The US have a huge territory, even near sprawling cities and the looong borders, and primary surveillance radars capable of seeing and tracking that kind of tar

The US have a huge territory, even near sprawling cities and the looong borders, and primary surveillance radars capable of seeing and tracking that kind of targets, even for military applications are not installed everywhere. You can't, too expensive in so many ways. You're not going to see those targets with long range military surveillance radars either, maybe in the first 10-20kms, maybe. That is the actual problem with drones. Easy to build, very hard to detect with actual infrastructure. Whomever finds a way to track them on the 'cheap' is gonna make a buck. Even if you made actual, existing radars, able to track them (which would be a tall-order) it won't be on the whole range and it won't be reliable, because some of those things maneuver like crazy. Say good bye to your clean 2d/3d-with-realistic-aerodynamics Kalman filters and say hi to very high rates of false alarm...