The system consists of two measuring units which are mounted on the roof, on the left and right side. This arrangement enables the detection of poles on both sides of the track.

Each unit consists of two laser distance measuring sensors which measure the vertical distance between the roof and the steady arms. When the laser beam hits a steady arm, the light is reflected and the receiving unit assesses the reflection. By analyzing the phase shift between outgoing and reflected light, the system determines the distance to the object. Each sensor performs approximately 62,000 distance measurements per second. So, when travelling at a speed of 300 km/h, the measuring distance is in the mm range.

To recognize the object as a steady arm and therefore as a pole, the following conditions have to be met:

• Both distance sensors of each unit measure the same distance.
• This distance is located within a given range.
• The distance remains constant for the given width of the steady arm.

When these conditions are fulfilled, the event is recognized as a pole and stored in the database together with the other measured data.

To assure precise measuring results, the two sensor units are temperature stabilized. The system is not influenced by ambient light. The laser beams are considered eye safe for distances greater than 2.5 m. As a safety precaution the scanner covers are closed at travelling speeds below 3 km/h.