Field artillery team
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In the land-based field artillery, the field artillery team is organized to direct and control indirect artillery fire on the battlefield. Since World War I, to conduct indirect artillery fire, 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, the field artillery team consists of some combinations of all of the these 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.
To understand the modern field artillery team concept, it is necessary to understand that modern artillery batteries shoot at targets measured in distances of kilometers and miles rather than the old 18th Century concept of meters and yards, representing a hundredfold increase in range. This dramatic range increase has been driven by the development of rifled cannons, improvements in propellants, better communications and technical improvements in gunnery computational abilities, that have been ongoing since the end of the 19th Century. Since a modern enemy is engaged at such great distances, in most cases, gunners no longer directly see their targets and so they can not directly engage the enemy with observed direct fire, thus there is a need for trained observers linked to the artillery units by modern communications to find and adjust fire on targets at great distances. In most field artillery situations, because of weather, terrain, night-time conditions, distance or other obstacles, the soldiers manning the guns can not see the target that they are firing upon. The term indirect fire is therefore used to describe firing at targets that gunners cannot see. In most cases, the target is either over the horizon or on the other side of some physical obstruction, such as a hill, mountain or valley. Since the target is not visible these gunners have to rely on a trained artillery observer, also called a forward observer, who sees the target and relays the coordinates of the target to their fire direction center. The fire direction center, in turn, translates those coordinates into first, a left-right aiming direction, second, an elevation angle, third, a calculated number of bags of propellant and finally, a fuse with a determined waiting time before exploding to be set (if necessary). The fuse is then mated to the artillery projectile.
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 maps, compass, binoculars and laser rangefinder/designators; then call back fire missions on his radio or other communication device. This position can be anywhere from a few hundred meters to 20–30 km distant from the guns. Modern day FOs are also trained in the rudiments of calling Close Air Support, Sea-borne Weapons and other weapons systems.
Using a standardized format, the FO sends either an exact target location or the position relative to his own location or a registered 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 a certain distance from the target, which varies based upon the weapon system being used and which requires extra precision from the guns). Once firing begins, if the rounds are not accurate the FO will issue instructions to adjust fire in four dimensions (Three physical; left/right, forward/back, up/down and one for time, when using timed fuses) and then usually call "fire for effect", unless his purpose in that fire mission has an objective other than suppression or destruction of the target. A "Fire For Effect" or "FFE" calls for all of the guns or tubes to fire a round; as opposed to the adjustment phase wherein only a single gun is firing.
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.
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. This 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.
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, fuze setting and propelling charge, bearing, elevation, and the method of adjustment or orders for fire for effect (FFE). Elevation (vertical direction) and bearing orders are specified in 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).
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 regimental 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 exercises, 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 on 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.
Direct fire exceptions to usual mission of artillery indirect fire
Artillery gunners are taught how to use direct fire to engage a target such as mounted or dismounted troops attacking them. In such a case, however, the artillery crews are able to see what they are shooting at. With indirect fire, in normal artillery missions, the crews manning the guns cannot see their target directly, or observers are doing that work for them. There have been exceptions to this situation, but even when US Marines assaulted Iwo Jima during World War Two, and gunners could see the impact of their rounds on Mt. Suribachi, the actual adjustment of their fires was accomplished by forward observers directly supporting and attached to infantry units, because they were in the position to see not only the enemy but to prevent friendly fire incidents and to coordinate shelling the Japanese with their infantry unit's movements.