While UAVs have been successful for their persistence (ability to stay over an area, with its sensors operating, for hours at a time), even better systems have been developed. These are called RAID, and they are basically a large UAV sensor payload mounted in a tall tower, or in an aerostat (a kind of blimp) that is tethered to the ground (with power supplied via the tether.) The first three RAID systems (one aerostat and two towers) were sent to Afghanistan in 2003. Since then 24 more RAID systems have been shipped overseas, mostly to Iraq. These provide excellent security for American bases, as they serve as 24/7 UAVs. Hundreds of video cameras have also been deployed in and around American bases, providing a degree of security never before seen. Things like RAID, and all those vidcams, are the main reason there have been so few successful attacks on American bases, especially compared to earlier wars.