Field artillery team
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In the land-based field artillery, the field artillery team is organized to direct and control indirect fire artillery fire on the battlefield. Since World War I, three distinct components have evolved in this organization, the forward observer (or FO), the fire direction center (FDC) and what is called the gun line (the actual guns themselves). On the battlefield, there will be combinations of all of the following elements. In other words 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.
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[edit] Components
[edit] Forward Observer (FO)
Because artillery is an indirect fire weapon, the forward observer must take up a position where he can observe the target using tools such as binoculars, laser range finders, and designators with call back fire missions on his radio. This position can be anywhere from a few thousand meters to 20-30 km distant from the guns. Modern day FOs are also trained in the rudiments of calling in both air strikes and naval gun fire.
Using a standardized format, the FO sends either an exact target location or the position relative to his own location or a common map point, a brief target description, a recommended munition to use, and any special instructions such as "danger close" (the warning that friendly troops are within 600 metres of the target, requiring extra precision from the guns). Once firing begins, if the rounds are not accurate the FO will issue instructions to adjust fire and then call "fire for effect."
The FO does not talk to the guns directly - he deals solely with the FDC. The forward observer can also be airborne and in fact one of the original roles of aircraft in the military was airborne artillery spotting.
[edit] FDC (Fire Direction Center)
Typically, there is one FDC for a battery of six guns, in a light division. In a typical heavy division configuration, there exists two FDC elements capable of operating two four gun sections, also known as a split battery. The FDC computes firing data, fire direction, for the guns. The process consists of determining the precise target location based on the observer's location if needed, then computing range and direction to the target from the guns' location. These data can be computed manually, using special protractors and slide rules with precomputed firing data. Corrections can be added for conditions such as a difference between target and howitzer altitudes, propellant temperature, atmospheric conditions, and even the curvature and rotation of the Earth. In most cases, some corrections are omitted, sacrificing accuracy for speed. In recent decades, FDCs have become computerized, allowing for much faster and more accurate computation of firing data.
[edit] Guns
The final piece of the puzzle is the "gun line" itself. The FDC will transmit a warning order to the guns, followed by orders specifying the type of ammunition and fuze setting, bearing, elevation, and the method of adjustment or orders for fire for effect (FFE). Elevation (vertical direction) and bearing orders are specified in milliradians or mils, and any special instructions, such as to wait for the observer's command to fire relayed through the FDC. The crews load the howitzers and traverse and elevate the tube to the required point, using either hand cranks (usually on towed guns) or hydraulics (on self-propelled models).
[edit] Parent battalion and US Army brigade/USMC regimental FDCs
FDCs also exist in the next higher parent battalion that "owns" 2-4 artillery batteries. Once again, an FDC exists at the US Army brigade or USMC regimetal level that "owns" the battalions. These higher level FDCs monitor the fire missions of their subordinate units and will coordinate the use of multiple batteries or even multiple battalions in what is called a battalion or brigade/regimental mission. In training and wartime execises, as many as 72 guns from 3 battalions may all be coordinated to put "steel on the target" in what is called a "brigade/regimental time or target" or brigade/regimental TOT for short. The rule is "silence is consent," meaning that if the lower unit does not hear a "cancel the mission" (don't shoot) or even a "check firing" (cease firing) order from the higher monitoring unit, then the mission goes on. Higher level units monitor their subordinate unit's missions both for both active as well as passive purposes. Higher level units also may get involved to coordinate artillery fire across fire support coordination boundaries (often parallel lines on maps) where one unit can not fire into without permission from higher and/or adjacent units that "own" the territory.
