GC: n
CT: The lidar (LIght Detection And Ranging) is a remote sensing instrument similar in principle to the radar but operating in the optical range. Depending on the desired measurement, lidar systems use various light-matter interactions such as Rayleigh, Mie and Raman scattering or fluorescence. Measurements of atmospheric ozone, temperature or aerosol are based on the first 3 processes. Generally, a lidar measurement consists in sending into the atmosphere a laser beam ; a small part of this laser radiation is scattered back to the ground, where it is collected by a telescope, detected by a photomultiplier tube and analysed by an electronic acquisition system. Range resolved measurements can be obtained using pulsed lasers. In order to measure the ozone vertical distribution, the Differential Absorption Laser technique (DIAL) is used. This technique requires the simultaneous emission of two laser beams characterised by a different ozone absorption cross-section.
S: Sparc – https://bit.ly/2T7NJKg (last access: 21 January 2019)
N: 1. Acronym from light detection and ranging.
2. Technique for determining the distance to an object by transmitting a laser beam, usually from an airplane, at the object and measuring the time the light takes to return to the transmitter.
3. The principle behind lidar is really quite simple. Shine a small light at a surface and measure the time it takes to return to its source. When you shine a torch on a surface what you are actually seeing is the light being reflected and returning to your retina. Light travels very fast – about 300,000 kilometres per second, 186,000 miles per second or 0.3 metres per nanosecond so turning a light on appears to be instantaneous. Of course, it’s not! The equipment required to measure this needs to operate extremely fast. Only with the advancements in modern computing technology has this become possible.
The actual calculation for measuring how far a returning light photon has travelled to and from an object is quite simple:
- Distance = (Speed of Light x Time of Flight) / 2
The lidar instrument fires rapid pulses of laser light at a surface, some at up to 150,000 pulses per second. A sensor on the instrument measures the amount of time it takes for each pulse to bounce back. Light moves at a constant and known speed so the lidar instrument can calculate the distance between itself and the target with high accuracy. By repeating this in quick succession the insturment builds up a complex ‘map’ of the surface it is measuring. With airborne lidar other data must be collected to ensure accuracy. As the sensor is moving height, location and orientation of the instrument must be included to determine the position of the laser pulse at the time of sending and the time of return. This extra information is crucial to the data’s integrity. With ground based lidar a single GPS location can be added for each location where the instrument is set up.
Generally there are two types of lidar detection methods. Direct energy detection, also known as incoherent, and Coherent detection. Coherent systems are best for Doppler or phase sensitive measurements and generally use Optical heterodyne detection. This allows them to operate at much lower power but has the expense of more complex transceiver requirements. In both types of lidar there are two main pulse models: micropulse and high-energy systems. Micropulse systems have developed as a result of more powerful computers with greater computational capabilities.
4. There are two basic types of lidar: airborne and terrestrial.
- With airborne lidar, the system is installed in either a fixed-wing aircraft or helicopter. The infrared laser light is emitted toward the ground and returned to the moving airborne lidar sensor. There are two types of airborne sensors: topographic and bathymetric.
- There are two main types of terrestrial lidar: mobile and static. In the case of mobile acquisition, the lidar system is mounted on a moving vehicle. In the case of static acquisition, the lidar system is typically mounted on a tripod or stationary device. Both lidar sensors consist of eye-safe lasers.
5. The first attempts to measure distance by light beams were made in the 1930s with searchlights that were used to study the structure of the atmosphere. In 1938, light pulses were used to determine the heights of clouds. After the invention of the laser in 1960, lidar was first done using airplanes as the platform for the laser beam. However, it was not until the arrival of commercially available Global Positioning System (GPS) equipment and inertial measurement units (IMUs) in the late 1980s that accurate lidar data were possible.
S: 1. LidarUK – https://bit.ly/2VUcWK9 (last access: 16 January 2019). 2. EncBrit – https://bit.ly/2RPxcNO (last access: 16 January 2019). 3. LidarUK – https://bit.ly/2VUcWK9 (last access: 17 January 2019). 4. ArcGis – https://bit.ly/2T5mBM9 (last access: 17 January 2019). 5. EncBrit – https://bit.ly/2RPxcNO (last access: 16 January 2019).
OV: Lidar, LIDAR.
S: TERMIUM PLUS – https://bit.ly/2HBghuz (last access: 21 January 2019)
SYN: laser infrared radar, light-based radar, laser radar, optical radar, lidar technique. (depending on field and context)
S: TERMIUM PLUS – https://bit.ly/2HBghuz (last access: 21 January 2019)
