@pgiulan@65dBnoise To measure all of the mechanisms on the rover, fiducials are installed all over the turret, on the end of the rover's arm, as well as on the tops of the steering actuators and on the top deck of the rover etc. The markings are a staple of machine vision systems that can serve two parallel purposes, to calibrate cameras and to calibrate various mechanisms on the rover. The one you refer to on Perseverance rover is one the smaller 'April Tags'.