The initial training to become a special forces operator is 3.5 years, the longest anywhere, but the training is also a continuous process. In the special forces, the members are imparted both basic and advanced training. They are taught specialised modes of infiltration and exfiltration, either by air (combat freefall) or sea (combat diving). Some trainees return to PTS to undergo the free-fall course, which requires at least 50 jumps from altitudes up to to pass. Both High Altitude Low Opening (HALO) and High Altitude High Opening (HAHO) techniques are learned. The ability to use the HAHO method and specially designed maneuverable parachutes called HAPPS (High Altitude Parachute Penetration System)/AMX-310 to conduct stealth insertions over distances up to is also perfected.