Components of a Lidar System
Introduction
A complete lidar system is made up of several components. All of the components work together to generate, record and georeference the data. The main components are:
- Lidar Source, Detector and Scanning Mechanism
- Timing Electronics
- Global Positioning System (GPS)
- Inertia Measurement Unit (IMU)
- Computer
The Optech Galaxy Lidar System Image Credit: OptechLidar System
Laser source and laser detector
The laser source generates the energy of the pulses. Near infrared wavelengths are used for most terrestrial lidar applications. Blue green wavelengths are used for bathymetry mapping as these wavelengths can penetrate water up to 40 meters depending on the water clarity. The lasers used in lidar system are low energy and are eye-safe. The laser detector or receiver detects the laser light pulses that are reflected back from the target objects. The scanning mechanism is designed to generate a consistent stream of laser pulses. The laser pulses are reflected off of a mirror (either rotating or scanning).
Timing Electronics
The timing electronics records the exact time the laser pulse leaves and returns to the scanner. The timing electronics have to be very precise to produce accurate data. Each pulse sent out can have up to multiple returns as it reflects off of objects on the surface. Each of the returns must be precisely timed to ensure an accurate measurements for each point.
Global Positioning System (GPS)
The Global Positioning System (GPS) records the precise X,Y,Z location of the scanner. To improve the accuracy most lidar systems use a fixed ground reference station or a Continuous Operating Reference Station (CORS). The data from the ground station or CORS has a known location ans is used to correct and improve the data collected by the sensor. The GPS data is later post-processed and the precise position of the sensor at approximately every second throughout the flight can be calculated, typically with minimal error (3 to 4 cm). The GPS together with the IMU (see below) allow for the direct georeferencing of the points.
Inertia Measurement Unit (IMU)
The Inertia Measurement Unit (IMU) Global Positioning System (GPS) records the precise X,Y,Z location of the scanner. The IMU contains an accelerometer, gyroscope, and magnetometer sensors that measure the velocity, orientation, and gravitational forces. The IMB constantly records the pitch, roll, and yaw of the aircraft. This data is used to determine the precise angle and location of the lidar system to ensure the distances to surfaces are correctly calculated.
Computer
A reliable computer system is required to make sure that all of the individual components of the system are working properly. The computer integrates the data from the laser system, the GPS and the IMU to produce the lidar point data. Most lidar systems include an integrated computer system along with software for the flight planning, integrating the IMG/GPS data and converting the sensor data into actual X,Y,Z coordinates.