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Table II. Errors of Missile Exposure Time.
Camera type
Shutter type
Random Modifications to original errors in time, camera /*sec
Askania
Mitchell Bowen
General Radio Fastax
Venetian blind
Rotating sector Rotating drum slit
None (Edgerton Flash) Rotating prism
Camera dials illuminated by Edgerton Flash Lamps Binary timing Finer slits in drum; 1000cycle timing pulses on film
1000-cycle timing pulses on film
3000
300 3
0.1 20
lines/mm, the highest frame speed over which acceleration data can be obtained with 35mm film would be 70 frame/sec.
In general, 5 to 10 acceleration values are desired from each camera. As a result, the Bowen cameras are used with the centerline of the 125-mm frame set approximately parallel to the portion of trajectory to be covered. On Bowen Film R = 10 lines/mm under most conditions. This value of R is low because of the movement of the missile image and the film during exposure.
So
Aa/S = V X F.S. /452,500
(11)
If Aa is 1 G, S = 10 and V = 1000, the highest usable frame speed is 45 per second. Data are usually obtained at the rate of 10 per sec or 30 per sec from Bowen cameras since missile velocities may be larger than 1000 fps.
But resolving power is not the only factor determining the accuracy of acceleration values. Under many conditions, it is necessary that the camera track the missile, so the accuracy with which the orientation of the lens and film can be determined is also important. With present camera mounts these orientations can be determined no better than ± 1 minute of arc and since high missile velocities require that the cameras be located some distance from the flight line in order that the operator can track the missile, these cameras
have random errors in missile position of the order of 1 to 3 ft. This invalidates the use of high frame rates for ballistic purposes.
Even for cameras which do not track the round, it is necessary to determine the camera orientation to a fairly high accuracy because errors in this orientation introduce bias errors in the data. The ± 1 minute of arc accuracy obtainable is sufficient to make bias errors inconsequential.
Besides resolving power and camera orientation there are still errors in the time of exposure of the missile. Consider the following example: If a missile is traveling 1 000 fps, to determine its position to ±0.01 ft the time of missile exposure must be known to the nearest 10 Msec.
The errors in time of exposure of the missile as best determined for the cameras in common use are given in Table II.
In general these errors in time of missile exposure have a random error effect on the accuracy of velocity and acceleration data. When cameras are used to track missiles, timing errors are inconsequential if the tracking is smooth. The 3000-Aisec error in Askania timing does not affect the accuracy of velocity and acceleration data if the operator is able to keep the missile in the center of the frame. But if he is not able to do so, the tracking error measured on
Griffin and Green: Accuracy Limitations
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