LiCAS sensing units are located inside the RTSR
(Rapid Tunnel Reference Surveyor) which is a "train" composed of six "cars".
The distance between the centres of neighbouring cars is 4.5m.
Each car is equipped with 4 CCD cameras and two beam splitters (BS) constituting
the straightness monitor.
The straightness monitor measures the transverse translation
(Tx,Ty) and transverse rotation (Rx,Ry) with respect to a
z axis defined by the laser beam passing through all cars in a vacuum pipe.
The laser beam is reflected back using the retro-reflector (RR) located in the last car,
illuminating the upper CCD cameras of the straightness monitors.
6 FSI lines placed in the same vacuum pipe between each pair of cars are responsible for
the distance measurement along the z axis (Tz).
In addition a clinometer located on each car
provides a measurement of rotation around the z axis (Rz).
