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
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...
. Aiming is performed by calculating azimuth
and elevation angles, and may include correcting the fall of shot by observing
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...
it and calculating new angles.
There are two dimensions in aiming a weapon:
- In the horizontal plane (azimuth); and
- In the vertical plane (elevation), which is governed by the distance (range) to the target and the energy of the propelling charge.
The projectile trajectory
A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...
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, tank guns and guns mounted in aircraft
In military tactics, close air support is defined as air action by fixed or rotary winged aircraft against hostile targets that are close to friendly forces, and which requires detailed integration of each air mission with fire and movement of these forces.The determining factor for CAS is...
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." 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 uses a high trajectory, which further distinguishes it from direct fire.
Indirect fire is most commonly associated with 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....
. It is also used with mortars
A mortar is an indirect fire weapon that fires explosive projectiles known as bombs at low velocities, short ranges, and high-arcing ballistic trajectories. It is typically muzzle-loading and has a barrel length less than 15 times its caliber....
and 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.
The original purpose of indirect fire was to enable fire from a ‘covered position’, one where the firers could not be seen by their enemies. Typically the position was just behind the crest of a hill from which the enemy could be seen. However, it is also used where visible targets may become obscured by dust, smoke or darkness. Modern indirect fire during daylight is usually because distance and or terrain make direct fire impossible.
During World War I covered positions moved further back and indirect fire evolved to allow any point within range to be attacked - firepower mobility
- without moving the firers. The concept of firepower mobility flowered with the arrival of radio communications that allowed target acquirers to be anywhere on the ground or in the air and communicate with the firers. It also enables many widely dispersed firers to concentrate their fire on one target.
The essence of traditional indirect fire is that the trajectory of the projectile cannot be altered once it has been fired. However, some projectiles with a guidance system operate by keeping to the trajectory they were fired on.
For several centuries Coehorn mortars were fired indirectly because their fixed elevation meant range was determined by the amount of propelling powder. It's 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.
It could also be argued that Niccolò Tartaglia
Niccolò Fontana Tartaglia was a mathematician, an engineer , a surveyor and a bookkeeper from the then-Republic of Venice...
's invention of the gunner's quadrant in the 16th Century introduced indirect fire guns because it enabled gunlaying by instrument instead of line of sight. This instrument was basically a carpenter's set square
A set square or triangle is an object used in engineering and technical drawing, with the aim of providing a straightedge at a right angle or other particular planar angle to a baseline....
with a graduated arc and plumb-bob
A plumb-bob or a plummet is a weight, usually with a pointed tip on the bottom, that is suspended from a string and used as a vertical reference line, or plumb-line....
placed in the muzzle to measure an elevation. There are suggestions, 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
The Burgundians were an East Germanic tribe which may have emigrated from mainland Scandinavia to the island of Bornholm, whose old form in Old Norse still was Burgundarholmr , and from there to mainland Europe...
in the 16th Century. The Russians seem to have used something similar at Paltsig in 1759 where they fired over trees, and their instructions of the time indicate this was a normal practice. These methods probably involved an aiming point
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....
positioned in line with the target. The earliest example of indirect fire adjusted by an observer seems to be during the defence of Hougoumont
Hougoumont was a fortified farm held by Wellington's army in the Battle of Waterloo. It may also refer to:* Hougoumont , a convict ship;...
in the Battle of Waterloo
The Battle of Waterloo was fought on Sunday 18 June 1815 near Waterloo in present-day Belgium, then part of the United Kingdom of the Netherlands...
where a battery of the Royal Horse Artillery
The regiments of the Royal Horse Artillery , dating from 1793, are part of the Royal Regiment of Artillery of the British Army...
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.
Modern indirect fire may date from the late 19th century. In 1882 a Russian, Lt Col KG 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
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
and the term goniometer
A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. The term goniometry is derived from two Greek words, gōnia, meaning angle, and metron, meaning measure....
had replaced "lining-plane" in English.
The first incontrovertible, documented use of indirect fire was on 26 October 1899 by British gunners during the Second Boer War
The Second Boer War was fought from 11 October 1899 until 31 May 1902 between the British Empire and the Afrikaans-speaking Dutch settlers of two independent Boer republics, the South African Republic and the Orange Free State...
. Although both sides demonstrated early on in the conflict that could use the technique effectively, in many subsequent battles, commanders nonetheless ordered artillery to be "less timid" and to move forward to address troops' concerns about their guns abandoning them. The British used improvised gun arcs with howitzers; 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
An inclinometer or clinometer is an instrument for measuring angles of slope , elevation or depression of an object with respect to gravity...
, a device using a spirit level
A spirit level or bubble level is an instrument designed to indicate whether a surface ishorizontal or vertical . Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some...
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 gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...
s provided a means of pointing gun or launcher in any required azimuth, thereby enabling indirect fire without using external aiming points. Coupled with electronic clinometers, it enables automated laying.
Before a gun or launcher could be aimed it had to be oriented in a known azimuth, or at least towards the target area. There were various methods for this. Initially, the angle between the aiming point and target area was deduced or estimated and this angle set on the azimuth sight, each gun then laid on the aiming point with this angle (sometimes corrected for each gun to keep them aimed roughly parallel to each other). However, for artillery
Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...
another instrument, called either a director (UK) or aiming circle (US), 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.
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. Fire may be either ‘adjusted’ or ‘predicted’. The latter (originally called ‘map shooting’) means firing data is calculated to include corrections for non-standard conditions. It also requires the target location to be precisely known relative to the gun location. Predicted fire
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...
was introduced in World War I.
Adjusting (originally ‘ranging’) means some form of observation is used to correct the fall of shot onto the target, there are several possible reasons for this:
- geospatial relationship between gun and target is not accurately known;
- good quality data for non-standard conditions is unavailable; or
- the target is moving or expected to move.
Adjusted and predicted fire are not mutually exclusive, the former may use predicted data and the later 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 US.