It's mostly GPS and inertial navigation. Probably integrated together with some kind of Kalman filter. The very last few seconds add in data from a radar alti

It's mostly GPS and inertial navigation. Probably integrated together with some kind of Kalman filter. The very last few seconds add in data from a radar altimeter. Per the post-launch press conference, this time they added some special paint to the landing pad to give a better radar return. I imagine the steel deck of the ASDS already gives a pretty good radar return.