Visual Signaling

by United States. Army. Signal Corps

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.