The heavy use of aircraft to parachute supplies to U.S. troops in Afghanistan has led to many improvements in technique. The latest one is XCDS (extract container delivery system), which speeds up the containers (actually pallets) of cargo leaving an aircraft (most often a C-130). This is done by adding a small deployed parachute to the pallet which catches the wind as the pallet rolls towards the open rear door and then rapidly pulls the pallet out. Normally gravity rolls the pallet out. But with XCDS the pallets move more quickly and land closer together. XCDS is used for low level drops (at an altitude of 200-400 meters) and each pallet carries up to a ton of cargo.
Two years ago the U.S. Air Force delivered, via parachute, a record amount of cargo to remote locations in Afghanistan. Some 34,500 tons was dropped, thereby avoiding the risks (from ambush, mines, and roadside bombs) of sending in trucks. In areas with few good roads using truck transport is dangerous even if there are no Taliban about. The amount dropped by parachute in 2011 was larger than the total weight (27,500 tons) for an earlier four year period (2006-9). That changed drastically in 2010, when 27,454 tons were dropped. That was a record, now there's a new one. Last year the U.S. began withdrawing troops from Afghanistan and that reduced the cargo dropped.
XCDS is one of several recent improvements in air delivery of cargo that addressed the accuracy problem. Other solutions to this problem included developing more accurate low altitude parachutes and more expensive, but very accurate at any altitude, GPS guided parachutes.
Low altitude/low velocity cargo parachutes are quite accurate when delivered from aircraft (or helicopters) flying low (under 400 meters/1200 feet) and slow. This type of parachute was a timely development because, in Afghanistan, there was a growing crisis with supplying the troops. This is especially true because two years ago, more and more American troops arrived and were dispatched to remote bases and outposts. There were soon over 300 American bases that had to be supplied either by truck or by air. There weren't enough helicopters to do this, and it was often too dangerous to do it by road. So air drops are increasingly favored. But even here, there are problems that had to be taken care of.
While low level drops are preferred, if there is a danger of hostile ground fire, a high altitude drop must be used. This option is difficult if accuracy is needed (because of the presence of hostile forces or very rough terrain). Air dropped supplies have landed, on average, within 185 meters of the aim point when dropped from higher altitudes. To address the accuracy problems GPS guided pallets that can land within 50 meters of the aim point were developed. When greater accuracy is needed (or it has to be done at night), a GPS guided parachute rig is used.
There are still problems with GPS guided parachutes. The big one is the many mountain peaks and ridges often encountered around the drop zone. The GPS guided rigs go for the spot on the ground. The GPS sees only a straight line, between where the GPS chute was dropped and the GPS location down there. There is no way to detect and avoid any mountainous terrain that's in the way. Because of this, airdrop supervisors and pilots have to carefully plan the drops. There are several solutions to this in the works, including flight planning software that will calculate the optimum altitude and location for making a drop. There are still problems with unpredictable winds (that overwhelm the guided parachute's ability to compensate).
Before the development of GPS guided air drops a large percentage of air dropped supplies were lost, either by falling into enemy hands or into things that destroyed them (especially water). With the GPS delivery systems, it's possible to do night drops, which is preferred when you don't want to alert nearby enemy troops. Often, you can accurately drop pallets without the GPS systems, if you have a large flat drop zone, daylight, and calm winds. But if conditions are difficult, you now have GPS guided drops. Otherwise, a low level, day time drop from a C-130 will get the job done.