The battery uses a variety of techniques to calculate the position of the enemy battery, then can lay fire upon it. If possible, a FO can access a position to call fire on enemy guns directly; either on the ground or in the air, or a battery can be located using counter-battery radar, which can be used to observe the fall of enemy shells and thus calculate their trajectory. A correctly calculated trajectory will reveal the location from which the shell was fired, which information can then be passed on to a friendly battery command post as a fire mission.