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The US Joint Direct Attack Munition (JDam) is a $21,000 tail kit that converts conventional 1,000 lb or 2,000 lb bombs into smart weapons.
 The position of the target is loaded before take-off or in flight - either by the crew or automatically from the aircraft's targeting system.
The bomb's guidance system is bolstered by GPS satellite positioning.
 Before being released, the weapon also gets position and speed information from the aircraft so it "knows" where it is in relation to the target. After release, it steers itself down.
 If the GPS satellite signals are interrupted on the way down it falls back on the inertial guidance - determining where it is in relation to its last "known" position.
 These bombs can be used even in bad weather, and are claimed to land within 10 metres (32ft) of their target.
More than one can be dropped simultaneously by one aircraft against different targets.