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FM 4-111
WAR DEPARTMENT
COAST ARTILLERY
FIELD MANUAL
ANTIAIRCRAFT ARTILLERY
POSITION FINDING AND CONTROL
ANTIAIRCRAFT SEARCHLIGHTS
FM 4-111
COAST ARTILLERY
FIELD MANUAL
ANTIAIRCRAFT ARTILLERY
POSITION FINDING AND CONTROL
ANTIAIRCRAFT SEARCHLIGHTS
Prepared under direction of the
Chief of Coast Artillery
UNITED STATES
GOVERNMENT PRINTING OFFICE
WASHINGTON : 1940
For sale by the Superintendent of Documents, Washington, D. C.
Price 10 cent«
WAR DEPARTMENT,
Washington, July 6, 1940.
FM 4-111, Coast Artillery Field Manual, Antiaircraft
Artillery, Position Finding and Control, Antiaircraft Search-
lights, is published for the information and guidance of all
concerned.
[A. G. 062.11 (5-3^0).]
By order of the Secretary of War:
G. C. MARSHALL,
Chief of Staff.
Official :
E. S. ADAMS,
Major General,
The Adjutant General.
ii
TABLE OP CONTENTS
Paragraph Page
Chapter 1. General 1-5 1
Chapter 2. Sound Location 6-9 3
Chapter 3. Acoustic Correctors.
Section I. General 10 9
II. Acoustic corrector for M2 sound
locator 11-18 10
III. Acoustic correctors Ml and M2 19-22 29
Chapter 4. Searchlights 23-29 36
Chapter 5. Searchlight Control 30-34 43
Chapter 6. Control Station 35-40 48
Chapter 7. Binaural Training Instruments 41-43 53
Index 57
III
FM 4-111
COAST ARTILLERY FIELD MANUAL
ANTIAIRCRAFT ARTILLERY
POSITION FINDING AND CONTROL, ANTIAIRCRAFT
SEARCHLIGHTS
CHAPTER 1
GENERAL
■ 1. Scope. — This manual treats of the position finding, con-
trol, and illumination phenomena pertaining to antiaircraft
searchlights. A knowledge of the basic principles described
in FM 4-110 and FM 4-112 will be helpful in understanding
the problem of locating and illuminating aerial targets. Per-
tinent definitions and symbols should be studied, and a thor-
ough understanding of the picture in space of the various
elements of data should be acquired. The training of anti-
aircraft artillery searchlight units is covered in FM 4-115.
■ 2. Illumination Problem. — From the description of fire
control instruments contained in FM 4-110 it is evident that
the success of the methods employed depends upon the actual
tracking of the target by the various instruments. During
the hours of daylight, under normal atmospheric conditions,
the target can usually be seen and tracked. This, however,
is impossible during the hours of darkness unless the target
is illuminated in some manner. The illumination must be
maintained during all the firing, if continuous pointed fire
is to be used. Of all the methods of illumination, search-
lights have been found to give the most satisfactory results.
■ 3. Location Problem. — A searchlight beam swinging aim-
lessly in the sky would illuminate the target only by chance.
Therefore, before the searchlight can be trained on the
target, the target must be located. The problem is to
determine the azimuth and angular height of the position
of the target. At the present time the only practical field
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COAST ARTILLERY FIELD MANUAL
method of locating the target is by sound. Various methods
of utilizing sound have been tried, the best results being
obtained by a horn collector with its acoustic tract leading
directly to the listener's ear.
■ 4. Control Problem. — The ability of the searchlight to
pick up a target and illuminate it continuously depends upon
a number of factors all of which tend to decrease the visi-
bility. Research indicates that the visibility of the target
increases as the operator moves away from the searchlight.
Approximately 90 percent of the advantage which can be
gained is obtained at a distance of about 50 feet. Manual
control of the searchlight is impractical at this distance,
hence electrical control is employed. The control problem
is that of pointing the searchlight in the direction indicated
by the sound locator, the operator teing located 50 feet or
more from the light source.
■ 5. Searching. — Even with the latest equipment and well-
trained listeners, the target seldom will be located exactly
at the azimuth and angular height indicated by the sound
locator. Because of the effect of atmospheric conditions on
the speed and direction of sound waves, it will be necessary
to search around the azimuth and angular height as com-
puted by the sound locator. However, the total effect of
the above-mentioned variables seldom exceeds 5° in azimuth
or angular height. This is well within the limits of search-
ing. So far no practical field method has been found for
determining the value of the atmospheric effects on the
sound waves. For this reason it is impractical to apply
corrections for these atmospheric effects prior to tracking
the airplane by sound.
2
CHAPTER 2
SOUND LOCATION
■ 6. Theory of Sound Location. — a. (1) All persons possess-
ing normal hearing with both ears have a faculty known as
the "binaural sense" which enables one to locate the approxi-
mate direction of a sound source. When the sound source is
directly in front of a person, the sound waves travel the
same distance in reaching both ears. (See fig. 1 in which
Rl equals Rr.) If, however, the head is turned to one side,
the sound waves will travel farther to reach one ear than the
other. (See fig. 1 in which Rl does not equal Rr.) Sound
travels through the air at a speed of about 1,100 feet per
second. Therefore, when the head is turned to one side,
the ear closest to the sound source receives the sound vibra-
tion slightly before the other ear. The binaural sense enables
the listener to distinguish this brief phase difference between
the reception of the sound by each ear. The listener turns
his head until the phase difference becomes imperceptible.
At this point the head will be pointed toward the sound
source.
(2) Normally the head is erect and the ears are in a hori-
zontal plane resulting in binaural sense in azimuth. How-
ever, if the ears are placed in the vertical plane, the eleva-
tion of the sound source can be determined just as readily.
The unaided ears can usually determine the direction of a
sound source within 10°.
b. The accuracy of the binaural sense is proportional to
the distance between the ears. This can be seen from figure
1. If the base line between the ears is made longer, the
difference between ranges Rl and Rr will become greater,
and the corresponding phase difference at which the sound
strikes each ear will become greater. Since the accuracy of
3
6
COAST ARTILLERY FIELD MANUAL
sound location is dependent upon the minimum phase differ-
ence that the ears can detect, extending ^he base line of the
ears in effect reduces the minimum phase difference that the
ears are able to recognize. The base line between the ears
Figure 1. — Binaural sense.
can readily be increased by the use of a pair of horns mounted
some distance apart and connected to the ears by tubes.
c. The horns serve another purpose. Sound is essentially
a vibration transmitted through some medium, generally air.
The sensation of sound usually is experienced by a person
when the vibrations are felt by the ear. The stronger the
4
ANTIAIRCRAFT SEARCHLIGHTS
6-7
vibration, the greater will be the sensation experienced by
the ear. The practice of cupping the hand over the ear in
order to hear faint sounds is well known. Horns will do the
same as the cupped hands except that they collect the vibra-
tions from a greater area than the hands. The intensity of
the sound is thus increased, and the listener is able to dis-
tinguish sounds which he would be unable to hear with the
unaided ear.
d. To summarize, all persons with normal ears possess
binaural sense. A listener possesses binaural sense in the
vertical plane if his ears are located in the vertical plane.
The accuracy of the binaural sense is increased and the in-
tensity of the sound magnified by the use of horns separated
by some distance.
■ 7. Sound Locator DEsiGN.^Sound locators are designed to
aid the listener's binaural sense. Basically they are all alike,
consisting of two pairs of horns mounted so that the base lines
of the pairs are perpendicular to each other, and one base line
is horizontal. This arrangement allows one pair of horns to
determine the location of the target in azimuth and the other
in angular height. The base lines of the horns are made
perpendicular by the design of the instrument. One base line
is made horizontal by the use of leveling jacks or screws.
o. To locate target. — (1) Sound locators now have either 3
or 4 horns. The 4-horn type uses one pair for azimuth deter-
mination and the other pair for angular height. In the 3-
horn type 1 horn is used by both listeners by dividing the
acoustic tract at the exit of the horn. It will be seen that
basically the 3-horn type functions the same as the 4-horn
type.
(2) The spacing of the horns varies in the different designs.
Theoretically, the accuracy of the instrument increases with
the length of the base line. The older models have a base
line length of 112 inches. Recent exhaustive tests have shown
that there is not much gained in accuracy if the length of
the base line exceeds 60 inches. The new models have the
shorter base line, thereby decreasing the weight and size of
the instrument.
239086"— 40 2
5
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COAST ARTILLERY FIELD MANUAL
(3) The older models of sound locators use the exponen-
tial horn, so called because the cross section area of the
horn at any point bears a definite relation to the length
of the horn at that point. Such a design collects the sound
within the frequencies of airplane noise and transmits it
to the acoustic tract without distortion.
(4) Recent extensive tests in the laboratory and in the
field have indicated that horn size and horn shape have
little influence on the range of the sound locator. Hence,
the latest models do not have the exponential horn but a
horn which has certain other characteristics. (See b (3)
below.)
b. To reduce ambient noises. — (1) The existence of am-
bient noises (that is, the general surrounding noise) serves
to decrease the range at which the sound locator can op-
erate. A sound locator will be used as much as possible at
its maximum obtainable range. The more distant the tar-
get, the fainter will be the sound. On the other hand, the
intensity of the ambient noises will remain fairly constant
regardless of the range. It is evident then that in order
to increase the range of the sound locator the ambient
noises must be reduced as much as possible. An analogy
will emphasize this point. The less noise there is in a
room, the more easily a quiet conversation can be heard.
(2) Ambient noises are classified as —
(a) Reverberation of horn structure.
(b) Effect of wind blowing on horns and supporting
structure.
(c) Gear and other noises transmitted through the struc-
ture itself to the acoustic tract.
(d) All air-borne noises except those in the direction of
the target.
(3) Ambient noises are reduced greatly by the design of
the latest type of sound locator. This type features sound
insulating and vibration dampening materials, streamlining
of the horns and supporting structure, and making the horn
definitely directional within a 30° cone in the direction of the
axis of the horn.
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ANTIAIRCRAFT SEARCHLIGHTS
8-9
■ 8. Sound Locator M2 (fig. 2) . — This sound locator em-
ploys three horns. For further description see FM 4-115,
TM 4-210, or the operator's manual issued with the sound
locator. For information on setting up, orientation, opera-
Figtjre 2. — Sound locator M2.
tion, and care see FM 4-115. For information on training
instruments and methods see chapter 7 of this manual and
FM 4-115.
■ 9. Sound Locator Ml Series (fig. 3) . — This instrument is
the 4-horn type with exponential horns. Several models
7
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COAST ARTILLERY FIELD MANUAL
Figure 3. — Sound locator M1A8.
(M1A1 to M1A8) are found in the service, all of which are
similar except for the types of trailers and acoustic cor-
rectors included therewith. For further description see
FM 4-115, TM 4-210, or the notes on materiel, issued with
the instrument. For information on setting-up, orientation,
operation, and care see FM 4-115. For information on
training instruments and methods see chapter 7 of this
manual and FM 4-115.
8
CHAPTER 3
ACOUSTIC CORRECTIONS
Paragraphs
Section I. General 10
II. Acoustic corrector for M2 sound locator 11-19
III. Acoustic correctors, Ml and M2 20-23
Section I
GENERAL
■ 10. General. — a. In the preceding chapter we have seen
how it is possible to locate a sound source in azimuth and
angular height by means of the sound locator. Consider a
specific example. An airplane is flying directly toward the
sound locator at a speed of 300 miles per hour and at an
altitude of 4,000 yards. At the instant when the plane is at
a horiEontal range of 8,000 yards (see fig. 4) , the slant range
to this point is 8,944 yards or 26,832 feet. Sound travels at
Sound Locator IR) 8000 YDS.
Figure 4. — Travel of target during sound lag time.
9
10-11
COAST ARTILLERY FIELD MANUAL
approximately 1,100 feet per second. It will take
26,832
• =24.4 seconds for the sound to reach the listener.
1,100
The time required for sound to reach the listener is called
sound lag time (£ s ) and is expressed in seconds. In 24.4
seconds the target will travel 24.4 times 150 or 3,660 yards,
approximately. That is, the target will actually be at a
horizontal range of 4,340 yards when the sound emitted at
8,000 yards is heard at the sound locator. The sound locator
would indicate the angular height of the target as being
470 mils, whereas it actually would be 760 mils. This illus-
tration shows only the vertical error due to sound lag because
the course considered was an incoming course. Any other
course would have errors because of sound lag in both
azimuth and angular height.
b. The above illustration indicates the necessity of calcu-
lating corrections for sound lag and of applying these cor-
rections before the data are transmitted to the control station
for use in directing the searchlight to the target. These
vertical and horizontal corrections for the travel of the target
during sound lag are called ax and s x , respectively.
Section II
ACOUSTIC CORRECTOR FOR M2 SOUND LOCATOR
■ 11. Sound Lag. — a. Consider the azimuth travel of the tar-
get as shown in figure 5, the target traveling in the direction
A-B. A listener at O will point the sound locator toward A
when the target is at B. The sound lag angle is S x . The dis-
tance the target travels during the sound lag (A-B) is equal
to ts times S where S is target speed expressed in feet per
second. The slant range to the apparent position of the
target 0~A can be expressed as ts times 1,100. Then
AB = t.XS = S
AO f„Xl,100 1,100
The speed of sound is considered constant, so the value of
the above ratio depends only upon the speed of the target.
b. Note that the magnitude of the slant range to the tar-
get does not affect the value of this ratio. This can be proven
10
ANTIAIRCRAFT SEARCHLIGHTS
11-12
graphically. Consider a course A'-B' parallel to A-B at one-
half the slant range. The sound lag will be one-half that
for the course A-B, but the travel of the target will be also
one-half A-B. The triangles OAB and OA'B' are therefore
similar, and the lateral sound lag angle is 5 X in either case.
■ 12. Wind Error. — In addition to the effect of sound lag, a
serious error may be introduced by wind. Consider a normal
u Sound Locator
Figure 5. — Sound lag angle, horizontal projection.
wind blowing in the direction indicated in figure 6. The
effect of the wind will be to change the apparent position of
the target from A to B. As in the case of sound lag and in
a similar manner
AB _W
( W= velocity of wind in f t./sec.) . The angle s 2m represents the
error due to wind. A wind not perpendicular to the direc-
tion OA can be resolved into a component parallel to AB and
11
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COAST ARTILLERY FIELD MANUAL
a component parallel to AO. The effect of the component
parallel to OA is so slight that it can be ignored.
■ 13. Parallax. — a. The sound locator for various reasons
may be placed at some distance from the searchlight. Such
displacement introduces a parallax error which may become
serious. Figure 7 shows the analysis of the parallax error.
The sound locator is at O, the searchlight at O', and the
true position of the target at B. If the azimuth of B from
Direction of
Sound Locator
FiGuitE 6. — Wind effect, horizontal projection.
O were used to set the searchlight in azimuth, the search-
light would be pointed at B'. An azimuth correction equal
to the angle DO'D' (known as the parallax correction)
must be applied to the searchlight data to have the search-
light point at B.
b. When the slant range OB is very great and the angular
height small, the parallax correction is practically negligible.
The error increases as the angular height increases, until
at a point directly over the base line between O and O' the
error in azimuth is 180°.
12
ANTIAIRCRAFT SEARCHLIGHTS
14
■ 14. Principle of Operation. — a. (1) The principle of the
acoustic corrector is illustrated in figure 8. The sound
locator is at O, the apparent position of the target at A,
and the true position of the target at B. AB is the travel
of the target during the sound lag time. The sound locator
determines the azimuth and angular height of the point A.
The acoustic corrector determines the sound lag corrections
S x (for azimuth) and ox (for angular height) which, when
o'
Figure 7. — Parallax error.
added to the azimuth and angular height of A, will give the
azimuth and angular height of B.
(2) In paragraph 11 it was shown that the ratio
AB
AO
varies only with the speed of the target. The ratio
AB
AO
and the direction AB determine the sound lag angles Si and ax.
239086°— 40 3
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COAST ARTILLERY FIELD MANUAL
b. (1) The acoustic corrector for the M2 sound locator
solves the problem of determining S x and <r x in the following
manner (see fig. 8) . The points O, A, and B are set up to a
reduced scale. The distance AB is obtained by estimating
the speed of the target. The miniature airplane course indi-
cator (see par. 16) automatically and continuously determines
the direction of AB. The point B is thus located to scale.
The sound lag angles are determined by sighting at the point
B from O in the miniature model and measuring the angular
displacement (lateral and vertical) of point B from point A.
SoundLocafor
Figure 8. — Principle of acoustic corrector. M2 sound locator.
(2) In the scale model used in the acoustic corrector, the
slant range AO is kept constant at 7.5 inches. AB is adjust-
able for target speeds from to 400 miles per hour. As previ-
ously stated in paragraph 11,
AB = S
AO 1,100
AO is constant, being 7.5 inches. For a target traveling 200
miles per hour, S will be equal to 293 feet per second. Then,
293X7.5
— 1 1 00 = ^ mcnes > approximately.
Likewise, when S equals 100 miles per hour, AB equals ap-
proximately 1 inch.
14
ANTIAIRCRAFT SEARCHLIGHTS
14-15
(3) Figure 9 shows a view of the acoustic corrector on the
M2 sound locator. The miniature airplane course indicator
indicates a direction parallel to the course of the target (see
par. 17). The target course computer (6) and multiplying
pantograph (1) are mounted so that they remain horizontal
while the yoke holding these mechanisms is elevated or
depressed with the horns of the sound locator. O'A' extended
will intersect the apparent position of the target. The length
of A'B' is adjusted by the knob (2) according to the esti-
mated speed of the target.
(4) For mechanical reasons it is impossible to sight from O'
to B', hence the multiplying pantograph is used to transfer
the position of B' to B where it can be viewed. The points
O, A, and B have the same significance as O, A, and B in
figure 8. OA is constant at 7.5 inches. AB is approximately
1 inch for each 100 miles per hour of target speed. It should
be noted that the use of the pantograph requires the offset
A' B' to be parallel to but opposite in direction to AB. The
true position of the target is therefore in the direction OB
and not O'B'.
■ 15. Sight Mechanism. — a. The ball (B) on the end of the
pantograph (1) is viewed through the sight (3) . (See fig. 9.)
The sight consists of a sighting tube and a mirror which
may be rotated in azimuth by a correction handle projecting
underneath the tube supporting arm. Motion of the mirror
in angular height is obtained by mounting the mirror on
a tilting table which is connected by gearing to a rotatable
knob on the end of the correction handle. By looking
through the sighting tube and positioning the mirror by
means of the correction handle and knob, the operator alines
the image of the ball (B) on the end of the pantograph with
the cross lines of the mirror. Thus the axis of the sight is
made to coincide with the line OB, figures 8 and 9. The
sight is connected to the sound locator so that its normal
(zero correction for sound lag) axis coincides with the line
AO. By positioning the sight so that the pantograph pointer
coincides with the cross lines of the mirror, the sound lag
angles S x and ax are introduced through differentials to
the gearing connecting the azimuth and elevation drives with
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COAST ARTILLERY FIELD MANUAL
Piguke 9. — Acoustic corrector, M2 sound locator.
16
ANTIAIRCRAFT SEARCHLIGHTS
15-16
the azimuth and elevation transmitters. (See fig. 10.) The
transmitters are therefore transmitting the azimuth and
angular height of the true position of the target to the con-
trol station.
b. The line of sight OB (fig. 9) if extended would pass
through the true position of the target. Therefore, when the
searchlight is in action, the pantograph pointer (B) should
be silhouetted against the searchlight beam. In daytime
practice, or at night when the target is in the beam, the
target should be visible beyond the pantograph pointer.
(This is true only when no parallax correction is applied.)
As an aid to daylight training a circle subtending 10° at the
Mirror
Mirror
Correction
Handle
To fl z. Transmitter
True Az.
To Elev .
Transmitter
True Elev
Apparent Az. TrueAz. Apparent Elev.
Figure 10. — Schematic sketch, sight mechanism.
operator's eye incloses the cross lines on the mirror. With
proper training the listeners should be able to keep the target
image within this circle when the pantograph pointer is cen-
tered on the cross lines.
■ 16. Miniature Airplane Course Indicator. — a. In paragraph
14& it was stated that the miniature airplane course indicator
automatically and continuously determines the direction AB
in figure 9.
b. (1) Figure 11 shows projected upon the horizontal plane
a target course L-M along which the target has the velocity
Vt. The sound locator situated at O is pointed in azimuth
in the direction OA, where A is the apparent target position.
17
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COAST ARTILLERY FIELD MANUAL
The angle which the target course makes with the sound
locator azimuth heading or vertical listening plane is repre-
sented by a.
(2) Considering only a straight course at constant altitude,
the target velocity Vt may be resolved into two horizontal
components Vr and Vl, the former lying in the listening plane
and the latter normal to the listening plane. In terms of
these two components, the course angle is
a: = tan -1 f^ (1)
-
Figure 11. — Projection of target's course on horizontal plane.
(3) Figure 12 is a projection upon the listening plane of
the same data whose horizontal projection is shown in fig-
ure 11. The sound locator elevation heading is OA where A
is the apparent target position as in figure 11. The angular
height of the sound locator is represented by e.
(4) The miniature airplane course indicator is controlled
by a caster connected to a ball analyzer which is mounted on
the sound locator in such a way that the axis of rotation of the
resolving ball remains horizontal. The two driving rollers, at
right angles to each other, are so located in the horizontal
plane that the one which corresponds to component Vr tends
18
ANTIAIRCRAFT SEARCHLIGHTS 16
to rotate the resolving ball in the listening plane, while the
second driving roller which corresponds to Vl tends to rotate
the resolving ball in the plane normal to the listening plane.
Figure 13 shows this arrangement schematically. The roller
corresponding to Vl is termed the lateral driving roller and
that corresponding to Vs. is termed the radial driving roller.
The resultant motion of the ball depends upon the relative
speeds of these two rollers. If the speeds of the lateral and
radial driving rollers are proportional to Vl and Vr, respec-
tively, the resulting plane of rotation of the ball is parallel
with the target course and the caster alines itself parallel
with the target course and indicates the correct course angle.
M
R A
Figure 12. — Projection on listening (or vertical plane) through
sound locator and apparent target position.
(5) The problem is to make the ratio of the speed of the
lateral driving roller to the speed of the radial driving
roller proportional to Vl/Vr. It is convenient to derive the
driving roller speeds from the azimuth and elevation angular
velocities of the sound locator due to tracking the target.
However, as will now be shown, the ratio of these velocities
is not proportional to Vl/Vr.
(6) Referring to figure 11, the azimuth angular velocity
of the sound locator due to tracking the target is
where R is the horizontal range applying to figure 11, as
shown in figure 12.
19
16
COAST ARTILLERY FIELD MANUAL
The elevation angular velocity of the sound locator due to
tracking the target is
Vr sin c
S«= D (3)
where e is the angular height and D is the slant range, as
shown in figure 12.
(7) If the lateral and radial driving roller speed ratio is
proportional to the azimuth and elevation angular velocity
20
ANTIAIRCRAFT SEARCHLIGHTS
16
ratio of the sound locator, then the course given by the
resolving ball is
tan-i~ (4)
Substituting (2) and (3) in (4)
tan~V= tan-i — ^ — (5)
Vr sin e
D
Expressing the horizontal range in terms of slant range,
R=D cos e, and substituting this in (5)
tan-'| 2 = tan-^_ os J.
^« Vr sin e
D cos e Vr sin e
Vr 1
= tan~ V X —. (6)
Vr cos e sin 6 '
(8) Equation (6) shows that when the driving roller speed
ratio is proportional to 2a/2 e , the resultant course given by
the resolving ball is not
tan->?~ (1) but is tan" 1 ^ X — (6)
Vr w Vr COS e sin e w
and it is therefore incorrect. To make the course correct we
must multiply the numerator of equation (6) by the factor
cos e sin e to cancel this factor in the denominator. We
then get the following relationship
a = tan~ 1 ^r cos e sin e (7)
which gives the correct course angle. This can be accom-
plished by inserting in the drive connecting the lateral
driving roller to the azimuth movement of the sound locator
a variable speed drive of the speed ratio characteristic,
cos e sin e.
(9) For practical purposes, however, since the useful op-
erating range of the sound locator lies between angular
239086°— 40 4 21
16
COAST ARTILLERY FIELD MANUAL
heights of from 15° to 75° we can replace the factor
cos e sin e with the constant 0.4 and still get a reasonably
accurate solution of course angle. Equation (7) now becomes
= tan-!
0.4=-"
2.5 S e
(8)
(9)
(10) For simplicity in design, in the apparatus as actually
constructed, equation (8) is satisfied by making the radial
driving roller speed 2.5 times faster than the lateral driving
roller speed for the same angular velocities of the sound
locator in azimuth and angular height.
Figure 14. — Ball analyser, plan view.
c. What has just been accomplished graphically, that is,
the determination of the direction of the course of the target
using the angular velocities in azimuth and angular height,
can be done mechanically by a ball analyser. (See fig. 14.)
The ball is free to rotate in any direction and is caused to
rotate by the two rollers EL and AZ. The axes of these two
rollers are at right angles and in the horizontal plane.
Thus any rotation given to the ball must be about an axis
in the horizontal plane. As the rollers EL and AZ revolve
in the direction shown in figure 14, the top of the ball will
be displaced in the direction F if the rates of the rollers
are equal.- If the AZ roller is stationary, the ball will roll
in the direction D. The direction of the rotation of the ball
22
ANTIAIRCRAFT SEARCHLIGHTS
16
is dependent therefore on the relative rates of turning of the
two rollers EL and AZ. Pressing on the ball is a caster
which is free to rotate about a vertical axis through the
point A. (See fig. 15.) The caster will swing about the
axis so as to point in the direction of rotation of the ball.
That is, the caster indicates the direction of the resultant
of the two vectors which are the rates of turning of the EL
and AZ rollers. The AZ roller is connected directly to the
azimuth drive of the sound locator. The elevation roller is
A B
Figure 15. — Miniature airplane course indicator.
connected through gearing to the angular height drive so
that the angular velocity is multiplied by 2.5 before it rotates
the roller. The caster is therefore indicating the direction
of the resultant of the two vectors 2 and 2.5 2 e which was
shown in b above to be the direction of the course of the
target.
d. A small model airplane is mounted on the caster so as
to give a realistic picture of the direction of the course. The
model plane also provides a method of introducing the target
speed. The pin B (fig. 15) is adjustable by turning the knob
23
16-17
COAST ARTILLERY FIELD MANUAL
on the model where the propeller would be located. A scale
graduated in miles per hour target speed is provided, allowing
speeds up to 400 miles per hour to be applied.
B 17. Correction for Wind. — a. In the preceding discussion
concerning the determination of the sound lag angles, the air
speed of the target has been assumed equal to the ground
speed.
b. In paragraph 12, it was shown that the wind acts in the
same manner as target speed to cause a difference between
the apparent position and the true position.
WIND Course of
Figure 16. — Wind correction.
c. A study of figure 16 will show how the acoustic corrector
can correct for wind. Assume first that there is no wind
and that the target is flying a rectilinear course as shown in
figure 16. The plane will travel from A to B during the
sound lag. Now consider a wind blowing in an opposite
direction to the course of the target. Then the listener will
determine the apparent position of the target as A'. The
target travel with respect to the ground will be from A to B'
as the head wind will reduce the ground speed of the target.
The distance between the apparent position and true posi-
tion has not changed due to the wind, that is AB=A'B'.
Both the sound and the target are borne by the same medium,
air. If the air is moving, as is the case when the wind is
24
ANTIAIRCRAFT SEARCHLIGHTS
17-18
blowing, both the sound and the target will be displaced equal
amounts.
d. It must be remembered that the ground speed of the
target is affected by the wind but that the air speed of the
target is entirely independent of the wind. From the above
discussion it is seen that the effect of wind is automatically
corrected for if we use the air speed of the target in making
the setting on the miniature airplane course indicator.
■ 18. Correction for Parallax. — a. The parallax error is de-
pendent upon the slant range to the target, the azimuth and
angular height of the target, and the direction and displace-
ment of the searchlight from the sound locator. When the
searchlight and sound locator are set in position, the dis-
placement becomes a constant.
b. In figure 17, the relation of target, sound locator, and
searchlight is shown as reproduced to scale in the acoustic
corrector. The pivot of the sight mechanism is at O. The
solid circle with radius AB represents the possible locus of
the pantograph pointer for some particular target speed.
The searchlight is at L. The distance AO is 7.5 inches, which
is the distance to scale that sound travels in 1 second (1,100
feet) . As
AB S
AO~ 1,100
(see par. 11) it follows that if AO is the distance that sound
travels in 1 second, AB must be the distance that the target
travels in 1 second. The distance AB remains constant so
long as the target speed is unchanged.
c. Let us assume that the course of the target (A-B, fig. 17)
is as shown parallel to the base line and that the apparent
position of the target is A. The true position of the target
is B. The azimuth of the point B from O must be corrected
by the parallax correction angle OCL (equals C'LC) if the
searchlight is to be directed at the point B by the data fur-
nished by the sound locator. If we could shift the pivot of
the sight from O to L and then aline the sight with the panto-
graph pointer, the parallax correction would be included in
the transmitted data to the control station.
d. Due to problems of construction, it is simpler to move
the pantograph pointer from B to B' rather than move the
25
18
COAST ARTILLERY FIELD MANUAL
pivot of the sight. The locus of the pantograph pointer for
a particular target speed is therefore the dotted circle of
radius A'B' equal to AB. This shifting is accomplished by
mounting the fixed elbow of the pantograph (8) on an ad-
justable offset pivot. (See fig. 9.) The axis of the offset is
parallel with the base line at the time of instrument orienta-
tion. It is always kept in such alinement by gearing con-
nected to the azimuth drive. In this manner A is offset to
A', and A' revolves in a circle about A as the sound locator
traverses in azimuth, the direction A A' being always parallel
to LO.
e. The true position of the target is actually in the direc-
tion OB, but the parallax correction makes the sight point in
the direction OB'. Since OB' is parallel to LB, the search-
light is trained on the true position of the target. If the
corrector operator were able to see the target in the mirror
of the sight, it would no longer be in line beyond the panto-
graph pointer (B', fig. 17) but at position B.
f. At 0° elevation the Z OBL= Z.OCL. As the angular
height increases (assuming slant range is constant) , Z OCL
and likewise Z OCX increase, but Z OBL remains constant.
When the target is directly over O the direction LC' will have
moved counterclockwise to lie in the direction LO. The
parallax correction has therefore increased from the Z OCL
to 90°, as the angular height increases from zero to 90°.
g. The offset AA' is chosen for an estimated slant range
at 0° elevation. As stated in paragraph 14b, the entire cor-
rector remains horizontal as the sound locator elevates or
depresses. Therefore the offset AA' is maintained horizontal
as the elevation of the line OA changes with the elevation
of the sound locator. Consideration of the figure will show
that BB' is always parallel to and equal to AA'.
h. In introducing the parallax correction, the vertical plane
through the sight is shifted from C to C, both of which are
in the horizontal plane. As the angular height increases, the
distance OC' decreases, thereby increasing the parallax cor-
rection angle CLC'. Hence it can be seen that the parallax
correction angle is changed to the proper value as the sound
locator changes elevation.
26
ANTIAIRCRAFT SEARCHLIGHTS
18
i. Figure 17 shows a course parallel to the base line in
which only parallax correction in azimuth is involved. The
construction of the instrument is such that AA' (and likewise
BB') is always parallel to the base line and horizontal. The
parallax correction has been introduced by shifting the
pantograph pointer (B) in direction and distance equal to
the displacement (to scale) of the searchlight from the sound
Figure 17. — Parallax corrections.
locator. Therefore, the parallax correction is made regard-
less of the direction of the course of the target, provided the
slant range to pick-up point is properly estimated.
j. The three factors affecting the parallax correction have
been discussed. The corrections in azimuth and angular
height are automatic. (See h and i above.) The direction
of the parallax offset is introduced by proper orientation when
the sound locator is set up and is thereafter automatically
27
18
COAST ARTILLERY FIELD MANUAL
maintained. Corrections for slant range and length of base
line are made by changing the offset of the pantograph. A
scale is provided for the offset graduated in arbitrary num-
bers from to 10, there being no offset for and the maximum
offset for 10. The value set on the scale is selected from figure
18, using as arguments estimated slant range to pick-up point
in feet, and base line length in feet.
BASE UNE-FEET
200 300 400 300 600 700 BOO 900 1,000
Figure 18. — Graph of parallax settings.
k. The estimation of the pick-up slant range is a matter
of experience. The factors affecting the problem in gen-
eral are —
(1) Type of target.
(2) Camouflage of target.
(3) Altitude of target.
(4) Atmospheric conditions at the moment (such as mist
or clouds) and the degree of darkness.
(5) Type of searchlight being used.
The above factors can be evaluated and an estimate made
of the approximate slant range at time of pick-up based on
past experiences under similar circumstances.
28
ANTIAIRCRAFT SEARCHLIGHTS
19-21
Section III
ACOUSTIC CORRECTORS Ml AND M2
■ 19. General. — a. In section II the acoustic corrector for
the M2 sound locator was discussed. This is the latest type
of acoustic corrector. Two other types will be found in the
service, the Ml and M2. Both the Ml and the M2 operate
on the same principle, but in other features they are quite
different. The principle of operation of the Ml and M2 is
totally different from that described in section II. Note that
the M2 acoustic corrector is not used with the M2 sound
locator.
b. The Ml and M2 acoustic correctors were designed for
use with the sound locators of the Ml series. Acoustic cor-
rector M1A1 is similar to the Ml except that it delivers data
in mils.
■ 20. Principle of Operation. — a. The angular velocities of
the target in azimuth and angular height are obtained by
the process of tracking the target with the sound locator.
These angular velocities 2 S and 2 e are multiplied by the
sound lag time (is) to obtain S x and a x , respectively.
b. The sound lag angles d x and ax are added, respectively, to
the azimuth and angular height of the apparent position of
the target as determined by the sound locator, giving the
true position of the target, which is then transmitted elec-
trically to the control station.
■ 21. Acoustic Corrector Ml (M1A1). — a. The principle by
which this device works has been explained in paragraph 20.
Figure 19 shows the instrument with the front cover re-
moved. It consists, essentially, of the following components:
(1) Two single unit electrical data transmitters.
(2) Sound lag drum and chart.
(3) Two prediction mechanims.
(4) Corrected azimuth and elevation scales.
b. Two single unit synchronous electrical data transmitters
are provided, one for azimuth and the other for angular
height, to transmit the true position of the target to the con-
trol station. The rotor of each is connected with the azimuth
or angular height drive of the sound locator. The transmitters
29
21
COAST ARTILLERY FIELD MANUAL
would therefore transmit the azimuth and angular height
of the apparent position of the target to the control station
if no sound lag correction angles were added.
c. The sound lag drum carries a chart upon which are
plotted curves of constant sound lag time (ts) using altitudes
as ordinates and angular heights of the apparent position as
30
ANTIAIRCRAFT SEARCHLIGHTS
21
abscissae. The drum is connected to the angular height
drive of the sound locator and is therefore positioned in
angular height of the apparent position. A pointer, which is
set at the estimated altitude of the target, indicates a curve
of sound lag on the cylinder.
d. (1) There are two prediction mechanisms, one for azimuth
and the other for angular height. The azimuth prediction
mechanism consists of a scale and two pointers. The inner
pointer is connected by a clutch to the azimuth drive of the
sound locator. Normally the clutch is disengaged. Pressure
on the operating button engages the clutch and the inner
pointer indicates the angle through which the sound locator
has traversed while the clutch was engaged. The clutch is
engaged by the operator for the period of time indicated by
the sound lag drum. It will be seen that we have performed
the multiplication SaXts which as shown in paragraph 20 is
S x . A knob is provided to zero the inner pointer after each
prediction.
(2) The outer pointer of the azimuth prediction mechanism
is geared to the frame of the azimuth transmitter. The
outer pointer is matched to the inner pointer, thereby rotating
the frame of the azimuth transmitter through the sound lag
angle (S x ). Since rotating the frame is equivalent to an
added rotation to the rotor, the sound lag angle CS X ) has
been effectively added to the azimuth of the apparent position
of the target. The azimuth transmitted to the control sta-
tion will be this sum.
(3) Arbitrary corrections in azimuth are introduced by dis-
placing the scale of the prediction mechanism. Therefore
the correction will not take effect until a new prediction is
made after the correction is applied.
(4) The curves on the sound lag chart are graduated with
values of one-third the actual sound lag time in order to
reduce the time necessary to make a prediction. For an
actual sound lag of 18 seconds, the clutch would be engaged
for 6 seconds. A suitable gearing is used between the predic-
tion mechanism and the frame of the transmitter so that
the full value of the sound lag angle (S x ) is applied.
(5) The prediction is not continuously computed. The
sound lag angle (Sx) computed by a prediction remains con-
31
21-22
COAST ARTILLERY FIELD MANUAL
stant until a new prediction is made and introduced by
matching the pointers.
(6) The prediction mechanism for angular height is identi-
cal with the azimuth prediction mechanism except that the
inner pointer is engaged with the elevation drive by the clutch,
and the matching of the pointers displaces the frame of the
angular height transmitter the amount of the sound lag
angle (ax).
e. The corrected azimuth and elevation scales indicate the
azimuth and the angular height being transmitted to the
control station, that is, the azimuth and angular height
of the true position of the target. The releasing knobs on
the top of the corrector clamp these scales to the rotors
of the data transmitters and provide a method of orienting,
■ 22. Acoustic Corrector M2. — a. The acoustic corrector
M2 is an improvement of the Ml corrector. The necessity
of using a stop watch to measure the sound lag when making
the prediction is eliminated. All operations are automatic
except setting in the estimated altitude of the target and
matching two sets of pointers. (See fig. 20.)
b. The instrument functions differently from the Ml. The
azimuth and angular height of the apparent position of the
target and the lateral and vertical angular rates (2 a and s e )
are transmitted from the sound locator to the acoustic cor-
rector. The elevation drive is connected to the rotor of
the elevation transmitter and, through friction drives, to the
pointer of the vertical rate indicator. The azimuth drive
is connected to the rotor of the azimuth transmitter and,
through friction drives, to the pointer of the lateral rate
indicator. The sound lag time computing cam is translated
along its axis in estimated altitude. The lift of the sound
lag time computing cam is proportional to l/t s . The lift of
the time cam follower moves (in translation) through a
rack and gear differential, a guide carrying two multiplying
cams (lateral and vertical). These cams are positioned (in
rotation) by movement of the lateral and vertical rate
matching knobs. The lifts of the multiplying cam follow-
ers are proportional respectively to the lateral (2 a ) and
vertical (2 e ) angular velocities of the target. The followers,
through racks and pinions, position the outer pointers of
the rate indicators.
32
ANTIAIRCRAFT SEARCHLIGHTS
22
c. What has been accomplished thus far may be explained
mathematically as follows:
_ D (sl ant range) , .
t H (sound lag time) - ^ (velocity of sound) W
^(estimated a l titude) ^
D ~sint' (angular height)
33
22
COAST ARTILLERY FIELD MANUAL
Substituting in (1)
'■=db (3)
Inverting (3)
{Lift of follower on time cam and] _ (rotation of time cam)
| translation of multiplying cams) (translation of time cam)
f angular height correction) r vert ical angularl v ,
a A required by travel of target > = 2„{ 6 Xt, (4)
(during sound lag time J I velocity J
Rearranging (4)
X. = a x Xl/t, (5)
f Lift of follower on vertical] frotation of . (translation)
I multiplying cam, and m-ll.. , muItiol v I v J of vertical I
Idication of index on verti-f ~ \ >' „ L„ P y I I multiplying
leal rate indicator J Ung Cam } (cam J
Note. — Equations (4) and (5) hold for the lateral elements
S x and 2a als - Tne reason for the use of the reciprocal of the
sound lag time l/t s is thus made apparent as it contributes toward
simplicity in design.
d. The actual measurement of the target's angular rates
(2a or Se) is accomplished automatically. The timing
mechanism consists of a small constant-speed (governor-
controlled) A. C. motor which drives a tripping cam at 6
revolutions per minute or 1 revolution in 10 seconds. The
elevating and traversing shafts from the sound locator are
connected to rate indicator pointers through friction
clutches. The tripping cam, through slides and levers, ac-
tuates pawls which alternately engage, for a period of 4
seconds, and disengage, for a period of 6 seconds, the ratchet
wheels which drive the rate indicator pointers (inner point-
ers on the rate indicator) . When the pawls are disengaged,
the tachometer pointers are engaged with their respective
elevating and traversing shafts and measure their angular
movements for 6 seconds. When the pawls are engaged,
the tachometer pointers remain stationary for 4 seconds, to
allow time for the operator to match the' indexes with them,
and then return to zero, after which the cycle of operation
34
ANTIAIRCRAFT SEARCHLIGHTS
22
is repeated. The chronometric cam is so designed that there
is a 5-second interval between the beginning of the cycles
of the lateral and vertical rate indicators, which permits
one operator to match alternately the two rate pointers.
The linear scale factors and gear ratios are such that the
operation of the mechanisms may be described as the peri-
odic multiplication (at 10-second intervals) of the average
angular velocity in mils per second ( y 6 of the angular move-
ment in 6 seconds) by the sound lag time in seconds.
e. The electrical transmitters which transmit azimuth and
angular height to the searchlight control station are built
into the instrument and function ,in the same manner as
those in the acoustic corrector Ml. The rotors are con-
nected to the azimuth and elevation shafts from the sound
locator. The stators are rotated through the sound lag
correction angles (& and ax) by the rate matching knobs.
Provision is made for arbitrary vertical and lateral cor-
rections.
/. The speed of the constant-speed motor may be checked
by counting the revolutions of the tripping cam over a 1-
to 5-minute interval using a stop watch. The speed may be
adjusted through an adjusting screw just below the top
cover plate.
g. Stops are provided which limit the motion of the op-
erating handwheels. The acoustic corrector M2 operates
within the following limits:
Slant range (D) 1,100 to 11,000 yards.
Altitude (H) 3,300 to 30,000 feet.
Angular velocity (2 a or 2 e ) 83 mils per second.
Sound lag correction (s x and ax) _ ±250 mils.
Arbitrary correction ±200 mils.
Prediction period 6 seconds.
Prediction interval 10 seconds.
35
CHAPTER 4
SEARCHLIGHTS
■ 23. General. — This chapter is devoted to searchlight phe-
nomena. Information pertaining to searchlights will be
found in TM 4-210 and the operator's manual issued with
the searchlight. Information on setting-up, orientation,
operation, and care will be found in FM 4-115.
■ 24. Contrast. — a. General. — (1) The range of a search-
light is the maximum distance at which an object in the
beam is visible. The visibility of the object does not depend
on its actual illumination but upon the contrast between
its illumination and the illumination of the surrounding field.
(2) The contrast between an object and the surrounding
field which is necessary for visibility depends upon the ap-
parent size of the object, that is, upon the angle which it
subtends at the eye of the observer and the area presented
toward him. When the object is close, a contrast of a few
percent is sufficient for visibility. When the object is far
away and subtends a small angle, a contrast of several hun-
dred percent is necessary to make the object visible.
(3) A specific example will illustrate the above fact.
Consider the sun as a searchlight which is illuminating the
moon and the planet Venus as well as the surrounding fields.
During daylight hours the contrast between the moon and
the surrounding field is about 2 to 1. The moon subtends
an angle of about 25 minutes. Venus, due to its greater
coefficient of reflection, has an actual contrast of about 12
to 1. Venus subtends an angle of about 1 minute. In
spite of the great difference in contrast, the moon can be
seen in daylight while Venus cannot. The apparent size
of the two objects, the moon and Venus, is the determining
factor in this case as to whether or not the object is visible.
(4) The area presented toward the observer affects the
problem of contrast in the case of airplane targets. As can
be seen from figure 21, an airplane presents less area toward
36
ANTIAIRCRAFT SEARCHLIGHTS
24-25
the observer as the angle of elevation decreases. The maxi-
mum area is presented at about 55°. Figure 21 is for a
biplane. For a monoplane the curve becomes a straight
line with a minimum value for 0° and increases to the
maximum value at the zenith. The amount of light re-
flected to the observer from both the target and the field
surrounding the target determines the contrast. The less
area the target presents, the less light is reflected and the
lower the contrast. A camouflaged plane may have a sim-
ilar effect, for the contrast is reduced if the amount of light
reflected from the plane is reduced.
i
i
1 11*1 1
v i i m i i
90° W 70° 60° 50° 40° 30° 20° 10° 0°
ZENITH HORIZON
Figure 21. — Angle of aspect.
b. Improving contrast. — The most obvious way of improv-
ing the contrast is to move the observer away from the search-
light beam and thus decrease the depth of illuminated at-
mospheric particles through which he must view the target.
This has the effect of decreasing the luminosity of the field
without appreciably decreasing the amount of light reflected
from the target. Figure 22 illustrates the value of moving
the observer away from the beam.
■ 25. Decrease in Illumination. — a. As the target increases
its range, the illumination falling on it decreases by reason
of two factors, atmospheric absorption and spreading of the
beam. Loss of light due to atmospheric absorption varies
37
25
COAST ARTILLERY FIELD MANUAL
\
with the weather — from 10 percent per mile in clear weather
to complete absorption in fog. Loss because of spreading of
the beam is the same for all conditions
tand follows the law of inverse squares,
that is, for each doubling of the distance
the illumination is reduced to one-quar-
ter of its former value. Spreading of the
beam is caused by the unavoidable fact
that the source of light is an area and
not a point and that the lines of light of
the beam must be slightly divergent.
(See fig. 23.)
b. The luminosity of the beam against
which the target is seen remains about
i \ the same irrespective of target range.
\ v This is because at all ranges the observer
I \ is looking through the same total depth
i \ of illuminated atmospheric particles in
\ \ front of and behind the target. There-
i \ fore, as the range to the target increases,
i \ we have the situation of less contrast be-
i \ tween the target and the field, and a
\ \ need for more contrast because of the
smaller apparent size of the target.
A. At light. B. 15-foot displacement. C. 50-foot displacement.
Figure 22. — Effect of location of observer on contrast.
These two factors impose sharp limitations on the range even
though extremely powerful beams are used. (See fig. 24.)
38
ANTIAIRCRAFT SEARCHLIGHTS
26
■ 26. Beam Candlepower.— a. The searchlight mirror in effect
magnifies the light source to the diameter of the mirror
Therefore, if it were not for the absorption of the light bv
he mirror front door, and shadow-making obstructions in
the searchlight, the beam candlepower would be equal to the
intrinsic brilliancy (candlepower per sq. mm.) of the light
source multiplied by the area of the mirror in square milli-
meters. Because of these absorptions and obstructions the
actual beam candlepower is about 60 percent of this product.
, 60"
60"
Figure 23. — Spreading of beam
b. The intrinsic brilliancy of the light source is not uniform
but is brightest at its center. likewise the beam also has a
central area of higher candlepower than the average over its
entire area. It has been found in practice that the range of
the searchlight is dependent upon the brighest central por-
tion. The value given for beam candlepower is computed by
multiplying the intrinsic brilliancy of the central area of the
light source by the area of the mirror and deducting 40 per-
cent for losses. If the mirror and the front door are perfectly
clean, the losses are considerably less than the 40 percent
which has been taken to cover average conditions in the field
RELATIVE
INTENiin'
m mm
39
27
COAST ARTILLERY FIELD MANUAL
■ 27. High Intensity Arc. — a, In order to obtain a beam
candlepower of 800,000,000 with an ordinary carbon arc, it
would be necessary to use a current of 350 amperes and a
mirror diameter of 125 inches. This is because the ordinary
Figure 24. — Falling off of illumination due to range.
carbon arc has an intrinsic brilliancy of 160 candlepower per
square millimeter.
b. We know from observation of the sun that an incandes-
cent gas is capable of much higher intrinsic brilliancy than
160 candlepower per square millimeter. The reason for this
40
ANTIAIRCRAFT SEARCHLIGHTS
27-28
is that the gas is translucent and light is derived from the
interior as well as the surface.
c. The incandescent ball of gas which forms the source of
light in the high intensity arc is derived from the rare earths,
cerium and lanthanum. By incorporating these earths in
the relatively soft core of the positive carbon in precisely the
right form and quantity and by forcing a high current density
through the carbon, the earths are volatilized and projected
into the crater.
d. To be useful as an intense source of light in the focus
of the mirror, this brilliant gas must be confined to a very
small volume. This is accomplished by compounding the
materials of the core of the positive carbon so that they
volatilize a little more readily than the hard carbon shell,
and thus form a crater in the end of the positive carbon. To
keep the crater uniform the positive carbon is rotated slowly.
To confine and compress the light-giving gas in the positive
crater, the negative carbon is so arranged that the electron
stream sweeps across and impinges on the gas.
e. The high intensity arc has an intrinsic brilliancy of 750
candlepower per square millimeter. With this brilliancy, the
beam candlepower of the searchlight is 800,000,000 (assum-
ing a loss of 40 percent by absorption) when used with a
60-inch mirror. The total current required is 150 amperes.
■ 28. Effect of Ottt-of-Focus Condition. — a. The most im-
portant cause of loss of beam intensity is out-of -focus condi-
tion of the light source. The curves in figure 25 show the
great loss in beam intensity caused by small errors in placing
the light source at the focus of the mirror. Curve A shows
the distribution of light in the beam when the light source is
at the focus. Under these circumstances the center portion
of the beam has a value of 800,000,000 beam candlepower
and the spread is slightly over 1°. An out-of -focus condition
of only Vb inch brings about the distribution of illumination
shown in curve B. The maximum beam candlepower is
reduced about 40 percent and the range at least 20 percent.
An out-of -focus condition of % inch brings about the dis-
tribution of illumination shown in curve C, which is ap-
proximately equivalent to the illumination of a 36-inch
searchlight.
41
28-29
COAST ARTILLERY FIELD MANUAL
8*I0 8
S7"I0 8
O6x|0 8
Qu
uj5x|0 8
U3x|0 8
J ?x|0 8
(O I0 8
V-
:±
-i
1
1°
'/2
Figure 25. — Effect of errors in focusing on beam intensity.
b. From the above discussion it is evident that special at-
tention must be given to keeping the light source within g'z
inch of the focus of the mirror. Such precision is better
accomplished by automatic means incorporated in the search-
light than by hand control.
■ 29. Other Losses op Beam Candlepower. — Two other im-
portant sources of loss of beam candlepower under field con-
ditions are sputtering or unsteadiness of the arc and deposits
on the mirror and front door. Unsteadiness of the arc, which
is caused almost entirely by excessive current density, is
avoided by not allowing the current through the arc to exceed
150 amperes. The deposits on the mirror and front door are
given off by the burning of the carbons. A ventilating sys-
tem must continuously sweep the mirror and front door
with fresh air but must not create a draft which will interfere
with the arc.
42
CHAPTER 5
SEARCHLIGHT CONTROL
■ 30. General. — In paragraph 24 it was shown that the con-
trast and consequently the visibility of the target are im-
proved by moving the observer away from the searchlight
beam. Two methods are provided for controlling the search-
light.
a. The extended hand controller which places the ob-
server about 15 feet from the searchlight beam.
6. The distant electric control in which the observer is
50 feet or more from the searchlight beam.
■ 31. Extended Hand Control. — This method is intended as
an emergency method of controlling the searchlight in case
of failure of the distant electric control. It consists of a
long rod inserted in a receptacle on the side of the searchlight.
By rotating the wheel at the end of the rod the searchlight
is elevated or depressed. By pushing on the end of the rod
in a horizontal direction the searchlight is traversed. As can
be seen from figure 22, some advantage is gained in visibility
by the 15-foot displacement of the observer.
■ 32. Distant Electric Control (D. E. C.) — a. The purpose
of the distant electric control is to provide a means of
elevating and traversing the searchlight from the control
station.
b. The distant electric control consists of a pair of trans-
mitters at the control station and a pair of motors mounted
on the searchlight. One motor drives the searchlight in
azimuth and the other in elevation, each motor being con-
trolled respectively by the transmitter at the control station.
The motors, transmitters, and method of operation of both
elevation and azimuth distant electric control are identical.
c. In the later types of searchlights, the D. E. C. motors are
operated directly by the transmitters at the control station.
43
32-33
COAST ARTILLERY FIELD MANUAL
Another type of D. E. C. motor may be encountered in the
service known as the brush shifting type. (See pars. 33 and
34.)
■ 33. Theory of Distant Electric Control — Step-by-Step
Transmitter (Direct) . — a. The transmitters may be described
as rotary switches which, by changing the current flow in
the field coils of the motors, accomplish a rotation of the
magnetic fields in a step-by-step process. The rotor being
of a constant polarity follows the rotating magnetic field in
the field coils and thereby imparts a motion to the search-
light.
b. The transmitter is shown schematically in figure 26.
It consists of a commutator with four segments, each sub-
tending approximately 75° of arc with insulation between
the segments. The polarity of the segments is alternately
plus and minus as shown in the figure. Three contact rollers
set 120° apart are kept in contact with the segments by spring
pressure. The rollers are rotated inside the commutator by
the observer's and zero reader (comparator) operator's hand-
wheels at the control station. Each roller is connected elec-
trically with certain stator windings of the D. E. C. motor
at the searchlight. It is seen that the current flowing
through each roller contact will be plus, zero, or minus
depending on the position of the roller on the commutator.
c. The D. E. C. motor stator has a distributed winding of 12
coils. The wiring of the stator is so arranged that the coils
form three groups of four coils (or poles) in each group.
(See fig. 26.) The rotor of the motor is polarized, having
four coils connected in series so that the adjacent poles
have unlike polarity. This causes the poles of the rotor
to have a strong constant polarity and gives the required
torque necessary to control the searchlight at all speeds. The
motor operates on direct current.
d. The manner of functioning is illustrated in the
schematic diagram, figure 26. In this figure the transmitter
and motor are shown in their relative positions as the trans-
mitter is turned through 30°. It will be seen that every 15°
change in the transmitter results in some change of the
current flow through the stator of the motor resulting in
44
ANTIAIRCRAFT SEARCHLIGHTS
33
Figure 26. — Schematic diagram, distant electric control step-by-step
transmitter (direct).
a shift of the magnetic field by 15°. The rotor follows the
shift of the magnetic field and is, therefore, turned through
15°. Each change of 15° by the transmitter is called a step,
45
33-34
COAST ARTILLERY FIELD MANUAL
hence the name "step-by-step" because the rotor is moved
by steps. A complete revolution of the transmitter takes 24
steps; likewise the rotor moves a complete revolution in 24
steps. . The motor is geared to the searchlight so as to move
the searchlight 6 minutes of arc per step or 2° 24' per complete
revolution of the motor.
■ 34. Theory of Distant Electric Control — Brush Shift-
ing Type. — a. This system employs a step-by-step transmitter
which moves the rotor of the brush shifting motor in exactly
the same manner as described in paragraph 33. However,
Az/muff)
Tra/n/ng
Motor
fser/es
«1> / UL.
/OOv.DC
Figure 27. — Principle of brush shifting type, distant electric
control.
the motor does not have sufficient torque to move the search-
light.
b. The brush shifting motors are operated on 35 volts
D. C. As the name implies, the motor shifts the position
of the brushes of the training motor in the same direction
as the rotation of the contact rollers of the transmitter.
The actual rotation is accomplished through a differential.
c. The training motors are 100-volt direct-current series
wound motors. The brushes are normally 90 electrical de-
grees from the brush position of the ordinary motor. In this
position there is no torque. The brushes are mounted in
the housing of a differential. Hence, when the brush shift-
ing motor operates under the control of the transmitter, the
housing of the differential and the training motor brushes
will be rotated from the no torque position to a torque
position. (See fig. 27.) The training motor, having torque,
46
ANTIAIRCRAFT SEARCHLIGHTS 34
commences to rotate. Its rotation has two effects: first, it
turns the searchlight, and second, it turns the other input
shaft (/) of the differential. The differential functions as
a canceling differential and the rotation of the input shaft
(/) backs off the amount of rotation which was initially
applied by the brush shifting motor through the input shaft
(ff). When the amount of rotation of shaft (/) equals the
amount of rotation of shaft (g), the output of the differ-
ential (which is the rotation of the housing (d) ) is zero.
The brushes which are carried by the housing have been
returned to the no torque position and the motor ceases to
rotate. See EM 4-110 for further information on differential
action. It is seen that the training motor has rotated the
same amount as the brush shifting motor which in turn
rotates the same amount as the step-by-step transmitter.
47
CHAPTER 6
CONTROL STATION
■ 35. General. — The control station establishes the connect-
ing link between the searchlight and the sound locator. It
usually contains the following items:
a. The distant electric control (see ch. 5) which is used to
traverse and elevate the searchlight in azimuth and angular
height.
b. Either a comparator (pars. 39 and 40) or a zero reader
(par. 36), which are devices employed to insure that the
searchlight is pointed at the azimuth and angular height
transmitted from the sound locator.
c. The later models (1934-1940) are provided with a mount
which holds the binocular night glass. These glasses aid in
picking up and tracking the target.
■ 36. Zero Reader. — This device indicates directional synchro-
nization between the sound locator and the searchlight, that is,
it indicates when the searchlight is directed at the true posi-
tion of the target as determined by the sound locator and
acoustic corrector. This is accomplished by electrically de-
tecting angular displacement of the searchlight from the
sound locator by means of a phase detector. Synchroniza-
tion is indicated by the centering of the needles of the azi-
muth and elevation zero reader meters.
■ 37. Theory op Operation of Zero Reader. — a. Figure 28
shows an A. C. self-synchronous transmitter connected to a
self -synchronous receiver in the usual manner (see FM 4-110)
except that the receiver rotor is not connected to the A. C.
power supply.
b. The transmitter rotor sets up an A. C. magnetic field
along the axis A-B. By transformer action, voltages are set
up between leads 1, 2, and 3 of the stator which, when con-
nected to the receiver stator, produce an A. C. magnetic field
along the axis A'-B'. When the transmitter rotor is rotated,
48
ANTIAIRCRAFT SEARCHLIGHTS
37
as is the case when transmitting angular data, the axis of
the field A'-B' in the receiver exactly follows the movement
of the transmitter rotor.
c. With the conditions as shown in figure 28, the field A'-B'
of the receiver is at right angles to the rotor winding and
hence no voltage is induced in the rotor. If the transmitter
rotor is set at a new position, the field A-B (and consequently
the field A'-B') will shift and the rotor of the receiver will
no longer be in the neutral (no voltage) plane. A voltage
will be induced in the receiver rotor which will be of such
Transmitter Receiver
(at Sound Locator) (at Searchlight)
60 cycle
A.C. Supply
Figure 28. — Principle of zero reader.
phase as to tend to move the rotor to the neutral plane.
Hence it will be seen that the phase of the voltage induced
in the receiver rotor will depend upon whether the search-
light leads or lags behind the sound locator. The voltage out-
put of the receiver rotor varies as the sine of the angular
displacement of the rotor with respect to the field A'-B'.
d. If the transmitter is geared to the sound locator and
the receiver is geared to the searchlight and adjusted so
that there is zero voltage when the sound locator and search-
light are in synchronism, then so long as the voltage output
of the receiver rotor (at the searchlight) is zero, synchronism
is maintained. When the two are not synchronized, the
49
37-38
COAST ARTILLERY FIELD MANUAL
receiver rotor will have a voltage output which varies as
the sine of the angular displacement of the transmitter, and
the phase of the voltage reverses as the searchlight lags
behind or leads the sound locator.
e. The reversible -phase voltage output of the receiver rotor
is fed into a simple phase detecting circuit, so that the lag or
lead of the searchlight may be indicated by the right or left
deflections of an ordinary zero center D. C. voltmeter. The
throw of the zero reader needle will be approximately pro-
portional to the angular displacement between locator and
searchlight.
/. Zero readers are furnished for angular height and for
azimuth. Both units are the same. The operators of the
zero readers keep the searchlight synchronized by using the
D. E. C. handwheels at the control station. The handwheel
rotation and deflection of the zero reader are correlated in
such a way that right deflection of the zero reader requires
counterclockwise rotation of the handwheel to center it and
vice versa. The operation is similar to that of steering a car,
and in practice the motion becomes instinctive.
gr. It should be noted that the above mechanism will indi-
cate zero readings when the searchlight is exactly 180° out
of synchronism in azimuth. This is detected by noting that
when the azimuth distant electric control handwheel is
moved clockwise, the needle moves off center to the left
instead of the right as it should. Synchronism is restored
by turning the searchlight by means of the azimuth D. E. C.
through 180°, at which time the zero reader will again be
centered.
■ 38. Phase Detecting Circuit. — o. The phase detecting
circuits for both azimuth and angular height are identical.
It is shown schematically in figure 29. It consists of two
small center-tapped resistors Rl and R2 of 500 ohms each.
Kl and K2 are similar 6-disk copper oxide rectifiers. A
small transformer supplies 10 volts A. C. polarizing voltage.
b. As the transformer and the transmitter at the sound
locator use the same source of power, the rotary converter,
the voltages will be in phase. Assuming that there is no
voltage output from the receiver rotor, Kl and K2 rectify the
50
ANTIAIRCRAFT SEARCHLIGHTS
38-39
polarizing voltage equally, and as the zero reader voltmeter
is connected across the points A-B of the same potential
there is zero deflection. When the searchlight lags behind
the sound locator, the voltage output of the receiver rotor
adds in phase with the polarizing voltage across Kl and
opposes the polarizing voltage across K2. Kl will pass more
rectified current than Kl, A is positive with respect to B, and
the meter will deflect, for example, to the right. When the
searchlight leads the sound locator, the voltage output of
4 |ki A
60 cycle
A C. Supply
Figure 29. — Phase detecting circuit.
the receiver rotor opposes the polarizing voltage across Kl
and adds in phase with the polarizing voltage across K2. B
is then positive with respect to A and the meter will deflect
in the opposite direction — to the left.
c. The phase detecting circuits are located on the search-
light. Meters to read the lag or lead of the searchlight are
located both at the searchlight and the control station.
■ 39. Comparator. — a. Another type of synchronizing device,
known as the comparator, will be found in the service. The
comparator uses the A. C. self-synchronous data transmis-
sion system. (See FNL 4-110.)
51
39-40
COAST ARTILLERY FIELD MANUAL
b. Inside the comparator case will be found four self-
synchronous data receivers. The rotors drive two sets of
concentric dials, one for azimuth and the other for angular
height. The outer dials in each set indicate 'the azimuth
and angular height as transmitted from the sound locator.
The inner dials in each set indicate the azimuth and angular
height of the searchlight. Azimuth and angular height
data transmitters are located in both the sound locator and
searchlight and automatically transmit the data to the com-
parator. The operators at the control station traverse and
elevate the searchlight by means of the D. E. C. until the
pointers on the dials are matched. At this time the sound
locator and searchlight are synchronized.
■ 40. Comparator, Mechanical (A. C). — A variation of the
comparator described in paragraph 39 is also in the service.
The searchlight dials of the comparator are driven by
direct gearing from the D. K. C. handwheels. Hence the
dials indicate the angular height and azimuth of the search-
light without using the data transmission system between
the searchlight and comparator. The sound locator dials
are positioned by data transmitted electrically from the
sound locator to the comparator as described in paragraph
39.
52
CHAPTER 7
BINAURAL TRAINING INSTRUMENTS
■ 41. General. — Binaural training instruments are necessary
for the following reasons:
a. Making the initial selection of listeners for further
training.
b. Developing and improving as a result of practice the
accuracy of the binaural sense.
c. Providing a means of training available at all times and
not dependent upon such factors as weather conditions, suit-
able targets, and availability of the other members of the
sound locator section. The use of binaural training instru-
ments in no way obviates the necessity of actual practice on
the sound locators with actual targets. Frequently, a train-
ing instrument will be the only available means of practice
for a listener.
■ 42. Binaural Trainer Ml. — a. The binaural trainer Ml
is still in service although no new units are to be manufac-
tured. The entire instrument has been incorporated in the
binaural trainer M2.
b. The binaural trainer Ml consists of an electric turntable
and magnetic pick-up to play phonograph records of aircraft
in flight, a receiver unit which serves as a source of sound, a
phase control unit which varies the length of the sound paths
to the ears, and a helmet for the use of the listener. The
complete unit is shown in figure 30. The unit is designed to
reproduce the conditions experienced in the operation of
sound locators insofar as binaural sense is concerned. A
numerical scale is provided which reads the error from exact
binaural balance.
c. The interna] construction is illustrated in figure 31. The
sound (transmitted electrically from the phonograph) is
reproduced in a receiver unit where it passes into the phase
53
42
COAST ARTILLERY FIELD MANUAL
ANTIAIRCRAFT SEARCHLIGHTS
42-43
control unit. The phase control unit is connected to the
two handwheels at opposite ends of the instrument so that
rotation of either handwheel will lengthen the acoustic tract
to one ear and shorten it to the other ear. This results in
a difference in distance from the sound source to each ear,
and as was shown in paragraph 6 results in a difference in
phase which the ears are able to distinguish. By varying the
relative lengths of the acoustic tracts, the listener gets the
same effect as turning his head with respect to the sound
source. The phase control unit is unbalanced by turning the
small handwheel. The listener then tries to center on the
sound by turning the large handwheel. In figure 31, the
phase control unit is shown balanced. For further informa-
tion on the binaural trainer Ml see the notes on materiel,
Sound Locators Ml Series, issued with the instrument. For
information on the selection and training of listeners see FM
4-115.
■ 43. Binaural Trainer M2. — a. This instrument is designed
for training of sound locator listeners under conditions which
very closely simulate those of actual aircraft tracking.
Changes in sound intensity, azimuth, angular height, and
contrast between aircraft sound and ambient noises can be
made to approach actual listening conditions. The listeners
work on the sound locator when using this training instru-
ment, or use the portion of the unit similar to the binaural
trainer Ml. (See par. 42.)
b. The out-of-doors portion of the instrument utilizes an
overhead wire along which a horn assembly is moved. The
horn assembly is actuated by an amplifier and reproduces a
phonograph record of an airplane in flight. By manipula-
tion of the volume control of the amplifier combined with
a variation in rate of movement of the horn assembly along
the overhead wire, the target can be made to approach, to
recede, and to execute other simple types of maneuvers.
■ Figure 32 shows the mechanism set up in position ready to
operate. As excess humidity will damage the horn assembly,
the device should not be operated in the rain or left out of
doors at night. The phonograph and amplifier operate on
55
43
COAST ARTILLERY FIELD MANUAL
^ IIOV. 60cycle
A C. Supply
Figure 32. — Schematic diagram, binaural trainer M2.
a 110- volt 60-cycle A. C. For further information see the
instruction manual issued with the instrument.
c. The indoor portion of the instrument consists of the
same devices used in the binaural trainer Ml. The electric
turntable and magnetic pick-up are used for both the indoor
and out-of-door portion of the instrument.
56
INDEX
Acoustic corrector: Paragraph Page
For M2 sound locator 14 13
Ml 19, 20, 21 29
M2 19,20,22 29,32
Ambient noises 7 5
Angle of aspect 24 36
Arc, high intensity 27 40
Ball analyser 16 17
Beam candlepower:
General 26 39
Losses 28, 29 41, 42
Binaural sense 6 3
Binaural trainer:
Ml 42 53
M2 43 55
Brush shifting motor 34 46
Cam:
Multiplying 82 34
Sound lag time computing 22 32
Candlepower :
Beam 26 39
Losses 28,29 41,42
Comparator 39 51
Comparator, mechanical (A. C.) 40 52
Conical horn, M2 sound locator 7 5
Contrast 24 36
Control :
Distant electric 32 43
Extended hand 31 43
Problem 4 2
Searchlight 30 43
Station 35 48
Correction :
Parallax 18 25
Wind 17 24
Decrease in illumination 25 37
Delta sub x (g x ) 10 9
Design, sound locator 7 5
Distant electric control:
Brush shifting, theory 34 46
Direct, theory 33 44
General 32 43
Effect of focus on beam intensity 28 41
Error :
Pbrallax 13 12
Wind 12 11
Exponential horn 7 5
Extended hand control 31 43
57
INDEX
Paragraph Page
Focus of beam 28 41
High intensity arc 27 40
Horn:
Design 7 5
Exponential 7 5
Illumination :
General 25 37
Problem 2 1
Indicator, miniature airplane course 16 17
Instrument :
Binaural trainer Ml 42 53
Binaural trainer M2 43 55
Intrinsic brilliancy of beam 26 39
Location problem 3 1
Losses of beam candlepower 28, 29 41, 42
Miniature airplane course indicator 16 17
Multiplying cam 22 32
Noises, ambient 7 5
Pantograph 14 13
Parallax :
Correction 18 25
Error 13 12
Phase detecting circuit 38 50
Pick-up slant range 18 25
Prediction mechanism for Ml acoustic corrector 21 29
Searching 5 2
Sight mechanism 15 15
Sigma sub 1 (o x ) 10 9
Slant range to point of pick-up 18 25
Sound lag :
Correction:
Ml acoustic corrector 10, 20, 21 9, 29, 29
M2 acoustic corrector 10, 20, 22 9, 29, 32
M2 sound locator 10, 14, 15 9, 13, 15
Drum 21 29
General 10, 11 9, 10
Time computing cam 22 32
Sound location, theory 6 3
Sound locator:
Design 7 5
Ml series 9 7
M2 8 7
Speed of sound 10 9
Spread of beam 25 37
Step-by-step receiver:
Brush shifting 34 46
Direct 33 44
Step-by-step transmitter 33 44
58
INDEX
Paragraph Page
Theory :
Distant electric control:
Brush shifting 34 46
Direct 33 44
Phase detecting circuit 38 50
Zero reader 36, 37 48,48
Training motor 34 46
Wind:
Correction 17 24
Error 12 11
Zero reader 36-38 48-50
o
59