The whole question, after careful consideration, was fully dealt with in “Training Note No. 16,” already mentioned.
The entire system of field signalling was divided under three main headings:
(i) Local.—Between tanks and tanks and tanks and the attacking infantry; also between the Section commander and the transmitting station, should one be employed.
(ii) Distant.—Between tanks and Company Headquarters, selected infantry and artillery observation posts, balloons, and possibly aeroplanes.
(iii) Telephonic.—Between the various tank headquarters and those of the units with which they were co-operating.
The means of signalling adopted were as follows:
For local signalling coloured discs—red, green, and white. One to three of these signals in varying combinations could be hoisted on a steel pole. In all thirty-nine code signals could thus be sent, e.g. white = “Forward”; red and white = “Enemy in small numbers”; red, white, green = “Enemy is retiring.” These codes were printed on cards and distributed to tank crews and to the infantry. Besides these “shutter signals” were also issued, but as they entailed both the sender and reader understanding the Morse code they were seldom used. The chief local system of communication was by runner, and it remained so until the end of the war.
Distant signalling was carried out by means of the Aldis daylight lamp, and as message-sending was too complicated a letter code was used, thus—a series of D.D.D. ... D’s meant “Broken down,” Q.Q.Q. ... Q’s “Require supplies.” Generally speaking, until November 1917, distant signalling was carried out by pigeons, which, on the whole, proved most reliable as long as the birds were released before sunset; at a later hour than this they were apt to break their journey home by roosting on the way.
In February 1917 Captain J. D. N. Molesworth, M.C., was attached to the Heavy Branch to supervise the training in signalling. This officer remained with the Tank Corps until the end of the war, and in 1918 was promoted to the rank of Lieutenant-Colonel and appointed Assistant Director of Army Signals in 1918. Under his direction classes in signalling were at once started and considerable progress was made in the short time available before the battle of Arras was fought.
In this battle the various means of communication laid down were put to the test of practical experience. The telephone system was described by the 1st Tank Brigade Commander as “heart-breaking.” “Many times it was totally impossible to hear or to be heard when speaking to Corps Headquarters at a distance of five to six miles.” Pigeons were most useful, the Aldis lamp was found difficult, and many messages were sent from tank to tank, and in some cases to infantry with good results, by means of the coloured discs.