Figure 3: A laser scanner emits pulses of laser light and records return time and intensity. Return time tells the distance between the aircraft and the surface. GPS satellites constantly track the latitude, longitude, and altitude of the aircraft. Using these xyz coordinates, the angle of the sensor, and the distance to the surface based on laser return time, a computer is able to determine the surface elevation. This image shows a whiskbroom scanner, which swings back and forth perpendicular to the flight direction to increase in the swath width. The readings will be in a zigzag pattern.
Image credit: U.S. Forest Service https://www.fs.fed.us/pnw/olympia/silv/lidar/