In the 90s I worked on hypersonic projectiles to intercept nuclear missiles. They were doing mach 6 out of the barrel and yes, the air around them was plasma. We had an on board control loop that used feedback from a LIDAR sensor. The window for the sensor had to be ruby so it wouldn’t melt.

The projectile was spun at 50 hz, so we got samples back at that rate. Even with the primitive chips at that time and a 50hz sample rate, we could control the projectile to within 1m of the target. This was with no ground communication and not even an inertial reference, so all control was in the frame of the projectile. My point is that while ground communication would have its uses, it’s not a fundamental problem with hypersonic projectiles. We can already get them where we want them to go.