FM-3-09 Fire Support and Field Artillery Operations Download
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Appendix B B-14 FM 3-09 30 April 2020 Figure B-12. Boundaries example PHASE LINE AND TRIGGER LINE B-44. A phase line is an easily identified feature in the operational area utilized for control and coordination of military operations (JP 3-09). Phase lines (see figure B-13 on page B-15) are usually easily identifiable from a ground or air vantage point and may include features such as ridgelines, tree lines, hilltops, roads and rivers. Phase lines can be used to identify limits of advance and to coordinate fires. B-45. A trigger line is a phase line located on identifiable terrain that crosses the engagement area—used to initiate and mass fires into an engagement area at a predetermined range for all or like weapon systems. (ATP 3-21.20). The commander designates a phase line as the trigger line for fire support attack assets. The trigger line's location is based on such variables as the time of flight for artillery shells, positioning of the guns, and the existence of quick-fire links. A trigger line's location varies from situation to situation. Its position reflects the distance that the enemy force is likely to traverse in the time it takes from when fires are requested to when artillery rounds impact (see figure B-13 on page B-15). This gives time for the fire support delivery systems to respond to the initial call for fire. Fires unit commanders must determine the appropriate planning response time for their unit based on their weapon system and training proficiency to recommend the placement of a trigger line.