It was discussed during one of the practice sessions. Karen thought it was dynamically measuring ride height but an engineer tweeted in to tell them it was measuring ground speed.
Interesting if thats what it is! I am no F1 engineer but I can think of lots of easier more reliable ways to do that. Its clearly optical and they are looking for detail given they are lighting the surface. Wonder if we will see anyone else running a similar set up?
Ground speed sensor, also happens to measure slip angle. Sensing ground speed and slip angle based on wheel rotation is apparently not accurate enough with the new tyres.