From time to time, the question comes up what have I to order for Big Sonic-Ski system with 4 sensors and what are the benefits of it. Assuming that a MOBA-matic II system with 3 sensors already exists, you´ll need the following components to extend your existing levelling system from 3 to 4 ultra-sonic sensors:

The sensors are positioned as follows:
Position the middle sensor (2) approximately level with the auger.
With same distance to middle sensor (2), position the two outer sensors (1/3) on the outer module elements.
Position the fourth sensor (4) centrally between the two first sensors in moving direction (F).

If the reference layer has already approx. 6 meters waves, these are almost copied 1 to 1 with a 13m Big-Ski and 3 sensors. The additional fourth sensor is placed so that it measures the opposite of the sine wave to equalize it.

The benefits are as follows:
Elemination of 4 to 6 meter waves
An more even smoother surface
Material saving because of smoother surface