A surface-to-air missile battery was an upgrade for the component tower. It was a major part of GDI defenses during the Second Tiberium War. The SAM Upgrade provided GDI bases with effective anti-air cover.
These were constructed through out GDI bases as their main means of anti-air defense. They had to be protected from ground threats however. The new AA Battery replaced the SAM upgrade, as it was cheaper and easier to maintain.