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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