system. Moreover, the missile must now know where it
was also. The "Thought Process" of the missile is as follows:
Because a variation has modified some of the information which the
missile had obtained, it is not sure where it is. However, it is
sure where it isn't and it knows where it was. It now subtracts
where it should be from where it wasn't (or vice versa) and by
differentiating this from the algebraic difference between where
it shouldn't be and where it was, it is able to obtain the
difference between it's deviation and it's variation, this
difference being called the Error.

The company also produces an extremely accurate command guidance
system, however this type of guidance doesn't lend itself to so
simple an explanation.