In the land-based field artillery

Field artillery

Field artillery is a category of mobile artillery used to support armies in the field. These weapons are specialized for mobility, tactical proficiency, long range, short range and extremely long range target engagement....

, the field artillery team is organized to direct and control indirect

Indirect fire

Indirect fire means aiming and firing a projectile in a high trajectory without relying on a direct line of sight between the gun and its target, as in the case of direct fire...

 artillery fire on the battlefield. Since World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, to conduct indirect artillery fire, three distinct components have evolved in this organization: the forward observer

Artillery observer

A military artillery observer or spotter is responsible for directing artillery fire and close air support onto enemy positions. Because artillery is an indirect fire weapon system, the guns are rarely in line-of-sight of their target, often located tens of miles away...

 (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.

Motivation

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. Because of this dramatic increases in range, in most cases, gunners no longer directly see their targets and so they can not directly engage the enemy with observed direct fire

Direct fire

Direct fire refers to the launching of a projectile directly at a target on a relatively flat trajectory. The firing weapon must have a sighting device and an unobstructed line of sight to the target, which means no objects or friendly units can be between it and the target...

. Typically, they use what is called indirect fire

Indirect fire

Indirect fire means aiming and firing a projectile in a high trajectory without relying on a direct line of sight between the gun and its target, as in the case of direct fire...

 whereby they can not directly see the target(s) that they are engaging. This dramatic range increase has been driven by the development of rifled cannons, improvements in propellants, better communications and technical improvements in gunnery

Gun

A gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol,...

 computational abilities. These technical developments have been ongoing since the end of the 19th Century. Since a modern enemy is engaged at such great distances, there is now the 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

Indirect fire

Indirect fire means aiming and firing a projectile in a high trajectory without relying on a direct line of sight between the gun and its target, as in the case of direct 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

Artillery observer

A military artillery observer or spotter is responsible for directing artillery fire and close air support onto enemy positions. Because artillery is an indirect fire weapon system, the guns are rarely in line-of-sight of their target, often located tens of miles away...

, 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

Indirect fire

Indirect fire means aiming and firing a projectile in a high trajectory without relying on a direct line of sight between the gun and its target, as in the case of direct fire...

 weapon, the forward observer must take up a position where he can observe the target using tools such as maps, compass, binoculars

Binoculars

Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

 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

Artillery battery

In military organizations, an artillery battery is a unit of guns, mortars, rockets or missiles so grouped in order to facilitate better battlefield communication and command and control, as well as to provide dispersion for its constituent gunnery crews and their systems...

 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

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. 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.

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 mils

Angular mil

An angular mil, also mil, is a unit of angle. All versions of the angular mil are approximately the same size as a trigonometric milliradian.-History:The milliradian was first identified in the mid nineteenth Century...

, 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

Hydraulics

Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...

 (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

Direct fire

Direct fire refers to the launching of a projectile directly at a target on a relatively flat trajectory. The firing weapon must have a sighting device and an unobstructed line of sight to the target, which means no objects or friendly units can be between it and the target...

 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

Indirect fire

Indirect fire means aiming and firing a projectile in a high trajectory without relying on a direct line of sight between the gun and its target, as in the case of direct 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

Iwo Jima

Iwo Jima, officially , is an island of the Japanese Volcano Islands chain, which lie south of the Ogasawara Islands and together with them form the Ogasawara Archipelago. The island is located south of mainland Tokyo and administered as part of Ogasawara, one of eight villages of Tokyo...

 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

Friendly fire

Friendly fire is inadvertent firing towards one's own or otherwise friendly forces while attempting to engage enemy forces, particularly where this results in injury or death. A death resulting from a negligent discharge is not considered friendly fire...

incidents and to coordinate shelling the Japanese with their infantry unit's movements.