Figure 5. Correlator Circuit 
going to the respective correlators are influential 
in controlling the input levels. 
The method of scanning the photographs is 
critical for optimum correlation. Only signals 
which can represent height differentials are use 
ful. For example, this implies that the image of 
a straight line (such as would be produced by a 
road running parallel to the line of camera separa 
tion) should produce a minimum signal because 
it does not contain any height information. The 
scan implied in Figure 1, (parallel to the line of 
flight) satisfies this requirement. For elements in 
the photography representing tilted ground areas, 
the appropriate scan is one representing the pro 
jection of such a scanning line on the tilted areas 
as seen in the two photographs. 
CORRELATOR CIRCUIT 
The correlators used in all Bunker-Ramo auto 
matic mapping equipments resemble a “quarter- 
square multiplier” or “phase-sensitive detector” 
as illustrated in Figure 5. To aid in this discus 
sion, the circuit is divided into an upper (heavy 
line) and a lower (light line) portion. 
In the upper portion, suppose the nonlinear 
elements D1 and D2 have current-voltage relation 
ships expressed by 
ii = ke i 2 
and 
i 2 = — ke 2 2 
where the sign reversal is obtained by reversing 
the element. Suppose further that the two trans 
formers have inputs e a and e b . Element D1 has 
an effective input voltage, supplied through equal 
summing resistances, R, of (e a + e b )/2, while 
element D2 has an input of (e a — e b ) /2. The cur 
rent into the integrating capacitor, C, is the dif 
ference of the currents from the two nonlinear 
elements: 
i. = k <^gs.)l] = ke>eii 
Because the voltage across a capacitor is the 
integral of the current into it, the output, taken 
from across the capacitor, is the desired integral 
of the product of the two input signals. 
The elements used for D1 and D2 are not 
square-law devices, but diodes which pass current 
for one polarity while passing a negligible current 
for the opposite polarity. The light (lower) por 
tion of the circuit (Figure 5) supplies the currents 
of opposite polarity. Thus, diode D1 supplies cur 
rent to the capacitor when e a + e b > 0, and 
diode D3 supplies a corresponding current for 
e a + e b < 0. Similarly, diode D2 supplies a neg 
ative current for e a — e b < 0 and diode D4 sup 
plies a negative current for e a — e b > 0. 
In practice, the balance of the correlator is far 
more important than its linearity as a multiplier. 
The transformers must cover the frequency range 
of interest, and the capacitor (with associated 
resistors) must have a desirable time constant. 
In the UAMCE the capacitor is replaced by an 
operational integrator; this integrates linearly 
until the capacitor is short-circuited by signals 
from the height counter or computer. One inte 
grator receives the combined output of two cor- 
4