Light detection and ranging (LIDAR) is a remote sensing technology that uses pulses of lasers to measure characteristics of objects and landscapes that are far away from the device. LIDAR can be used to measure distance, speed, and rotation as well as chemical composition and concentration. The technology is often used to measure the speed of vehicles to enforce speed limits and can also be used to measure distances (at a crime scene, for example). LIDAR is also useful for surveying and terrain-contour mapping; its wide range of applications include agriculture, forestry, oceanography, archaeology, geology, and meteorology.
Like radar, LIDAR sends out pulses of electromagnetic radiation and measures the radiation that is reflected back to detect objects. The relatively high-energy, short-wavelength light (ultraviolet, visible, or near-infrared light) that LIDAR uses resolves images more clearly than the weaker, long-wavelength radio waves used by radar. In addition, lasers produce a tight beam of light that does not spread out as it travels, so LIDAR is much better than radar at targeting specific areas. For example, LIDAR can create a high-resolution map or accurately measure the composition of a small region in the atmosphere.
A LIDAR system onboard a plane in flight or an orbiting satellite can be used to collect data about the surface of Earth. The basic components of such a system would include a laser emitter–receiver scanning unit, which would emit, receive, and then process the pulses of light; and a global positioning system (GPS) and inertial measurement unit, which determine the location of the laser scanner (as well as the movement of the aircraft) to give a precise position. The laser scanner sends pulses of light to the ground and measures the time it takes for each pulse to reflect back to it. Because the speed of light is known, the time measurements can then be used to calculate the distance that the pulse traveled. This information, combined with the information about the location of the laser scanner at the time of each measurement, provides researchers with the data to create a 3-D map of Earth's surface.
In this case, the target object was the ground. However, the pulse of light may hit more than one surface on its way to the target, returning multiple data points (such as the height of a bridge or vegetation). There are methods to account for these multiple returns, such as choosing to use only the "last return," which would be the ground surface.
LIDAR can also be used to study the composition and structure of the atmosphere. By measuring changes in the wavelength of the light caused by phenomena such as scattering and absorption by molecules in the air, researchers can identify and measure concentrations of gases.