[stephen]

A brief description of RFC two seater training procedures in 1918:

Communication from ground to air was accomplished by physical signals. The spotting plane was required to stay within sight of the artillery battery in order to view various letters of the alphabet, which would signify specific messages from the battery's crew such as their state of readiness, etc. They would reveal what was called a "Panneau" signal to indicate their readiness to begin an operation.

The air to ground communication would then begin, accomplished via a one way wireless set that used Morse code at eight words per minute to a four gun battery. Each gun would fire in succession, observed by the two seat crew, after which they would correct the fire by a clock code. "A2," for example: "A" means the fire landed 50 yds from the target, "2" means that the fire landed at 2 o'clock from the target. This system continued, sometimes for hours, ranging one gun at a time, until the target had been bracketed, after which a "G" morse code signal was sent to instruct the battery to fire at will to destroy the target.

EYE IN THE SKY 1918 by Philip Brereton Townsend is an indispensable book for such studies, though it is extremely difficult to find. Hope that helps.