Indirect fire

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Indirect fire means aiming and firing a gun 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 elevation angles, and may include correcting the fall of shot by observing it and calculating new angles.

Description

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 is also affected by atmospheric conditions, other factors and the difference in altitude between the firer and the target. Direct fire sights may include mechanisms to compensate for some of these. Handguns and rifles, tank guns and guns mounted in aircraft are examples of direct fire weapons.

Indirect fire means that aiming does not rely on directly viewing the target through the sights. The implication is that azimuth and/or elevation ‘aiming’ is done using indirect methods. In practice and in the modern sense it usually means both. Hence indirect fire means applying ‘firing data’ to azimuth and elevation sights and laying these sights. This entry does not include detail about the production of firing data.

Calling and adjusting indirect artillery fire on a target unseen to the soldiers manning the guns, a modern United States example

Indirect fire is most commonly associated with field artillery. It is also used with mortars 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. This entry does not consider shipborne systems.

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. 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 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. This entry does not deal with indirect fire with guided projectiles or missiles.

History

It may be argued^[who?] that 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.^{[original research?]}

It could also be argued^[who?] that Niccolò Tartaglia's invention of the gunner's quadrant in the 16th Century introduced indirect fire guns. 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^[who?] that indirect fire was used by the Burgundians 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 positioned in line with the target.

Modern indirect fire dates 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 and the term goniometer had replaced "lining-plane" in English.

Both sides made some use of indirect fire during the Second Boer War. However, the is no evidence to suggest that either side used goniometric sights. The British used improvised gun arcs with howitzers^[1]. 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, thereby enabling indirect fire without using external aiming points. Coupled with electronic clinometers, it enables automated laying.

Related Issues

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 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 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.  

See also

• Gun laying

Footnotes

References

• Chris Bellamy (1986). Red God of War - Soviet Artillery and Rocket Forces. Brassey's. ISBN 0080312004.