In Call of  Duty: Modern Warfare 3 for the PC, PS3, Wii, and Xbox 360, you unlock the different air support in Survival Mode as you progress in levels.
Here is the air support you can unlock in Survival Mode, and what level must reach to unlock them.
Predator Missile – $2500
Air Strike – $2,500, unlocked at level 4
Delta Squad – $3,000, unlocked at level 14
Riot Shield Squad – $5,000, unlocked at level 21
