Field artillery (United States)
In the United States military, land-based field artillery typically consists of three distinct components: the forward observer (FO), the fire direction center (FDC), and the firing unit, sometimes referred to as the gun line. The field artillery fires process involves the coordination of all of these elements; for instance, there may be multiple FOs calling in fire on multiple targets to multiple FDCs and any component may be in communication with some of the other elements depending on the situational requirements.