text: FM 7-100.1________________________________________________________________________________
  at the operational level play a major role in gathering, integrating, and
  disseminating information to tactical units. Continuous surveillance of
  surrounding airspace ensures current data on the enemy air situation.
  9-113. The OPFOR uses electronic and electro-optical means and visual
  observation to conduct air surveillance. The air defense forces have a passive
  early warning system based on a combination of radar systems and observers
  that will serve them well in the early stages of combat operations against
  even the most modern opponents. In later stages, the OPFOR accepts that it
  will either lose many of its airborne and/or ground-based early warning systems
  or at least not be able to use them with optimum effectiveness. When technical
  early warning systems are not available, the OPFOR is prepared to continue air
  defense operations while relying primarily on observers for air surveillance.
  Radars
  9-114. Air defense radars fall into two general categories: surveillance and
  fire control. The category of surveillance radars includes early warning,
  target-acquisition, and height-finding radars. Some fire control radars also
  have limited target-acquisition capability.
  9-115. National-, operational-, and tactical-level surveillance and target-
  acquisition radars detect and monitor targets. The radars then provide the
  necessary data for engagement. Radars work as part of the IADS rather than as
  separate units. Air defense planners at all levels integrate radars into an overall
  system of coverage.
  9-116. Operational-level early warning units deploy their radars as close as
  possible to the supported ground force unit’s battle line, in order to detect
  enemy aircraft at maximum radar range. National-level early warning
  units can establish a second line of radar posts behind the first line of op-
  erational-level systems, in order to give depth. The types and capabilities
  of the systems employed and whether the battlefield is linear or nonlinear
  will determine the distances between lines.
  9-117. Both national- and operational-level air defense forces maintain
  reserves to expand coverage as the operation develops, to replace casualties,
  or to establish a new line of radar posts. When the OPFOR is preparing for
  offensive operations, operational-level early warning radars remain inactive
  as part of C3D, and the national-level reserve radars deploy forward.
  9-118. In many cases, long-range surveillance radars in early warning units
  at the national and operational levels can gather target information long be-
  fore the enemy aircraft come into the range of air defense firing units. These
  radar units pass preliminary target data to air defense commanders and
  their firing batteries. Commanders then select the weapon system that can
  best engage a given target. The early warning units also pass warning infor-
  mation to operational and tactical maneuver units and air defense firing units.
  9-119. SAM systems and AA guns may have their own radars in the firing
  positions or mounted on self-propelled systems. However, higher-level radars
  can gather the information without unnecessarily exposing the air defense
  firing units to detection and subsequent neutralization by enemy forces. This
  practice reduces the vulnerability of battery radars and radar-equipped gun