The emitted laser pulse has a certain footprint size when it reaches the earth surface which depends on the sensor specific beam divergency. If a laser pulse hits the vegetation a part of the signal may be reflected by the top of the vegetation while an other fraction of this pulse may penetrate the vegetation and gets reflected by the terrain. The runtime of the first returning signal fraction can be measured separately from the last returning one. This yields to a distinction between first pulse data which contains mainly vegetation surface information, and last pulse data which contains more information on the terrain.
Due to a filtering process that separates the non terrain laser hits from the last pulse data, airborne laser scanner data enables the generation of high resolution accurate digital terrain models (DTM).