The JSTARS aircraft which has been around for 20+ years is kind of interesting as a forerunner. It can track 600 ground targets (vehicles) over 50,000 sq km using radar. This was 20 years ago, and that is the unclassified portion. We don't really know how it's improved since then, or how many simultaneous targets it can actually track in 2012.
During the DC Sniper incident, they were debating posse comitatus and whether or not to bring one of these craft into action to track cars in the vicinity of a reported sniper attack.
During the DC Sniper incident, they were debating posse comitatus and whether or not to bring one of these craft into action to track cars in the vicinity of a reported sniper attack.