Absolute control of the scale factor in GP2 laser gyroscope: Toward a ground based detector of the lense-thirring effect

The sensitivity achieved by large laser gyroscopes opens the perspective of observing in a ground laboratories very thin relativistic effect related to the Earth rotating mass (gravitomagnetic effect or Lense-Thirring effect). The required accuracy asks for a strict control of the ring cavity geometry. Here we present a control procedure that can be applied in order to solve this task.