A military artillery observer or spotter is responsible for directing artillery

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...

 fire and close air support

Close air support

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...

 (ground attack by aircraft) onto enemy positions. Because artillery is an indirect fire

Indirect fire

Indirect fire 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...

 weapon system, the guns are rarely in line-of-sight of their target, often located tens of miles away. The observer serves as the eyes of the gunners, calling in target locations and adjustments to the Fire Direction Center (FDC) via radio

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

 or (less commonly) landline

Landline

A landline was originally an overland telegraph wire, as opposed to an undersea cable. Currently, landline refers to a telephone line which travels through a solid medium, either metal wire or optical fibre, as distinguished from a mobile cellular line, where transmission is via radio waves...

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