THE SIGNAL STATION.
LOCATION OF STATIONS.
In field operations tactical considerations will usually prescribe
within certain limits the number and general location of signal
stations. The general directions for deployment being given, the
signalman will be called upon to demonstrate his skill in the selection
of particular locations most conducive to the efficient service of
information.
[Illustration: FIG. 17.--Field signal station.]
_General considerations._--Considering all things, the best location
for a signal station is one which affords maximum visibility and at the
same time minimum exposure to hostile observation. These conditions,
apparently paradoxical, can be more or less reconciled by the exercise
of ingenuity on the part of the signalist. A good theoretical knowledge
of the special requisites of signal sites, together with the ability
to apply it to the conditions arising in any given case, will result
in securing the best obtainable locations.
The first essential of the signal station is visibility, the second
being that of concealment from hostile observation. In acquiring a mean
between conflicting requirements, the following special considerations
in the selection of stations should be considered.
_Backgrounds._--Backgrounds are important factors in the selection of
signaling sites.
Sky backgrounds are desirable as affording strong contrast and are
therefore conducive to celerity in the transmission of signals. They
are rare and can only be secured when stations are located on the exact
crest of ridges, on mountain peaks, or on lands which bound the horizon
of view from the other stations. Stations with sky backgrounds, while
affording the best facilities for transmission, are little adapted to
the requirement of secrecy.
Dark backgrounds are far more common and more easily obtainable than
sky exposures. They afford the maximum means of concealment from
hostile observation, but materially reduce the range, speed, and
accuracy of signal transmission.
Mixed or broken backgrounds are those which display varied colors
behind the signals. Backgrounds of this description do not accord with
either of the essential requirements of the signal station and should
be avoided whenever possible.
In general, sky backgrounds should always be selected for signal
stations when conditions are such that the requirement of secrecy can
be dispensed with; if, on the other hand, there is reason to fear that
the signals may be intercepted by the enemy, dark backgrounds should
invariably be chosen, even though the disadvantages they impose, render
them less desirable visually.
_Azimuth of stations._--The azimuth of signal stations should, if
possible, be such that the visual lines of information should intersect
the vertical plane through the apparent course of the sun, at a
considerable angle. Stations located so as to be unavoidably viewed
from these directions during portions of the day are very liable to
appear enveloped in a haze, and telescopes, if turned upon them, are
filled with dazzling light. If the location of stations on or close to
the sun line is unavoidable, sites affording sky exposures should be
chosen. Exposures of this kind obviate to a great extent the difficulty
of sun haze and should be secured when this difficulty is encountered
and it is impracticable to change the azimuth of the station.
_Altitude._--The location of signal stations at high altitudes will
tend to obviate difficulties arising from smoke, haze, and dust. The
undulation of the atmosphere noticeable on a hot summer's day is
always less at a distance from the earth's surface, and it is often
practicable to read signals from a tree or housetop when they would be
unintelligible from the ground. This air undulation is less over spots
well shaded than those exposed to the glare of the sun, a fact that
should be borne in mind in all telescopic examinations. Another reason
for locating stations at high altitudes is because the cool night air,
the smoke and dust of the day, and heavy mists lie close to the ground,
filling the depressions and lowlands, while the higher points remain
in view. Stations on high ground are then equally well adapted to day
and night signaling. Sites and selections of this kind of terrain will
not only often preclude the necessity for changes of location, but also
will allow the continuous working of the station when signals made from
lower positions would be invisible. In foggy or murky weather peaks and
mountain tops are usually enveloped in mist, and under these conditions
stations should be situated on lower ground.
_Determination of background color._--The color of the background of a
station is that color against which the signals appear to be displayed
when viewed from the distant station. Having chosen a point entirely
in view of the station or stations to be communicated with, and having
fixed the exact position of the signaling apparatus, the color of the
background should be determined as carefully as conditions of terrain
will permit. If the elevation of the distant station is without doubt
greater than that of the home station it is safe to assume that the
color of the background will be that of the objects directly around and
behind it. On the other hand, if the distant station unquestionably
occupies the lower position, a sky exposure will usually result. In
locating stations it is very difficult, if not impossible, especially
at long ranges, to determine the color of the background as viewed from
the distant station when the stations are approximately on the same
level. This can only be done by proceeding in front of the home station
and taking such a position that it can be viewed with the eye on the
line of sight between the stations. The telescope should be established
over the initial point of the home stations and directed on the distant
station. The observer for background should proceed to a point where
his head is in the center of the field of the telescope. Looking back
at the home station from this point, the color of the objects about and
just behind the initial point will be the color of the background. The
correct determination of background color from the vicinity of home
stations is usually difficult and unsatisfactory, and it is considered
the best method to establish communication with the distant station by
simultaneously using several kinds of signaling apparatus, that kind
producing the most intelligible signals being retained for continued
use.
_Choice of apparatus._--Sunlight conditions permitting, the heliograph
will ordinarily be used for day signaling on account of the advantages
of the great range and speed afforded by it. When its use is prohibited
by weather conditions, the flag will be substituted for it. The white
flag will be used against dark and the red against sky or broken
backgrounds. The distant station is the better judge as to which color
flag is best suited to given conditions and the color indicated by it
should invariably be used. For night signaling, the acetylene lantern
is usually employed. Long-range night signaling should be done with
the searchlight if available. The employment of the semaphore, in
daytime, and the Ardois system, at night, will be confined to more or
less permanent stations. Rockets, shells, night fires, etc., are only
employed for special or emergency signals.
_Miscellaneous considerations._--For various reasons stations should
not be located at or near camp grounds. These localities usually
afford mixed backgrounds, and the presence of dust and smoke and
the interference caused by moving bodies of troops and trains will
militate against the efficient transmission of signals. Stations
located in vicinities of this kind are also subject to annoyance from
noise and visits of unauthorized persons. Signal stations should be
convenient for messenger service and hence as near commonly traveled
roads as the physical contour of the country will permit. Locations
for signal stations should be so selected that the visual lines do not
cross traveled roads, camps, etc., as dust and smoke in the daytime and
lights at night are factors in determining the visibility of signals.
Signal stations can if necessary be artificially concealed by erecting
screens constructed of limbs of trees, etc., about the flanks and rear.
Sheltered positions should be utilized in windy weather.
_Intervisibility table._--The following table shows the extent of
horizon for different heights above the sea level--that is, it shows
how far one can see an object which is itself at the level of the sea:
----------------------------+-----------
| Distance
Height of the eye above sea |in statute
level. | miles.
----------------------------+-----------
10 feet | 4
15 feet | 5
20 feet | 6
30 feet | 7
40 feet | 8
50 feet | 9
60 feet | 10
70 feet | 11
85 feet | 12
100 feet | 13
115 feet | 14
130 feet | 15
150 feet | 16
200 feet | 18
230 feet | 20
300 feet | 23
350 feet | 25
500 feet | 30
700 feet | 35
900 feet | 40
----------------------------+-----------
A formula to determine approximately the limits of visibility from
a given height is as follows: The square root of the height of the
station in feet multiplied by 1.26 equals the distance in miles at
which the signal is visible.
Hence, an observer whose eye is 30 feet above the sea can distinguish
an object 7 miles distant, provided it is at the sea level; but if the
object is itself 15 feet above the sea he can make it out 7 + 5 = 12
miles off.
FINDING A STATION.
To find a signalman near any known station, note with the unaided eye
some prominent landmark near which the looked-for person or object is
supposed to be, and direct the telescope upon the place, as sight is
taken over a gun barrel, covering the object; if the eye is now placed
at the eyeglass of the telescope, the prominent or directing landmark
will be found in the field of view. It will be easy then to scale the
country near the marker until the signalman is found. This method is
often necessary at night, when only a point of light is seen far off
through the darkness, and the telescope must be turned upon it. When
the compass bearing of the object sought for is known, the telescope
may be aligned by a line drawn with the proper compass bearing.
Commencing then with the view at the horizon, the telescope is slowly
moved from side to side, taking in fresh fields of view each time a
little nearer to the observer, until the whole country shall have been
observed from the horizon to quite near the station. When the general
direction only of the object can be given and it is sought for, the
whole landscape in that direction to the horizon should be divided into
sections by imaginary lines, the limits of these sections being bounded
between visible landmarks through which the bounding lines are supposed
to pass. Each section should be scrutinized little by little until the
glass has been passed over every spot. Such search will seldom fail to
be successful.
The magnetic bearings of all stations with which another station has
worked should be carefully noted and made matter of record in the
office directly concerned, so that advantageous use may be made of this
data. In addition, guide lines may be established by driving two stakes
firmly into the ground and close to each other. A prolongation of a
line through the center of one post and marked on the adjacent one will
strike the distant station. Under each line should be written the name
of the station which it marks.
Signalers upon permanent or semipermanent stations will examine, from
time to time, every prominent point within signal distance, to see if
communication is attempted therefrom.
Attempts to attract the attention of a known station, in order to
be successful, must be persistent. They should never be abandoned
until every device has been exhausted, and they should be renewed
and continued at different hours of the day and night. It must be
remembered that efforts which have failed because the observer's
attention has been drawn in another direction may at any other moment
be successful if the observing glass chances to bear on the calling
signals.
During the whole time that signals are being made to attract attention
the calling station must watch closely with the telescope the station
called. The watch should not be relaxed until communication is
established or the station ordered abandoned.
[Illustration: FIG. 18.--Signal Corps heliograph station.]
OPERATION OF STATIONS.
_Personnel._--At signal stations where continued operation is required
at least a squad or "set of fours" is required. Physical and mental
exhaustion always result from continuous signal duty, and as
alertness of mind and body is an indispensable factor in the prevention
of errors, two reliefs of signalmen should be furnished each station
whenever practicable. The senior officer or enlisted man is in charge
of the station and is responsible for efficiency and discipline. He
will require from each man a strict and entire attention to his own
immediate duties, and permit no conversation that will distract the
men at work. He will be careful not to allow persons to loiter about
the station or within the hearing of the words called out to the
signaler. The assignment of men should be such that a continuous watch
for signals is kept and the responsibility for neglect to promptly
answer calls determined. Of the station men, one is the sender,
whose duty it is to transmit all signals to contiguous stations.
Another, the receiver, attends the telescope and reads and calls off
the signals displayed at the distant station. A third man acts as
recorder, alternately calling off the outgoing message to the sender or
transcribing the incoming message repeated by the receiver.
_Calls and personal signals._--Each station will be assigned a call
consisting of one or two letters. Each and every operator will also
have a personal signal of like character. Station calls or personal
signals when once given or assumed will not be changed except by order
of higher authority. Every station should at all times have on hand
a list of all calls and personal signals liable to be encountered in
station working. The general call suited to attract the attention of
any station whose regular call is unknown will always be a signal
represented by the letter "A" in the Morse or the letter "E" of the
Army and Navy Code.
_Opening communication._--To open communication with any distant
station whose call is known, signal the call repeatedly, occasionally
signing the call of the home station. If the regular call of the
station sought is unknown the general call above prescribed should
be used. As soon as the call is observed the called station will
acknowledge receipt by "ii ii," or "I understand," signing thereafter
its station call. These preliminaries completed, the stations are ready
for working.
It is sometimes difficult to secure the attention of stations
at unexpected hours. The force may not be strong enough for an
uninterrupted watch. To provide, so far as possible, for this
contingency, it may be concerted that if communication is required
at unusual time, or is of pressing importance, certain flags
shall be displayed, rockets discharged, smokes shown, or other
attention-compelling signals used.
When a number of stations are in view from one station and it is
desired to send a message to all or more than one station, some
preconcerted signal, as a rocket, a red light, or some peculiar flag or
torch signal, should be designated as a signal for general attention.
Upon noticing this signal all the called stations reply, and then
observe the calling station. This plan is useful when two or more
stations can, at the same time, read the signals from the one station,
and thus together receive any information to be transmitted from it.
When a signal station is to communicate with two or more stations, a
telescope should be firmly fixed bearing on each, when practicable, and
so far apart that those communicating with one station will not disturb
the other party.
_Commencing the message._--Every message is invariably commenced by the
signal "Hr" or "Anr." Sometimes at the commencement of communication
a preface will be sent in order to give some preparatory information
to the receiving station regarding the number or character of messages
about to be sent. For example, "Hr 8," means "I have eight for you" or
"Hr ck 300" means a three hundred word message follows.
_Sending and receiving._--Before the commencement of a message, care
should be taken that all the letters and characters thereof are
entirely and correctly understood by the signalman whose duty it is
to call the same to the sending operator. The message is read off by
the "reader," who first calls off a word and then spells it out letter
by letter. The "reader" should observe the signals of the operator
and invite his attention to any apparent errors. When the last letter
of a word is announced this fact will be communicated to the sending
operator.
At the receiving station the man at the telescope will call off each
letter as received and not wait until the completion of a word. On
reaching the end of a word announcement of this fact will be made to
the recorder.
_Breaking._--If the sending operator discovers that he has made
an error which will probably render the sense of the message
unintelligible at the receiving station, he will make the signal "BK"
and recommence the message, beginning at the last word correctly sent.
When the receiving station fails for any reason to get correctly what
is being sent, the sending station is interrupted by the signal "GA,"
followed by the last word correctly received. The message will then be
recommenced by the sending station at the point indicated.
_Discontinuance of transmission._--When all the messages on file at any
station have been sent the signal "NM" in Morse or "Cease signaling"
in the army and navy system, according to which code is authorized,
will be the concluding signal of the sending station. When a signal
station is operated only during the daytime, the signal "GN" will be
transmitted after all business filed up to the hour designated for
closing has been dispatched.
_Acknowledgment of receipt._--No message will be considered sent until
receipt for the same has been acknowledged. This is effected by making
either the "I understand" of the army and navy or the "OK" of one of
the Morse systems, depending upon the one authorized. In every case
the receiving operator's signal is signed after acknowledgment. When a
number of messages are continuously sent, one acknowledgment for all
will suffice and will be so understood. In receiving messages nothing
should be taken for granted and nothing considered as seen until it has
been positively and clearly in view.
_Station records._--Records kept at field signal stations will be
confined to original files of messages sent and carbon copies of
messages received. Ordinarily the only available stationery will be
the United States Army Field Message Book. Station records will be
invariably preserved as part of the station equipment until orders for
their disposition are given by higher authority. Whenever a station is
in imminent danger of capture, all records should be destroyed in the
discretion and under the direction of the operator in charge.
_Formation of signals._--Make signals with regularity; do not send one
word rapidly, the next slowly; adopt such a rate of speed as can be
read by the distant signaler without causing him to "break" frequently.
Make a distinct pause between letters. It is time gained to do so; it
is a loss of time and an annoyance to run letters together. Nothing
so distinguishes the good from the indifferent operator, visual or
telegraph, as this. When signals are being made with a flag, a fraction
of a second will be ample. In using the lantern or heliograph, the
pause between letters should be relative to the time of display of
the elements, longer than with the flag. To prevent any entangling of
the flag upon its staff, skillful handling, acquired by practice, is
necessary. It is accomplished by making a scoop of the flag against the
wind, the movement describing an elongated figure 8, thus ∞.
The motions should be made so as to display in the lateral waves the
whole surface of the flag toward the point of observation.
In using the heliograph, if the receiver sees that the sender's mirror
needs adjustment, he will turn on a steady flash until answered by a
steady flash. When the adjustment is satisfactory, the receiver will
cut off his flash and the sender will resume his message.
_Repeating the message._--It may happen that very important messages
received by signals must be verified by repeating back from the
receiving station, signal by signal, each signal used by the sending
station in conveying the message. There can be no error in signals thus
verified, and the correct transmission of the message is made certain.
For such verification each signal must be repeated by the receiving
station as soon as it is made at the sending station.
_Signal practice._--Full efficiency of the signaler can be maintained
only through constant practice, and those in charge of Signal Corps
troops should see that sufficient practice be had to insure that
accuracy and rapidity in handling messages which is so essential in
time of war.
Instruction should commence with the study of the principles of
signaling and the theories of their general use, and the pupil should
be well grounded in this study before practice is begun. He should
so memorize the alphabets to be used that no letter combination will
require thought to determine its meaning.
Daily inspections should be made to insure that all signaling
instruments, appliances, and materials are in readiness for instant
use. Defects in the apparatus annoy the sender; to a greater extent
they annoy the person to whom the messages are imperfectly sent, and
delays result that may have serious consequences.