Explain base defense placement.

2.3-Campaign-Design.md

@@ -54,4 +54,42 @@ To specify that a control point should be owned by the player when the campaign
 
 ### Front lines
 
-Front lines are defined by placing a CJTF Blue M-113 at an airfield and defining a route to the connected control point. The front line will follow the _waypoints_ of the vehicle group (not the path, so using on-road waypoints will not cause the front line to follow roads automatically), so waypoints should be placed at each location that the front line should turn to avoid terrain. The route should end near the airfield that the front line connects to. The beginning and ending waypoints will automatically snap to the nearest airfields when the campaign is generated.
+Front lines are defined by placing a CJTF Blue M-113 at an airfield and defining a route to the connected control point. The front line will follow the _waypoints_ of the vehicle group (not the path, so using on-road waypoints will not cause the front line to follow roads automatically), so waypoints should be placed at each location that the front line should turn to avoid terrain. The route should end near the airfield that the front line connects to. The beginning and ending waypoints will automatically snap to the nearest airfields when the campaign is generated.
+
+### Base defenses
+
+Airbases will randomly generate a set of base defenses at the beginning of the campaign. The campaign generator currently uses the following algorithm when deciding what base defenses to spawn:
+
+* Spawn 1 EWR
+* Spawn 1 garrison
+
+Then the generator will create between 2 and 5 (evenly weighted) additional defenses with the following probabilities:
+
+* The first has a 1/2 chance of being a SAM, 1/6 chance of being a SHORAD, and 2/6 chance of being a garrison.
+* The second through fifth groups habe a 1/3 chance of being a SHORAD and a 2/3 chance of being a garrison.
+
+This means that if the campaign designer wishes to avoid randomly positioned base defenses, they must define at least:
+
+* 1 EWR location
+* 7 air defense locations (SAMs and SHORADs differ by unit type but share placement locations)
+* 6 garrison locations
+
+More can be placed to allow more variety in each campaign. When generating base defenses, locations will be selected from the presets at random. If no more preset locations remain for the given type, the generator will fall back to random positioning. Random positioning can result in units being placed under water, on buildings, on steep mountain slopes, etc, so it should not be relied upon.
+
+Base defense locations will automatically be associated with the closest control point to them.
+
+#### EWR
+
+To define a potential EWR location, place a CJTF Blue EWR 55G6.
+
+One EWR will always be placed at each control point.
+
+#### Air defenses
+
+To define a potential air defense location, place a CJTF Blue S-300PS SR 64H6E within a 2 NM radius of the control point. Units placed outside that radius will be regular objective locations rather than base defenses.
+
+The campaign generator will use these locations to spawn all ranges of air defenses, from an SA-10 to AAA. Precisely what is spawned depends on whether the generator uses the location for a SAM or SHROAD, the faction's available equipment, and random selection of that equipment during campaign generation.
+
+#### Garrisons
+
+Garrisons are ground units placed at the airbase that defend against the encroaching enemy front line. To define potential locations for garrison groups, place a CJTF Blue SA-19 Tunguska 2S6.