Distance is the principal problem encountered in desert communications. Radio is used extensively, as wire is laid only when there is time--an element often lacking in desert operations. Radio presents a unique problem of security, because radio communication is like shouting from place to place--all who will can listen. This has necessitated the development of various codes and devices for the secret transmission of data by radio.

b. Visual Signals

Although visual signals are not used extensively for transmitting artillery data, flag signals are employed by both sides for short messages and for identification, especially in small tank units. Recently, the Germans had radio sets in a ratio of one set to three tanks. The three operated as a unit, flag signals being used to control the tanks which had no radio. Great use has also been made of pyrotechnic signals. Recently, two signals were used by the Germans to identify their tanks to German aircraft: a Very signal of three white stars, and an orange-colored smoke. Large flags have sometimes been used for the same purpose. Rockets have been used in profusion at night, apparently both to rally forces which were scattered when dark fell, and to confuse and harass the enemy. It is not known what method the Germans use to identify tanks to friendly antitank and artillery weapons. When the British used the method of approaching friendly troops with turrent guns pointing to the rear, the Germans were quick to adopt the same method for purposes of deception, in order to approach close to hostile weapons.

c. Wire

Although the speed of operations in the desert may on occasion preclude the use of telephone lines, a greater degree of security and dependability is achieved by using wire. Almost all lines are laid on the ground. Motor vehicles traveling across the desert constantly are running over the wires. The results of bruising of wire are not so serious in the desert, since ordinary field wire operates better over the desert surface than it would over damp or moist ground. Also, laying and picking up wire are much less difficult in the desert than in swampy or wooded country. Of course, overhead wire circuits are more desirable when the situation becomes at all static. Communication over long field lines in most cases is good in the desert.

The wide dispersion of guns has made necessary the use of an enunciator³ system between the executive and the individual sections of the firing batteries. Such a system permits the executive to coordinate and command his guns in such a manner as to control rapidly the guns for effective concentrated fire.

d. Radio

Radio is the most important means of communication in the desert. During the summer of 1941 one British armored division conducted its entire communication network by radio. Every command vehicle had a receiving set. Each artillery troop has three No. 11⁴ sets using one principal frequency, and, for emergency use, one switch in "frequency."

Each troop of this armored division was part of a mobile column, which furnished No. 9 command sets. Switch frequencies were in the overlap band of No. 10 and No. 11 sets, permitting use of No. 9 in displacement by a half-troop in case no extra infantry No. 11 set was available for the purpose. Artillery troops normally operated with two OP's using No. 11 sets. A third OP could be manned for emergency use by diverting a set from the infantry of the mobile columns.

Only one radio set could be provided to each gun position. This may have been caused by the fact that British radio sets are heavy and cannot be removed from the vehicle in which they are mounted. The range of the No. 11 set--voice, 15 miles--has been considered adequate for all troops used during the summer of 1941.

e. Codes

During active operations all messages below the division are usually sent in the clear. Christian names of tank and unit commanders and prearranged code names for places are used. Although there is little intentional enemy interference with artillery communications, there are active and efficient Axis radio-interception intelligence units.

The use of plain language even when accompanied by code names and enciphered place names enables radio interception to be employed effectively. By keeping a careful record of all names, key words, and numbers, both the Italians and the Germans have been able to bring their order-of-battle information up to date by a process of sifting and cross indexing. Officers' names, either family name or given name, are the principle keys used in identifying intercepted messages. Captured German documents indicate that the careful compilation of names made by the Germans has enabled them to work out British code names. In addition to names, references to the personnel arm, such as "Gunner Smith," or "Rifleman Jones," have helped the Axis forces to identify said units.

One of the most interesting methods of enabling map references to be sent in the clear with security is the "thrust line" method used by the Germans. (This method is similar to the code described in FM 18-5, "Organization and Tactics of Tank Destroyer Units," June 16, 1942, paragraph 231 b (2) (e).) It consists of a line drawn upon a map which theoretically may run in any direction but which actually usually extends in the proposed direction of advance or down the axis of a reconnaissance unit.

The line, which begins at a fixed point and continues indefinitely in the required direction, is usually divided into centimeters for convenience. To give a map reference, a perpendicular is dropped from the reference point to the thrust line. Measurements are then taken from the point of origin to the point where the perpendicular cuts the thrust line, then along the perpendicular to the reference point. Since the point may lie on either side of the thrust line, the second figure must be prefaced by either "right" or "left", as one looks toward the enemy.

A typical reference would be "6 right 3." (See fig. 15.) The figures are always in centimeters; therefore, the actual distance on the ground will vary with the scale of the map used. The scale may start with an arbitrary figure, and have dummy figures interspersed, or it may start with the number of the thrust line when there are several in a given area. These devices make the code difficult to break rapidly.

Figure 15.--The "thrust line"

Instruments have been found consisting of a transparent ruler graduated in millimeters, with a shorter ruler similarly graduated and fixed to slide up and down at right angles to the longer ruler. Practiced operators can give references very quickly.