Lidar

Lidar is a sophisticated remote sensing technology used to determine distances by measuring light. The name is an acronym for light detection and ranging. It functions by targeting an object or surface with a laser beam. The system then measures the time it takes for the reflected light to return to a receiver.

This process allows for the creation of highly accurate 3D models of various environments. Lidar is vital because it provides high-resolution data for many scientific and industrial fields. It can map everything from complex forest structures to the deep ocean floor.

The mechanism of lidar relies on the constant speed of light. To find the distance to a target, the system uses a specific formula. It calculates the distance by multiplying the speed of light by the time the light spent traveling to the object and back, then dividing by two.

The light used can be in the ultraviolet, visible, or near-infrared spectrum. Instead of simple reflection, lidar often relies on backscattering. This occurs when light hits a surface and scatters back toward the sensor. Different types of scattering, such as Rayleigh, Mie, Raman, and fluorescence, are used depending on the goal.

Lidar systems are categorized by how they detect energy and how they pulse. There are two main detection schemes: incoherent and coherent detection. Incoherent detection, or direct energy detection, measures changes in the amplitude of reflected light. Coherent detection is more complex and measures Doppler shifts, which are changes in the phase of the light.

Systems also use different pulse models. Micropulse systems use intermittent bursts of energy, often around one microjoule. These are frequently "eye-safe," meaning they can be used without special safety precautions. High-power systems are used for atmospheric research to measure things like cloud density, temperature, and wind.

Different applications require specific hardware and wavelengths. For example, airborne topographic mapping often uses 1,064 nm diode-pumped YAG lasers. However, bathymetric lidar, which maps underwater depths, uses 532 nm lasers. This is because the 532 nm wavelength penetrates water with much less attenuation, or loss of signal, than the 1,064 nm wavelength.

To scan an area, the laser must be directed. Some systems use a rotating mirror to scan the scene. Others use microelectromechanical mirrors, known as MEMS, which are tiny mirrors that can reorient a laser beam.

To ensure accuracy, sensors on aircraft or satellites often use a Global Positioning System (GPS) and an inertial measurement unit (IMU) to track their exact position.

The concept of using light to probe the atmosphere was first envisioned by E. H. Synge in 1930. He imagined using powerful searchlights for this purpose. The modern era of lidar began in 1961 under Malcolm Stitch at the Hughes Aircraft Company. They developed a system called "Colidar," which stood for coherent light detecting and ranging.

This system was designed for satellite tracking shortly after the invention of the laser. The first practical terrestrial tool was the Colidar Mark II in 1963. This was a large, rifle-like rangefinder used for military targeting. It had a range of 11 km and an accuracy of 4.5 m.

Lidar has reached incredible milestones in space exploration. In 1971, the Apollo 15 mission used a laser altimeter to map the surface of the Moon. This helped the general public understand the precision of the technology. More recently, the Ingenuity helicopter used lidar for navigation during its flights on Mars.

NASA also views lidar as a key technology for future missions. It will help enable the safe, autonomous landing of both robotic and crewed vehicles on the Moon.

Today, lidar is integrated into many modern technologies. Autonomous cars use lidar sensors to navigate complex streets and avoid obstacles.

In agriculture, lidar can be used to analyze the yield rates of different fields.

It is also essential for atmospheric physics and meteorology. Scientists use it to study clouds, aerosols, and even single molecules in the air. From mapping ancient archaeological sites to tracking passengers in airports, lidar continues to expand its reach.