Sketch of the basic geometry of the GLAO scheme with the six laser beacons serving as reference for the adaptive optics. Located at a finite height above the telescope, the light from the laser beacons travels down on a cone through the Earth’s atmosphere. Turbulent layers located close to the ground will be sampled equally by all three beacons on each side, while on the high layers the footprints are completely separated and smaller than the illumination from the science object at infinity. The wavefront measurements from the high layers will average out over time, and the adaptive optics correction therefore is strongest for turbulent layers close to the ground.