The U.S. Air Force is testing a new system (AMSTE, or Affordable Moving Surface Target Engagement) that allows the updatingof the aim point of GPS smart bombs as they fall. This enables JDAM (and other types of smart bombs) to be used against moving target. The system uses the more accurate differential GPS (which gets signals from ground stations as well as satellites). Differential GPS is accurate to within four inches of the aim point. As a practical matter, this has consistently brought the one ton bomb down within 20-30 feet of the target. This is close enough for a one ton bomb to destroy most moving targets (like missile launchers or electronic equipment vans.)
The system uses two or more JSTARS aircraft to precisely locate the ground target. A bomber would drop the smart bomb and then get update data from the JSTARS to update the bomb's aim point. There are also plans to put a ground search radar similar to JSTARS into a large UAV like Global Hawk.
The system could be ready for combat use in a year, but more extensive testing and equipment refinement will probably mean AMSTE won't be in action for two or three years.