The elapsed time between emission and arrival is used to compute the distance between the sensor and the target by dividing the recorded time by two and multiplying it by the group velocity of the light pulse (approximately 3 x 10 8 m/s).
The short laser pulse (typically a few ns in duration) travels from the sensor through the atmosphere and is then reflected by one or more objects on the ground that are illuminated by the laser beam. The airborne sensors are typically mounted on a fixed-wing aircraft (usually augmented with simultaneous digital imagery and aimed at large areas) or a helicopter (typically used for smaller areas with high-resolution mapping). Most commercial airborne sensors are based on the LiDAR principle of pulse round-trip time measurement. This section mainly discusses airborne applications of LiDAR systems. These collection techniques are popular within the surveying and engineering communities since they can enable the accurate development of railroads, roadways, bridges, buildings, breakwaters, and other shoreline structures. LiDAR data is also collected from ground-based stationary and mobile platforms for street mapping and autonomous driving applications. LiDAR systems are often placed in aircraft where data can be rapidly collected over large areas. LiDAR has become an established method for generating dense and accurate elevation data across landscapes, shallow-water areas, and project sites. Furthermore, this technique can generate high-resolution 3D topographic surface information more rapidly due to its ability to penetrate vegetation. Compared to traditional photogrammetric approaches, LiDAR is less sensitive to the current weather, time of the year, or time of the day during which data is collected. LiDAR can generate a dense, three-dimensional (3D), geo-referenced point cloud, i.e., a set of data points in space, for the reflective terrain landscape when combined with a Global Positioning System (GPS) and an Inertial Measurement Unit (IMU). By measuring the round-trip travel time of the emitted laser pulses, the LiDAR system can determine the distance between the sensor and the mapped terrain. The pulses encounter the terrain and a portion of the laser energy is reflected back to a sensor located near the source.
In this technique, a laser source emits pulses that are directed towards the target of interest, such as a terrain landscape. LiDAR is an active remote sensing technique that is similar to RADAR but, instead of using radio waves as a radiation source, it uses laser pulses.
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