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Accuracy Limitations on High-Speed Metric Photography
By AMY E. GRIFFIN and ELMER E. GREEN
Parameter limits in high-speed camera design and physical characteristics of photographic images limit the assessment accuracy of high-speed metric film records. The errors in film measurements range from 2 to 75 microns. Proper design of field instrumentation is required to minimize effects of reading errors in a measurement system for ballistic data.
Uses of Metric Cameras
The measurement system on the ground ranges at the U.S. Naval Ordnance Test Station, Inyokern (NOTS), was designed primarily to obtain ballistic data on rockets and guided missiles. High-speed photography, which is here defined as photography at more than 500 frame/sec, is a useful tool for studying missile flight phenomena such as flame characteristics but is seldom applicable to the study of ballistic parameters which are derived from missile trajectory and attitude data. High-speed cameras are not useful for these purposes because, in general, their accuracy limitations are too severe to permit their use at their high frame rates as metric instruments, i.e. instruments from which data such as velocity, acceleration and direction of motion (which are usually obtained
Presented by Amy E. Griffin on October 9, 1952, at the Society's Convention at Washington, D.C., by Amy E. Griffin and Elmer E. Green, Assessment Div., Data Reduction Branch, U.S. Naval Ordnance Test Station, Inyokern, China Lake, Calif.
by differencing successive position determinations) can be obtained.
If an unsuccessful missile flight is caused by factors inducing vibrations or moments resulting in instability, the first photographic evidence of this may be some physical change in the missile such as the breaking of a stabilizing fin. To pinpoint the time of this occurrence photographically to desired accuracy may require frame speeds of 1000 per sec or higher, but a mere determination of the moment of failure is inadequate for flight analysis. In addition to knowing the exact time of failure it is necessary to study closely the behavior of the missile previous to this time to determine whether the fin was substandard or whether the failure was caused by unexpected stresses on the fin. If the latter is the case, data are required to determine whether the unexpected stresses were caused by aerodynamic forces larger than anticipated or by inability of the missile to achieve flight stability under the anticipated conditions.
December 1952 Journal of the SMPTE Vol. 59
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