The FDC is located in the headquarters section of the mortar platoon. Through the FDC, the platoon commander is able to control and quickly mass the fire of his entire unit. The FDC is normally located at or near the firing position and maintains contact with the guns by use of wire communications. Calls for fire are reported from all sources directly to the FDC where the targets are plotted on the firing chart (plotting board). From this chart, firing data is prepared by the computer (a member of the FDC) and announced to the mortar crews as fire commands. The FDC can be operated by the computer alone; however, it is desirable to include a radiotelephone operator, so the computer can be dedicated to computing firing data and issuing fire commands.

PLOTTING BOARD

The M17 plotting board is a fire-control instrument that helps in computing firing data by providing the range and direction from the mortar position to the target. The M17 plotting board is sturdy, easy to operate, accurate, and suitable for use in the field under adverse atmospheric conditions. The plotting board consists of a pivoted disk of transparent plastic and a removable range scale arm, both attached to a flat base grid. The M17 plotting board is carried in a durable nylon case, as shown in figure 14-24.

Figure 14-25.-Launcher 84-mm, M136 (AT4), Heat.