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Collateral damage from the use of indirect fire in populated areas – can it be avoided?

Analysis / Humanitarian Action / Urban warfare 11 mins read

Collateral damage from the use of indirect fire in populated areas – can it be avoided?

Collateral damage is the unintended damage inflicted on humans, structures, animals and the natural environment that are not the target of the attack. The lack of precision in the delivery of explosive weapons may be the main source of collateral damage affecting the civilian population.

In this post, Ove Dullum, Principal Scientist at the Norwegian Defence Research Establishment, describes the sources of such inaccuracies and the tools available to reduce them.

Let’s imagine that a less experienced army unit is ordered to use heavy mortars to bomb a group of industrial buildings two kilometres away, where an enemy unit is believed to be hiding. Adjacent to these houses are residential buildings, including a school. The first salvo of 18 rounds is fired with sloppy aiming procedures and just one of the rounds hits on target. Two rounds detonate in the school area, and the rest in the residential area. The damage is massive – sadly, a common scenario in contemporary armed conflicts.

Explosive warhead is the kind of munition that inflicts the highest rate of damage on the battlefield. In most cases, it consists of a steel container filled with a high explosive substance that, upon ignition, ejects a high number of high-velocity fragments causing damage to soft objects, including human bodies. It also creates a strong pressure wave that may damage infrastructures and inflict human injuries.

In wartime, explosive warheads are mainly delivered in three different ways:

  • Guided fire, with different degrees of automation such as GPS-navigation, remotely controlled, beam riding, or homing onto an illuminated spot. This method may be used from a wide range of distances.
  • Direct fire, launched with a gun, in which the operator can see their target in their aiming devices. The firing distance is usually limited to around three kilometres.
  • Indirect fire, launched from a platform where the operator cannot see the target, but where the warhead is launched with a direction and velocity that makes it most likely to hit near the target. The firing distance may be up to 40 kilometres.

Guided fire and direct fire will be quite accurate and will usually hit the intended target with a margin of error of just a few metres. Indirect fire is the mode that is most error prone. In the worst cases, the magnitude of the error may exceed the launcher-target distance by several percent, which may extend to several hundred metres. In addition, indirect as well as direct fire have a yield that may cause damage several hundred metres away.

With this in mind, let’s consider indirect fire more closely.

Indirect fire

By its nature, indirect fire is designed and intended to have area effects; in other words, its purpose is not to destroy small targets, like a house, a certain vehicle or another small military object, but to inflict damage over a larger area hosting a group of soldiers, fortifications and infrastructure. An area of one acre is considered a very small target for indirect fire and requires high accuracy and precision to avoid collateral damage outside that area.

For artillery, a typical target may extend to an area of several hectares. For reasons that are discussed below, the target area may never be less than one hectare (100 x 100 metres) when firing at long distances, say 20 kilometres or more.

The launching platforms for indirect fire are:

  • Tube artillery, mainly army howitzers ranging in caliber from 105 to 155 mm
  • Rocket artillery ranging in calibre from 105 mm up to 230 mm
  • Mortars, mainly 81/82 mm and 120 mm calibre
  • Aircraft delivered unguided bombs, ranging in weight from 250 to 2000 lbs.

The nature of errors and how minimize them

Let’s take artillery, gun- or rocket-launched, to illustrate the nature of errors. Artillery rounds are usually launched in salvos, each containing three to six warheads from each gun platform, many more from rocket launchers. All are fired within a period of a couple of minutes.

Errors in delivery can be classified as two different types:

  • Precision or consistency, where the lack thereof is characterized by the spread of the impact points within a salvo. These errors are mostly due to variations in manufacture between the individual rounds, and how the launcher responds during fire.
  • Accuracy or bias, which is the distance between the mean point of impact of a salvo and the intended aim point. Such errors are mostly due to inherent limitations of various measurement devices, including the aiming devices.

The connection between these terms is illustrated in the figure below.

Figure 1: The relation between the different terms

When the firing distance for tube artillery exceeds 20 kilometres, the meteorological parameters – especially the variability of the wind in time and space – are the main factors determining the accuracy of fire. Rocket artillery is very susceptible to wind, especially during the boost phase. Even from a ten-kilometre range, wind errors begin to dominate.

However, if time allows, the firing units may ‘adjust fire’, by which they observe the impact points of the first rounds and adjust the aim point accordingly, thereby minimizing the bias. There is obviously a danger of inflicting collateral damage in this process, but it largely improves the accuracy of the following round. Omitting this process may have disastrous consequences in terms of collateral damage.

Accuracy and precision are often measured as a percentage of the firing distance. This percentage will, in general, increase with the firing range and will also depend on manufacturing details of both ammunition and launcher.

Mortars and rifle grenades are less accurate and precise than artillery, but they are used at relatively short distances and the actual miss distance will therefore be smaller.

Lethal areas

The miss distance caused by indirect fire cannot be seen as isolated from the area of effectiveness of a round. Often, this area is also called the lethal area, although the term ‘lethal’ is not meant literally. Simply speaking, this can be considered as the area surrounding the impact point, in which a soldier will be incapacitated, i.e. will no longer be able to serve as a soldier, permanently or temporarily. Likewise, any civilian within that area will surely be in need of some medical assistance, if they are still alive.

  • For direct fire, the margin of error is smaller than the radius of the lethal area. Thus, precision is less relevant, as the extent of collateral damage is determined by the lethal area. Since these areas overlap, the total affected area is limited, and almost independent of the number of rounds fired.
  • For indirect fire, the radius of the lethal area is usually less than the typical miss distance created by accuracy and precision combined. Here, the overlap is limited, and the total lethal area, as well as the number of rounds fired, will determine the extent of damage.

The figure below illustrates these points.

Figure 2: Direct vs. indirect fire. Red star represents aim point; black dot represents impact point; yellow circle represents lethal areas.

The size of the lethal area varies a lot, depending on the size of the warhead as well as several environmental factors. Artillery rounds have a lethal area of 500 – 1,000 square metres; aircraft bombs will have several thousand square metres. Outside of this area, damages, injuries and even death may take place, but the probability is reduced with distance.

Minimizing collateral damage

The commander of a firing unit may have information at their disposal to minimize the extent of collateral damage. It is in their interest to minimize for several reasons: one, inflicting collateral damage is beyond the objective of their task; two, firing many rounds takes time and will make their unit exposed to enemy counterfire; and three, firing too many rounds depletes the unit’s ammunition resources and wears down materiel.

In a modern army unit, the commander will plan his task upon the following information: the position, size, and shape of the target; the number and the type of enemy units inside the target; the extent to which the target should be damaged (harassment, neutralization, destruction or annihilation); the lethal area of their ammunition against the target; limitations in their access to launchers, ammunition and crew; meteorological information that enables them to estimate the accuracy; the expected precision of the fire based on distance and muzzle velocity of rounds; the presence of civilians or civilian objects adjacent to the target; the possibilities to adjust fire and to observe the firing by forward observers or drones.

Aircraft bombing issues

A bomber aircraft pilot is, to some extent, in the same situation as the artillery commander. In some cases, the pilot may see the target and drop their load with reasonable accuracy, as in a kind of direct fire mode. However, in order to avoid attacks by anti-aircraft fire, the pilot would release the bombs from a very high altitude or from a distance. An example of the latter is known as toss bombing, during which the bombs are released at a low altitude as the aircraft pulls up, hurling the bombs into a long ballistic trajectory. High altitude bombing may take place at up to 15,000 metres above ground.

It is hard to give a definite measure of the accuracy of air-dropped bombs, but they are affected by the same effects as artillery, i.e. wind, manufacturing details, and disturbances during release. In addition, there will be inaccuracies in the attitude of the aircraft at the moment of release, like speed, altitude and dive/loft angle. In most cases, the miss distance of a bomb will be larger than that of artillery.


Modern defense forces are manned by experienced commanders and motivated soldiers using advanced equipment. They will also have access to advanced guided missiles that, in the long term, may be more cost-effective, since they are able to destroy a target at first attempt and thereby inflict minimal damage outside the target. Such forces may also adapt to strict rules of engagement in order to minimize collateral damage on civilian life and property.

Some armed forces may still have old equipment and inexperienced soldiers that may have limited knowledge and respect of international humanitarian law. However, even in a modern unit, such negative attitudes may be present.

For all kinds of indirect fire applying rounds without any guidance, accuracy and precision are inherently less good. In a populated area, the inevitable consequence is that they may cause a considerable amount of collateral damage to civilian life and property. However, those in charge of such weapons may have access to several tools; by using these, the risk of such damage can be limited, but never completely eliminated.

In light of the wide area effects of indirect fire, and the high risk of collateral damage when it is employed against targets in populated areas, it is imperative that armed forces take mitigation measures to increase the accuracy and reduce the lethal area of their weapons, in addition to other measures to prevent or reduce collateral damage. If the risk remains too high despite such mitigation measures, commanders should consider using alternative weapons and tactics to better protect civilians.

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