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Engagement Area Development
and Battlefield Calculus

by LTC Lance Moore
Fire Supporters at the Combat Maneuver Training Center (CMTC) repeatedly fail to adequately develop targets for engagement areas (EAs) during the planning process. Engagement area development requires detailed analysis, planning, and preparation of observers, triggers, and targets.

Fire Support Observer/Controllers (O/Cs) routinely observe two problems with EA development:

1. Fire support officers (FSOs) and observers fail to establish a rational trigger-shooter plan.

2. Observer targeting inside the EA is not adequately developed to achieve battlefield success and commander's intent.

Successful EA development is vital to achieving commander's intent. It compels the FSO to consider such factors as the number of indirect fire assets available, training proficiency of observer/firing unit, the enemy's direction/rate of march, trigger and intercept points, terrain analysis, anticipated enemy actions, and the amount of time the enemy can be expected to remain inside the EA. The process requires forethought, analysis, and, in many instances, mathematical calculations. "Wargaming" issues, such as competing fires, communications, fixing systems, observation plan, terrain analysis, and clearance of fires procedures, are complex. The fire support O/C team refers to this collective process of wargaming and mathematical calculations as "battlefield calculus" (see figure 1).


Establishing a Trigger-shooter Plan. Successful EA development relies heavily on the effective use of battlefiled calculus. The commander's intent for fires usually requires fire supporters to develop a plan to attrit the enemy prior to its entry into an EA and its destruction once it enters the EA. To accomplish the implied task of engaging a moving target throughout the depth of the battlefield, observers must be proficient at engaging moving targets, i.e., establishing a trigger-shooter plan.

Even if a fixing system is emplaced and an EA established, the enemy (targets) will begin to move after being engaged inside the EA either to "push-through" or disperse.

To engage a moving enemy, observers must determine an intercept point (planned target location) and a trigger point. The intercept point is usually a target located along a road at a terrain chokepoint (a fixing system may or may not be available). Although observers usually choose a logical intercept point, selection of a trigger point is often less rational. Frequently the observer selection process consists of nothing more than approximating an appropriate trigger distance before the intercept point and then selecting a prominent terrain feature nearby which can be seen from a pre-selected observation post.


1. Ignore the "interpolation" approach to trigger selection. It is usually ineffective.

2. Instead, base your trigger point selection on a time-distance formula.

3. In choosing a trigger point, always consider:

a. The intended path of the target,

b. Target speed,

c. Time of flight (TOF), and

d. Call for fire (CFF) transmission time.

4. Use this formula to determine the minimum distance the trigger point must be before the intercept point:

DISTANCE = (Transmission Time + Fire Mission Processing Time + TOF) x Target Speed

The best method for determining appropriate times is a combination of historical data based on FSO/observer experience and specific data provided during the rehearsal process. For example, the average transmission time for a CFF is 5-10 seconds. Fire mission processing time is dependent on target type (priority, on-call, or target of opportunity). Personal experience suggests priority targets take 60 seconds, on-call targets require 120 seconds and targets of opportunity require 160 seconds. TOF can be provided by the BN FDC during the field artillery rehearsal. FM 6-30, Observed Fire, provides target speed for enemy movement.

Proper calculation provides the observer with an exact distance from which to establish a trigger point. Unfortunately, the trigger point may not always coincide with a prominent terrain feature that can be observed day and night.


1. When the trigger point does not coincide with a prominent terrain feature that can be observed day and night, use a VS-17 aircraft marking panel for marking trigger points.

2. Burn low-grade fuel or charcoal inside a drum so it can be detected by thermal devices and consequently serve as a marking mechanism (trigger point) for night operations.

Observers who are not positioned to see the intercept and trigger point are simply not capable of executing the initial CFF and subsequent adjustments. Therefore, they are incapable of achieving commander's intent (attrition or destruction of the enemy).


1. Establish a trigger point on rational data (time-distance formula), not interpolated and selected based on prominent terrain features.

2. Ensure that observers finalize their observation posts after the selection of intercept and trigger points.

Adequate development of targeting inside the EA to achieve battlefield success and commander's intent. The observer must identify how long the enemy will be inside the EA and how to accomplish commander's intent. Once again, battlefield calculus can provide the answer. By definition, an EA is in effect once a target is fired. This principle applies whether the enemy is inside a "kill sack" with an emplaced fixing system (maneuver, obstacle, or FASCAM) or on the move.

Once the enemy is inside the EA, the observer must deliver the appropriate volume and type of fire to achieve commander's intent. The type of fire depends on vehicle types and density. The volume of fire is directly related to the amount of time the enemy is inside the EA. Consequently, the observer must estimate the length of time the enemy can be engaged with indirect fire and anticipate actions of the enemy upon engagement (push-through or dispersion). This knowledge enables the observer to plan an appropriate fire mission(s) with the right volume of fire.

An observer request for "continuous fires" is usually because the enemy disperses after being engaged. Although the first fire mission may be effective, repeat fires without target grid adjustments are wasted.


1. Observers must develop targets to achieve commander's intent.

  • Determine the length of time the enemy will be inside the EA.

  • Anticipate enemy actions.

2. Use the following formula to determine the amount of time an enemy can be expected to remain inside an EA. This formula enables you to calculate the EA as a point target or a kill sack: Time in EA for MRB = (60 divided by Rate of March) x (Length of EA) + (Length of MRB) x (MIN per KM)

Figure 2 graphically depicts the methodology for EA development. An enemy Motorized Rifle Battalion (MRB) is attacking east to west along a high speed avenue of approach with restricting terrain to the north and south. The enemy rate of march is 20 kmph (3 minutes per kilometer). The average length of an MRB in attack formation is 1.4 kilometers. The length of the EA is 2.4 kilometers. Observers 1 and 2 have an available fixing system (obstacle) with direct fire coverage. The intercept point is selected and coordinated as priority target AA0071. The battalion FDC computes the TOF as 42 seconds. Using the formulas provided, the observers determine that the trigger must be 2.14 kilometers prior to target AA0071. It is marked with a VS-17 aircraft marking panel. The observers anticipate that the MRB will be inside the EA for approximately 11 minutes. Consequently, using a sustained rate of fire for an M109 howitzer, there is sufficient time to potentially deliver 12 battalion volleys. Of course, the actual volume of fire must be based on the other factors of battlefield calculus (terrain analysis, anticipated enemy actions, competing fires, shift times, etc.). However, equipped with the above data, the FSO and observer can rationally determine the most effective means to attack the target and achieve commander's intent.


Proper development of EAs during the fire support planning process is vital to achieving commander's intent and battlefield success. The adage that the devil is in the details applies to all aspects of fire support planning and EA development. The details are incorporated into battlefield calculus. Battlefield calculus incorporates tactical knowledge (tactics, techniques and procedures), analysis and science. The result is an EA capable of fire support success.

Always remember the three keys to developing successful EAs:

1. Choose no more than five or six EAs in which to influence or shape the battlefield.

2. Use battlefield calculus to develop those EAs.

3. Rehearse, rehearse, rehearse!


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