A sensor on the aircraft records the time it takes to receive the backscattering of pulses , and that is used to compute distances to the forest canopy and to the soil beneath .
During a forest survey , an aircraft-borne lidar sweeps a beam that fires about 70000 pulses a second over the canopy . A sensor on the aircraft records the time it takes to receive the backscattering of pulses , and that is used to compute distances to the forest canopy and to the soil beneath .