July-August - Air Defense Artillery School

Individual racer ontro
By Major George W. Race, Coast Artillery Corps
Individual Tracer Control mav be defined as a svstem of
machine gun fire control wher~by the individuaf gunnel
places his fire on the target as a result of his own observation
of the tr:tcer stream. l.T.C. in its pure form takes no
cognizance of the presence of sights on the machine gun.
Firing must be continuous in order to pro\'ide an unbroken
tracer stream; interruption of the tracer stream (firing in
bursts) is destructive to this fom1 of fire control, due to the
fact that in such a case the gunner must begin his adjustment
all over again at the beginning of each burst.
TRACER OBSERVATION
The science of efficient observation and e\'aluation of the
tracer stream deserves discussion, without regard to the
tracking, leading and firing based thereon. Efforts to this
end are tremendously hampered by two factors; first, the
total absence of depth perception ability (stereoscopic vision)
at ranges exceeding 300 yards, and secondly, the curve,
both real and apparent, of the tracer stream in space.
Depth perception has been defined as the ability of an individual
to determine, by the use of his two eyes, which of
two objects is the farther from him. Stereoscopic vision is
actually only one of several factors in depth perception and
depends entirely on the angle subtended by the distance
between the eyes. At about 500 yards range, this angle
becomes so small as to be useless; consequently, at longer
ranges the relative position of objects is unconsciously determined
through other factors, as comparative sizes, superimposition,
amount of visible detail, etc. From this standpoint
it would become necessary to compare the tracer and
the airplane target, as seen together, in order to determine
which was at the longer range, or to be more exact, in order
to pick a point on the tracer's path having the same range
as the airplane. This is obviously impossible; even at very
short ranges it is doubtful whether or not depth perception
is effective when applied to a source of light having neither
sharp outline nor definite shape.
J f the tracer stream were laterally straight in appearance
the limitations of depth perception would have little effect
on the problem, it being then necessary only to pass the
line of the tracers through the target. Unfortunately,
however, the tracer stream presents a definitely curved
appearance, both in the lateral and the vertical plane. See
Fig. 1.
The vertical curve of the tracer stream is a real curve.
It is due to the actual gravity drop of the projectile below
the line of bore and warrants no further discussion.
The lateral curve of the tracer stream is both real and
fictitious. The real element of the curve is produced by
traverse of the gun during firing. Tracers fired successively
with a constantly changing azimuth are viewed simultaneously.
each individual tracer being at a different range
and azimuth from the others when viewed; thus by polar
coordinates an actual curve is plotted in the sky. This
curve is produced and clearly seen merely by traversing a
firing gun, without a target. The fictitious element of a
lateral CUf\'eis produced by motion of the target; the targe:
in space is the reference point upon which the eyes ar~
focused: consequently the target nppears stationary, its lal
ernl motion manifesting itself as lateral motion of the in
dividunl trncers in n renrward direction. This optical illu
sion is npparent even in the case of a single tracer fired
from n stationary gun, ns long as a moving target is visibl
together with the tracer. Tracers from slow firing gum.
exhibit a lateral CUf\'e due entirely to this optical illusion
while the tracer stream from a machine gun invariabl
exhibits a lateral curve from both sources, provided that
moving target is used. Actually, when firing is done on
crossing courses the vertical and lateral CUf\'es combine t
give a diagonal curving effect as viewed from the gun
\ Vhen firing is done on a constant altitude incoming course
the two CUf\'es combine to give a very sharp CUf\'e in the
vertical plane. On an incoming course that is diving on th
gun, the only visible curve is the actual vertical cun'c du
to gravity drop of the bullet. I
LINE SHOTS I
Line shots are properly defined as shots intersecting the
line of sight from gun to target; line shots intersecting the
line of sight nt a point near the target also intersect the
target's line of flight: therefore, for practical purposes. line
shots arc best defined as shots crossing target's line of High
at proper elevation. In general, tracers may be considcrt'l!
line shots when they appear to be (l) going into the targe
(2) passing around the target and eclipsed by it, nnd (3 I
passing short of the target and superimposed against it.
In l.T.c. either with or without the aid of sights. the
primary necessity is to make the tracers appear as line sho"l
'.
C. Jk.,.!'ca/ d La,l""",/ C""W3
c;",..6,n,/ (r=r we ..... )
6. Lor~ral Cl/rve (p/an)
~ ",,,,
Int:o-n,;,,y co .....# (c-..n.....#)
d. Sp=ia/ COr./0,./~ '-'"
191/C''''r_rorr ,;" V~i