A laser instrument maintains the course of the positioned gantry module. A laser beam generator emitting a vertical beam is placed at the end of the experimental field. A laser beam catcher with photo diodes is mounted onto a positioned gantry module replaceable support. Course deviations were assessed in two cases: when laser, straight-course maintaining mechanism was mounted at the same end of the spar as positioning trundle; and when it was mounted at the middle of the spar. Furthermore, we estimated how course deviations vary when changing the interaxial distance of laser catcher photodiodes. An electromechanical transmitter was used in the tests. The gantry module is positioned by a special positioning trundle and is run by a 12 V electric motor through a worm self - braking reduction gear.