- 13 -
where
i = j for the autocorrelation function
and
i / j for the cross-correlation function
and the functions fi(t) and fj(t) are assumed to be stationary random pro
cesses with a delay of T existing between the two functions.
The instrumentation of the autocorrelation function clearly in
volves multiplication of the two electrical signals, fi(t) and f j (t +T) /
resulting from synchronous scanning of the diapositives and integration
of the resultant product over a finite time interval. Delay term T is re
lated to the linear translation (differential parallax) existing within the
conjugate terrain areas. The correlation of electrical signals derived
from synchronous scanning of areas free of parallax results in a maximum
value of the correlation function, as given by
While the output of the above function is clearly maximized for
identical input signals, the form of the output is not easily adapted to a
positioning servo system.
An instrumentation of the above equation yields an analog voltage
level functionally related to the degree of correlation. Such an instrumen
tation yields useful data with regard to the degree of correlation, and as
such is utilized as a velocity control signal in the automatic contouring
modes .
The even property of the autocorrelation function may be modified
to yield a null output, for identical input signals, which is more adaptable
to a servo control system. The addition of a wide-band 90-degree phase
shift network develops an orthogonal relationship between the electrical
signals being processed from bcth video channels, thereby yielding a null
output for signals derived from areas free of differential parallax.
The stereoplotter's ability to automatically trace contours from
the stereomodel is based upon the continuous detection of regions free of
X-axis parallax. Based upon the correlator measurements obtained in