LiDAR systems are composed of three main components: the scanner, the laser, and the GPS receiver. Other elements such as the photodetector and optics are also essential for data collection and analysis. Organizations use helicopters, drones, and airplanes to acquire LiDAR data. The laser source generates the energy of the pulses.
For most terrestrial LiDAR applications, near-infrared wavelengths are used. Blue-green wavelengths are used for bathymetric mapping since they can penetrate water up to 40 meters, depending on the clarity of the water. The lasers used in LiDAR systems are low-energy and safe for the eyes. The laser detector or receiver detects the pulses of laser light that are reflected from target objects. The scanning mechanism is designed to generate a constant flow of laser pulses.
The laser pulses are reflected in a mirror that is either rotating or scanned. The timing electronics record the exact time when the laser pulse leaves and returns to the scanner. Timing electronics must be very precise to produce accurate data. Each pulse sent can have multiple returns as it is reflected by objects on the surface. Each of the returns must be precisely timed to ensure an accurate measurement for each point.
The Global Positioning System (GPS) records the precise X, Y, Z location of the scanner. To improve accuracy, most LiDAR systems use a fixed ground reference station or a continuously operating reference station (CORS).The data from the ground station or CORS has a known location and is used to correct and improve the data collected by the sensor. Subsequently, the GPS data is further processed and the precise position of the sensor can be calculated approximately every second during the flight, usually with a minimum error (3 to 4 cm). The GPS together with the Inertia Measurement Unit (IMU) allow direct georeferencing of the points. The IMU contains accelerometer, gyroscope, and magnetometer sensors that measure speed, orientation, and gravitational forces.
The IMU constantly records the pitch, roll, and orientation of the aircraft. This data is used to determine the precise angle and location of the LiDAR system to ensure that distances to surfaces are calculated correctly. Airplanes and helicopters are commonly used platforms for acquiring LiDAR data in large areas. Two types of LiDAR are topographic and bathymetric. Topographic LiDAR usually uses a near-infrared laser to map land while bathymetric LiDAR uses green light that penetrates water to measure elevations of seabed and riverbeds.
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