APR strip| Flying height| Distance between | Distance in miles |Mean Residual
No. in feet. terminal control |to internal check |errors in feet
in miles. control from sou- | at internal
thern end of APR |control zones
1 1 P2 1 2
2/3 7,790 57 37 - 0 -
3/4 7,790 61 2 35 0 +3
4/5 8,500 61 34 57 +5 -4
5/6 8,500 81 59 - 0 -
6/7 8,500 82 23 54 -2 -6
7/8 8,500 52 26 - +6 -
8/9 10,000 45 13 - 0 -
After reduction of the APR strips, the corrected height values of the
selected 35mm p.ps were used to set up the overlaps individually in a series of
Wild A8 plotting machines on which the plotting of topographical detail and
contours was carried out. It was soon discovered that by using the lateral APR
lines {flown below 10, 000ft.) the lack of sympathy between the control points
used to set the overlap was seldom more than 3ft. whereas,directly the control
obtained from the central (high level) profile was introduced,discrepancies of
10ft. or more were not infrequent and there was a general tendency for the
central profile to read too high.
Of the high-level points showing unduly large errors one in every five
proved to be due to 'side-lobe! effect since, if readings were made over nearby
water surfaces or damp ground instead of at the p.p. of the 35mm photo, the
read-off heights were then sensibly correct. At any particular flying height
the effect of the side-lobe is to increase the area-spread,over, which reflections
are obtained, by approximately thirty-six times. If the centre of the beam
happens to fall on dry ground while a side-lobe impinges on water, the echo
from the side-lobe is likely to be stronger than the vertical reflection, in which
case the slope-distance of the side-lobe is recorded and not that of the centre
of the main beam.* For this reason, from 20, 000ft. altitude, any readings
taken within say 1, 000ft. of a waterline are suspect and should be discarded.
Aircraft tilt will almost certainly have accounted for the greater share of
the remaining errors (at 20, 000ft. 10 of tilt will produce an error of 3ft. and
2° tilt an error of 12ft.).
Another correction to be made is for earth curvature which at an altitude
*Diagram D shows the strength indecibels of the radar beam for varying degrees
off axis
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