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appears to have originated in England during the Second World War. Of 
course, none of their reports were published. Their techniques were 
communicated to the National Research Council in Canada and similar 
tests were begun there. The wavy lines drawn on the film by steady 
lights as used initially permitted only evaluation of angular vibration 
in roll, i.e., about the fore and aft axis of the aircraft. By replac- 
ing the steady lights with lights flashed rapidly at an accurately 
known rate, the possibility of measuring pitch and yaw components was 
added. This was first done with lights flashed by means of rotating 
sectors, as reported in 1951. When similar studies were resumed much 
more recently electrically flashed discharge lamps were used instead. 
The General Radio Stroboslaves operating at 300 flashes per second had 
only enough intensity to permit good imagery for a 153 mm focal length, 
f/5.6 lens, if the aircraft was flown at about 400 feet above ground. 
This low altitude complicated flight operations because of air 
traffic control regulations.’ To avoid this difficulty, the optical 
systems shown in figure 2 were designed, and built, to concentrate 
practically all the light from the flash tubes in an upward cone of 31° 
semi-angle. These permitted operation at 1000 feet above ground with 
ample image density on the negatives. 
Subsequently, to permit testing of inertial systems for recording 
camera position and orientation, arrangements were made to tie the 
flashing lamps to the CHU Canadian time signals, and a third flashing 
lamp was added to provide enough data to permit calculation of the six 
elements of exterior orientation of the camera at each flash. 
3.1 MEASUREMENT DIFFICULTIES 
This method of measuring angular motion of cameras in flight has 
several disadvantages which should be mentioned. 1) The ground instal- 
lation is fixed, requiring the aircraft to come to it. 2) The ground 
operation is costly in man-hours. 3) Flying conditions are not typical 
of aerial survey operations. 4) Data analysis is time-consuming and 
hence the length of time sampled is small. 5) Accuracy is limited by 
pointing accuracy on the images of the point light source on the film, 
with granularity a significant factor. 6) Accuracy of angular velocity 
values can be improved by increasing the time between flashes, but data 
on high frequency vibrations may be lost. If an alternative measuring 
method could be developed, based on instrumentation which could be used 
in an aircraft without a ground reference, it would be most helpful in 
improving knowledge of vibration problems. 
4. RESULTS OF FLIGHT TESTS 
Figures 3 to 10 show in graphical form some examples of vibration 
data obtained in flight tests. The plots are of angular velocities of 
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