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The method of insertion used by the Special Air Service depends on the mission, for example, an aggressive strike on a target is unlikely to require a covert infiltration and such a mission would use mobility troop or helicopters for insertion. When covert infiltrations are necessary one of the Special Air Service' most common infiltration methods is by parachute, this is where the men of Air Troop (sometimes known as Free Fall Troop) come in.
The men of Air Troop can parachute in from friendly air space and glide into enemy territory and drop in behind enemy defences whilst remaining undetected. Once on the ground the patrol (normally consisting of four-men) will often be separated, so they will all gather at a pre-determined RV point (rendezvous) before moving out on their patrol.
"The parachutist's personal equipment includes oxygen breathing kit, main controllable chute, reserve chute, helmet with headset, altimeter, plus equipment for his mission on the ground." (A-Z of the S.A.S. p.75)
The Special Air Service will normally parachute from the back of a C130 Hercules plane at heights up to "almost 11km above sea-level" (Fighting Skills of the S.A.S. p.168.) and use one of two insertion methods, HALO and HAHO.

HALO - High Altitude Low Opening

HALO is the most widely used method by the Special Air Service, the patrol jump from the plane at around 10,000metres and free fall to 760m where their parachute automatically opens and they land. The advantage with HALO is they parachute from so high up the plane is usually invisible to the eye and the patrol descends a long way in a very short time, therefore presenting less of a target to unfriendly enemy patrols. Equally as important is that it is easier for the patrol to stay together as their chutes open quite close to the ground, they don't drift as far from the landing zone (LZ), this is extra important as most insertions are done at night (Fighting Skills of the S.A.S. p.169.)

HAHO - High Altitude High Opening

HAHO jumps give the Special Air Service the ability to infiltrate behind enemy lines with total covertness. The trooper jumps from the plane at around 10,000m but opens his chute as soon as he's clear of the plane (around 8500m.) The patrol then glides to the ground taking around 70-80minutes to land but they can glide up to 30km using their GPS equipment to pin point the LZ. This means the enemy can't hear the plane and the plane does not necessarily need to be in enemy air space for the troopers to get behind the enemy lines.