Most airborne LIDAR systems are made up of the LIDAR sensor, a GPS receiver, an inertial measurement unit (IMU), an onboard computer and data storage devices.
The LIDAR system pulses a laser beam onto a mirror and projects it downward from an airborne platform, usually a fixed-wing airplane or a helicopter. The beam is scanned from side to side as the aircraft flies over the survey area, measuring between 20,000 to 150,000 points per second. When the laser beam hits an object it is reflected back to the mirror. The time interval between the pulse leaving the airborne platform and its return to the LIDAR sensor is measured. Following the LiDAR mission, the data is post-processed and the LIDAR time-interval measurements from the pulse being sent to the return pulse being received are converted to distance and corrected to the aircraft’s onboard GPS receiver, IMU, and ground-based GPS stations. The GPS accurately determines the aircraft’s position in terms of latitude, longitude and altitude which are also know as the x, y and z coordinates. The LiDAR sensor collects a huge amount of data and a single survey can easily generate billions of points totalling several terabytes.
An IMU is used to determine the attitude of the aircraft as the sensor is taking measurements. These are recorded in degrees to an extremely high accuracy in all three dimensions as roll, pitch and yaw – the vertical and horizontal movements of the aircraft in flight. From these two datasets the laser beam’s exit geometry is calculated relative to the Earth’s surface coordinates to a very high accuracy.
The initial LiDAR data can be further enhanced using additional post-processing, some of which can be automated and some are manual. Further processing utilises the multiple return signals from each laser pulse. By evaluating the time differences between the multiple return signals the post-processing system can differentiate between buildings and other structures, vegetation, and the ground surface. This process is used to remove surface features to produce bare earth models (DTM) and other enhanced data products.
It is also possible to do selective feature extraction, for example, the removal of trees and other vegetation to leave just the buildings.
source credit: LiDAR UK