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Indirect fire is aiming and firing a projectile without relying on a direct line of sight between the gun and its target, as in the case of direct fire. Aiming is performed by calculating azimuth and inclination, and may include correcting aim by observing the fall of shot and calculating new angles.
Indirect fire is most commonly used by field artillery and mortars (although field artillery was originally and until after World War I a direct fire weapon, hence the bullet-shields fitted to the carriages of guns such as the famous M1897 75 mm). It is also used with missiles, howitzers, rocket artillery, multiple rocket launchers, cruise missiles, ballistic missiles, naval guns against shore targets, sometimes with machine guns, and has been used with tank and anti-tank guns and by anti-aircraft guns against surface targets.
There are two dimensions in aiming a weapon:
The projectile trajectory is affected by atmospheric conditions, the velocity of the projectile, the difference in altitude between the firer and the target, and other factors. Direct fire sights may include mechanisms to compensate for some of these. Handguns and rifles, machine guns, anti-tank guns, tank main guns, many types of unguided rockets, and guns mounted in aircraft are examples of weapons primarily designed for direct fire.
NATO defines indirect fire as "Fire delivered at a target which cannot be seen by the aimer." [1] The implication is that azimuth and/or elevation 'aiming' is done using instrumental methods. Hence indirect fire means applying 'firing data' to azimuth and elevation sights and laying these sights. Indirect fire can be used when the target is visible from the firing position. However, it is mostly used when the target is at longer range and invisible to the firer due to the terrain. Longer range uses a higher trajectory, and in theory maximum range is achieved with an elevation angle of 45 degrees. [2] [3] [4]
It is reasonable to assume that original purpose of indirect fire was to enable fire from a 'covered position', one where gunners can not be seen and engaged by their enemies (that and as the range of artillery lengthened, it was impossible to see the target past all the intervening terrain). The concealment aspect remains important, but from World War I equally important was the capability to concentrate the fire of many artillery batteries at the same target or set of targets. This became increasingly important as the range of artillery increased, allowing each battery to have an ever-greater area of influence, but required command and control arrangements to enable concentration of fire. The physical laws of ballistics means that guns firing larger and heavier projectile can send them farther than smaller-calibre guns firing lighter shells. By the end of the 20th century, the typical maximum range for the most common guns was about 24 to 30 km, up from about 8 km in World War I.
During World War I, covered positions moved farther back and indirect fire evolved to allow any point within range to be attacked, firepower mobility, without moving the firers. If the target cannot be seen from the gun position, there has to be a means of identifying targets and correcting aim according to fall of shot. The position of some targets may be identified by a headquarters from various sources of information (spotters): observers on the ground, in aircraft, or in observation balloons. The development of electrical communication immensely simplified reporting, and enabled many widely dispersed firers to concentrate their fire on one target.
The trajectory of the projectile could not be altered once fired, until the introduction of smart munitions.
Indirect arrow fire by archers was commonly used by ancient armies. It was used during both battles and sieges. [5]
For several centuries Coehorn mortars were fired indirectly because their fixed elevation meant range was determined by the amount of propelling powder. It is also reasonable conjecture that if these mortars were used from inside fortifications their targets may have been invisible to them and therefore met the definition of indirect fire.[ original research? ]
It could also be argued[ according to whom? ] that Niccolò Tartaglia's invention of the gunner's quadrant (see clinometer) in the 16th century introduced indirect fire guns because it enabled gunlaying by instrument instead of line of sight. [6] This instrument was basically a carpenter's set square with a graduated arc and plumb-bob placed in the muzzle to measure an elevation. There are suggestions, [7] based on an account in Livre de Canonerie published in 1561 and reproduced in Revue d'Artillerie of March 1908, that indirect fire was used by the Burgundians in the 16th century. The Russians seem to have used something similar at Paltzig in 1759 where they fired over trees, and their instructions of the time indicate this was a normal practice. [8] These methods probably involved an aiming point positioned in line with the target.[ citation needed ] The earliest example of indirect fire adjusted by an observer seems to be during the defence of Hougoumont in the Battle of Waterloo where a battery of the Royal Horse Artillery fired an indirect Shrapnel barrage against advancing French troops using corrections given by the commander of an adjacent battery with a direct line of sight. [9]
Modern indirect fire dates from the late 19th century. In 1882 a Russian, Lt Col K. G. Guk, published Field Artillery Fire from Covered Positions that described a better method of indirect laying (instead of aiming points in line with the target). In essence, this was the geometry of using angles to aiming points that could be in any direction relative to the target. The problem was the lack of an azimuth instrument to enable it; clinometers for elevation already existed. The Germans solved this problem by inventing the lining-plane in about 1890. This was a gun-mounted rotatable open sight, mounted in alignment with the bore, and able to measure large angles from it. Similar designs, usually able to measure angles in a full circle, were widely adopted over the following decade. By the early 1900s the open sight was sometimes replaced by a telescope and the term goniometer had replaced "lining-plane" in English.
The first incontrovertible, documented use of indirect fire in war using Guk's methods, albeit without lining-plane sights, was on 26 October 1899 by British gunners during the Second Boer War. [10] Although both sides demonstrated early on in the conflict that they could use the technique effectively, in many subsequent battles, British commanders nonetheless ordered artillery to be "less timid" and to move forward to address troops' concerns about their guns abandoning them. [10] The British used improvised gun arcs with howitzers; [7] the sighting arrangements used by the Boers with their German and French guns is unclear.
The early goniometric devices suffered from the problem that the layer (gun aimer) had to move around to look through the sight. This was very unsatisfactory if the aiming point was not to the front, particularly on larger guns. The solution was a periscopic panoramic sight, with the eyepiece to the rear and the rotatable top of the sight above the height of the layer's head. The German Goertz 1906 design was selected by both the British and the Russians. The British adopted the name "Dial Sight" for this instrument; the US used "Panoramic Telescope"; the Russia used "Goertz panorama".
Elevations were measured by a clinometer, a device using a spirit level to measure a vertical angle from the horizontal plane. These could be separate instruments placed on a surface parallel to the axis of the bore or physically integrated into some form of sight mount. Some guns had clinometers graduated in distances instead of angles. Clinometers had several other names including "gunner's level", "range scale", "elevation drum" and "gunner’s quadrant" and several different configurations. Those graduated in ranges were specific to a type of gun.
These arrangements lasted for most of the 20th century until robust, reliable and cost-effectively accurate gyroscopes provided a means of pointing gun or launcher in any required azimuth and elevation, thereby enabling indirect fire without using external aiming points. These devices use gyros in all three axes to determine current elevation, azimuth and trunnion tilt.
Before a gun or launcher can be aimed, it must be oriented towards a known azimuth, or at least towards a target area. Initially, the angle between the aiming point and target area is deduced, or estimated, and set on the azimuth sight. Each gun is then laid on the aiming point, with this angle in order to keep them aimed roughly parallel to each other. However, for artillery another instrument, called either a director (United Kingdom) or aiming circle (United States), became widespread and eventually the primary method of orienting guns in most if not all armies. After being oriented and pointed in the required direction a gun recorded angles to one or more aiming points. Such early directors were the progenitors of the later general class of directors.
Indirect fire needs a command and control arrangement to allot guns to targets and direct their fire. The latter may involve ground or air observers or technical devices and systems. Observers report where shots fall so that aim may be corrected. In the First World War an important task for aircraft — both heavier-than-air or balloons — was artillery spotting. In naval use several ships may be shooting at the same target, making identifying the fall of shot from a particular ship difficult; different-coloured dyes for each ship were often used to help with spotting. [11]
Fire may be "adjusted" or "predicted". Adjusting (originally "ranging") means some form of observation is used to correct the fall of shot onto the target; this may be required for several possible reasons:
Predicted fire, originally called "map shooting", was introduced in World War I. It means that firing data is calculated to include corrections for the current prevailing conditions. It requires the target location to be precisely known relative to the gun location.
Adjusted and predicted fire are not mutually exclusive, the former may use predicted data and the latter may need adjusting in some circumstances.
There are two approaches to the azimuth that orients the guns of a battery for indirect fire. Originally "zero", meaning 6400 mils, 360 degrees or their equivalent, was set at whatever the direction the oriented gun was pointed. Firing data was a deflection or switch from this zero.
The other method was to set the sight at the actual grid bearing in which the gun was oriented, and firing data was the actual bearing to the target. The latter reduces sources of mistakes and made it easier to check that the guns were correctly laid. By the late 1950s, most armies had adopted the bearing method, the notable exception being the United States.
Artillery are ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during sieges, and led to heavy, fairly immobile siege engines. As technology improved, lighter, more mobile field artillery cannons developed for battlefield use. This development continues today; modern self-propelled artillery vehicles are highly mobile weapons of great versatility generally providing the largest share of an army's total firepower.
The howitzer is an artillery weapon that falls between a cannon and a mortar. With their long-range capabilities, howitzers can be used to great effect in a battery formation with other artillery pieces, such as long-barreled guns, mortars, and rocket artillery.
An inclinometer or clinometer is an instrument used for measuring angles of slope, elevation, or depression of an object with respect to gravity's direction. It is also known as a tilt indicator, tilt sensor, tilt meter, slope alert, slope gauge, gradient meter, gradiometer, level gauge, level meter, declinometer, and pitch & roll indicator. Clinometers measure both inclines and declines using three different units of measure: degrees, percentage points, and topos. The astrolabe is an example of an inclinometer that was used for celestial navigation and location of astronomical objects from ancient times to the Renaissance.
An artillery observer,artillery spotter, or forward observer (FO) is a soldier responsible for directing artillery and mortar fire support onto a target. An artillery observer usually accompanies a tank or infantry unit. Spotters ensure that indirect fire hits targets which those at a fire support base cannot see.
A fire-control system (FCS) is a number of components working together, usually a gun data computer, a director and radar, which is designed to assist a ranged weapon system to target, track, and hit a target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more accurately.
Counter-battery fire is a battlefield tactic employed to defeat the enemy's indirect fire elements, including their target acquisition, as well as their command and control components. Counter-battery arrangements and responsibilities vary between nations but involve target acquisition, planning and control, and counter-fire. Counter-battery fire rose to prominence in World War I.
A milliradian is an SI derived unit for angular measurement which is defined as a thousandth of a radian (0.001 radian). Milliradians are used in adjustment of firearm sights by adjusting the angle of the sight compared to the barrel. Milliradians are also used for comparing shot groupings, or to compare the difficulty of hitting different sized shooting targets at different distances. When using a scope with both mrad adjustment and a reticle with mrad markings, the shooter can use the reticle as a ruler to count the number of mrads a shot was off-target, which directly translates to the sight adjustment needed to hit the target with a follow-up shot. Optics with mrad markings in the reticle can also be used to make a range estimation of a known size target, or vice versa, to determine a target size if the distance is known, a practice called "milling".
In ballistics, the elevation is the angle between the horizontal plane and the axial direction of the barrel of a gun, mortar or heavy artillery. Originally, elevation was a linear measure of how high the gunners had to physically lift the muzzle of a gun up from the gun carriage to compensate for projectile drop and hit targets at a certain distance.
Field artillery is a category of mobile artillery used to support armies in the field. These weapons are specialized for mobility, tactical proficiency, short range, long range, and extremely long range target engagement.
In field artillery, the accuracy of indirect fire depends on the use of aiming points. In air force terminology the aiming point refers to holding the intersection of the cross hairs on a bombsight when fixed at a specific target.
Gun laying is the process of aiming an artillery piece or turret, such as a gun, howitzer, or mortar, on land, in air, or at sea, against surface or aerial targets. It may be laying for direct fire, where the gun is aimed similarly to a rifle, or indirect fire, where firing data is calculated and applied to the sights. The term includes automated aiming using, for example, radar-derived target data and computer-controlled guns.
Base end stations were used by the United States Army Coast Artillery Corps as part of fire control systems for locating the positions of attacking ships and controlling the firing of seacoast guns, mortars, or mines to defend against them. A British equivalent was the position finding cell.
A director, also called an auxiliary predictor, is a mechanical or electronic computer that continuously calculates trigonometric firing solutions for use against a moving target, and transmits targeting data to direct the weapon firing crew.
Predicted fire is a tactical technique for the use of artillery, enabling it to fire for effect without alerting the enemy with ranging shots or a lengthy preliminary bombardment. The guns are laid using detailed calculations and surveys to increase aiming accuracy from the first round.
A plotting board was a mechanical device used by the U.S. Army Coast Artillery Corps as part of their fire control system to track the observed course of a target, project its future position, and derive the uncorrected data on azimuth and range needed to direct the fire of the guns of a battery to hit that target. Plotting boards of this sort were first employed by the Coast Artillery around 1905, and were the primary means of calculating firing data until WW2. Towards the end of WW2 these boards were largely replaced by radar and electro-mechanical gun data computers, and were relegated to a back-up role.
In the U.S. Army Coast Artillery Corps, the term fire control system was used to refer to the personnel, facilities, technology and procedures that were used to observe designated targets, estimate their positions, calculate firing data for guns directed to hit those targets, and assess the effectiveness of such fire, making corrections where necessary.
The 14-inch M1920 railway gun was the last model railway gun to be deployed by the United States Army. It was an upgrade of the US Navy 14"/50 caliber railway gun. Only four were deployed; two in the Harbor Defenses of Los Angeles and two in the Panama Canal Zone, where they could be shifted between the harbor defenses of Cristobal (Atlantic) or Balboa (Pacific).
Direct fire or line-of-sight fire refers to firing of a ranged weapon whose projectile is launched directly at a target within the line-of-sight of the user. The firing weapon must have a sighting device and an unobstructed view to the target, which means no obstacles or friendly units can be between it and the target. A weapon engaged in direct fire conversely exposes itself to direct return fire from the target.
The depression range finder (DRF) was a fire control device used to determine the target's position by observing range and bearing and to calculate firing solutions when gun laying in coastal artillery. It was the main component of a vertical base rangefinding system. It was necessitated by the introduction of rifled artillery from the mid-19th century onwards, which had much greater ranges than the old smoothbore weapons and were consequently more difficult to aim accurately. The DRF was invented by Captain H.S.S. Watkin of the Royal Artillery in the 1870s and was adopted in 1881. It could provide both range and bearing information on a target. The device's inventor also developed a family of similar devices, among them the position finder, which used two telescopes as a horizontal base rangefinding system, around the same time; some of these were called electric position finders. Some position finders retained a depression range finding capability; some of these were called depression position finders. Watkin's family of devices were deployed in position finding cells, a type of fire control tower, often in configurations that allowed both horizontal base and vertical base rangefinding. Watkin's system included automatic electrical updating of range and bearing dials near the guns as the position finders were manipulated, and a system of remotely firing the guns electrically from the position finding cell. The improved system was trialled in 1885 and widely deployed in the 1890s. Functionally equivalent devices were developed for the United States Army Coast Artillery Corps and its predecessors, called depression position finders or azimuth instruments depending on function, adopted in 1896 and deployed widely beginning in the early 1900s as the Endicott program of modern coastal defences was built. These devices were also used by both countries to control submarine (underwater) minefields.