7 
The video signals from each 
of the two PMT's are processed 
in the video correlator to produce 
signals representing the following 
five quantities: 
(1) X parallax 
(2) Y parallax 
(3) X terrain slope error 
(4) Y terrain slope error 
(5) Degree of correlation 
The X and Y parallax infor 
mation is obtained from the two 
video input signals by means of 
an orthogonal cross-correlator. 
After appropriate normalization, 
the two inputs are first passed 
through electrical networks which, 
in effect, cause one signal to be 
phase-shifted 90 degrees with re 
spect to the other. The signals 
are then multiplied together. The 
result is an output which is zero 
when the two images are aligned 
as well as possible. For small 
amounts of mis-alignment, the 
output is proportional to the time displacement of the video inputs. 
This signal is resolved into its X and Y components by multiplying, 
in separate multipliers, by the instantaneous X and Y scan velocities. 
The two-displacement components, after suitable filtering, thus 
represent measures of X and Y parallax. 
The X and Y terrain slope measurements are derived from the 
variation of X parallax as measured over the scan pattern. Since the 
scan pattern is already compensated for terrain slope, the resulting 
outputs are proportional to the X and Y components of the error in the 
pattern-compensation process. These signals are used by the computer 
to further refine the slope compensation. 
Figure 4 - AP-2 coorelator cabinet, 
containing the scan generator and 
video correlator