Airborne forces are ground combat units carried by aircraft and airdropped into battle zones, typically by parachute drop or air assault. Parachute-qualified infantry and support personnel serving in airborne units are also known as paratroopers. The main advantage of airborne forces is their ability to be deployed into combat zones without land passage, as long as the airspace is accessible. Formations of airborne forces are limited only by the number and size of their transport aircraft; a sizeable force can appear "out of the sky" behind enemy lines in merely hours if not minutes, an action known as vertical envelopment. Airborne forces typically lack enough supplies for prolonged combat, so they are utilized for establishing an airhead to bring in larger forces before carrying out other combat objectives. Some infantry fighting vehicles have also been modified for paradropping with infantry to provide heavier firepower. Due to their necessary slow descent, paratroopers are vulnerable to anti-air fire from ground defenders, but combat jumps are at low altitude (400–500 ft) and normally carried out a short distance away (or directly on if lightly...