text: Chapter 9
  9-66. Commanders position forces within a perimeter to decrease the possibility of an enemy force
  simultaneously suppressing the inner and outer perimeter forces with the same fires. Friendly forces within
  the perimeter provide mutual support. In open terrain, they cover gaps on the outer perimeter between units
  with fires. They do not allow gaps between defensive fighting positions when the unit is in restrictive terrain
  with restricted fields of fire and observation. This may mean that a unit defends along a narrower frontage
  than on more open terrain. Commanders may also have to employ all subordinate units on the line formed by
  the perimeter. Units ensure that outer perimeter positions have rearward protection from inner perimeter
  weapons if an inner perimeter is established.
  9-67. Units can assign combat vehicles supporting defense firing positions on the perimeter to cover the most
  likely mounted avenues of approach. Based on their mobility, combat vehicles can also form the unit’s
  reserve. Vehicle commanders select and prepare alternate and supplemental firing positions and routes to and
  from them. If a perimeter has several mounted avenues of approach leading to it, units may elect to hold
  combat vehicles in hide positions until an enemy force approaches. Units prepare routes, firing positions, and
  range cards for all positions. Small-unit leaders must ensure that vehicles do not destroy communications
  wires when vehicles move from one position to another.
  9-68. The need to hold or protect terrain features—such as bridges, airfields, or landing zones—from enemy
  observation and fires may restrict the positioning of units within a perimeter. These factors, as well as the
  inability to achieve depth, make a perimeter defense vulnerable to penetration by heavy enemy forces. Units
  reduce these vulnerabilities by—
  * Developing information collection plans that provide early warning.
  * Positioning anti-armor systems on restrictive terrain to concentrate fires on armor approaches.
  * Providing as much depth as the diameter of the perimeter to allow the proper placement of security
  elements and the reserve and the designation of secondary sectors of fire for anti-armor weapons.
  * Constructing obstacles to fix or block enemy forces, so friendly units can effectively engage them.
  * Using obscurants and military deception.
  9-69. If isolation from other friendly units drives commanders to form a perimeter, such as when conducting
  echelon support area security or when encircled, functional and multifunctional support and sustainment
  elements from other units may seek to take advantage of that perimeter’s protection. The base commander
  gives these elements defensive missions based on their capabilities within the base formed by the perimeter.
  Units coordinate and integrate any fire support provided from outside the perimeter into the overall defensive
  plan. This extra fire support conserves the ammunition of units within the perimeter.
  9-70. Units normally employ reconnaissance assets, such as a scout platoon, outside the perimeter to provide
  early warning. They augment perimeter security with squad sized or smaller observation posts forward of the
  perimeter provided and controlled by units positioned along that perimeter. These security elements position
  to observe avenues of approach. Patrols examine areas not under observation by stationary elements. Any
  security forces operating outside the perimeter coordinate their passage of lines into and out of the perimeter
  with the appropriate perimeter units.
  9-71. The reserve may be a designated unit or a provisional force organized from available personnel and
  equipment. The reserve forms a second line of defense behind the perimeter forces. Ideally, the reserve is
  mobile enough to react to enemy action along any part of the perimeter. Units position the reserve to block
  the most dangerous avenue of approach and assigns on-order positions on other critical avenues. They may
  task combat vehicles initially occupying firing positions on the perimeter with the mission of reinforcing the
  reserve.
  Control Measures
  9-72. Commanders, in a perimeter defense, use control measures to designate the trace of the perimeter, BPs,
  contact points, and lateral and forward boundaries. They can use engagement areas (EAs), target reference
  points (TRPs), final protective fires (FPFs), and principal direction of fire as fire control measures. Units
  designate checkpoints, contact points, passage points, and passage routes for use by local reconnaissance,
  surveillance, and security elements operating outside the perimeter boundary. Figure 9-11 illustrates some of
  these control measures.