5 
Figure 6. Height-Error Sensor 
relators connected differentially, as described in 
the next section. 
DEVELOPMENT OF 
HEIGHT-ERROR SIGNALS 
An error in estimating the height gives rise to 
a corresponding time offset in the appearance of 
corresponding imagery in the two scanning sig 
nals. This, in turn, results in a level of correlation 
which is as much dependent upon the image con 
tent as on the error; the output of such a corre 
lator provides no clue to the altitude error. 
To obtain a useful height-error signal, two-cor 
relators are used in conjunction with time delay 
units connected (as shown in Figure 6) so that 
correlator 1 has a peak output for errors on the 
low-altitude side while correlator 2 has a peak for 
errors on the high-altitude side. These, along with 
the corresponding difference output, are shown in 
Figure 7. It will be observed that the difference 
is zero for aZ = 0 (coincident signals), with a 
positive output for AZ > 0 and negative output 
for AZ < 0; thus, the difference output is appro 
priate for use as a height-error signal. 
The UAMCE has a high-acuity and a low-acuity 
height-error sensor. The outputs of these are 
appropriately summed to provide a net height- 
error signal. 
Figure 7. Output of Differential Correlators