S-400 is an upgrade of the older S-300. Very similar, although the former may require internet connectivity and that's probably what worries NATO. There are a

S-400 is an upgrade of the older S-300. Very similar, although the former may require internet connectivity and that's probably what worries NATO. There are a myriad of versions of the S-300, from the original SA-10 introduced in the late 70's to the newest MPU version.