Indirect fire

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 elevation angles, and may include correcting aim by observing the fall of shot and calculating new angles.

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 (although rocket artillery and ballistic missiles, etc, are indirect), 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." 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.

Indirect fire is most commonly associated with field artillery (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 75mm). 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.

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