The Signal Station_

The history of war teems with occasions where the interception of dispatches and orders written in plain language has resulted in defeat and disaster for the force whose intentions thus became known at once to the enemy. For this reason, prudent generals have used cipher and code messages from time immemorial. The necessity for exact expression of ideas practically excludes the use of codes for military work although it is possible that a special tactical code might be useful for preparation of tactical orders. >> 39-94-01-01

Locations 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.

>> 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:


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.

Operation of Stations_

The capture of messengers is no longer the only means available to the enemy for gaining information as to the plans of a commander. All radio messages sent out can be copied at hostile stations within radio range. If the enemy can get a fine wire within one hundred feet of a buzzer line or within thirty feet of a telegraph line, the message can be copied by induction. Messages passing over commercial telegraph lines, and even over military lines, can be copied by spies in the offices. On telegraph lines of a permanent nature it is possible to install high speed automatic sending and receiving machines and thus prevent surreptitious copying of messages, but nothing but a secure cipher will serve with other means of communication. >> 39-94-01-01

>> 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.

Codes and Ciphers // _51-00-00-00

A code is a list or collection of arbitrary words or groups of letters to each of which some ordinary word, proper name, phrase, or sentence is assigned for meaning. Ciphers embrace all means whereby writings may be transcribed into secret terms. All ciphers employ some distinct method for transcription, which method is termed a key. In practice the key is usually applied directly in enciphering and reversed in deciphering messages. >> 39-94-01-01

Codes in Use // _51-00-00-00

The codes of the Western Union and Postal Telegraph companies are examples of well-known codes suited to general commercial use. Besides these, many special codes have been formulated, so as to embody technical expressions especially adapted to use in particular lines of industry. The War Department Code is a military code adapted to the special needs of the military establishment in peace and war. >> 39-94-01-01

Employment of Codes // _51-00-00-00

Codes are primarily intended for economy, but they may also be readily employed to secure secrecy. When used solely for economy, the coded message is said to be plain code; that is, the word or phrases of the message are coded by direct reference to their respective code equivalents. Thus plain code is readily translatable to anyone in possession of a code book. When secrecy is desired, some method of enciphering or key is employed in such a way that only persons in possession of it can in conjunction with the code book decipher it. In such case the message is said to be in cipher code. >> 39-94-01-01