LiDAR stands for light detection and ranging and it is a technology that appeared in the 1960s, shortly after the invention of the laser. In this post we will explain its working principle, the main types of LiDARs and the most important applications.
LiDAR technology consists in sending a laser beam to the target and measuring the reflected light with a photodetector to determine the distance to the target and this way generate a precise map of the surrounding environment. The main advantages of LiDAR are that it can provide precise position over large areas and that it is fast, making possible to collect information with a speed and a degree of detail that it would not be possible otherwise.
LiDAR works based on two sets of measurements. The first piece of information is the position (the scanning system can be fixed or moving) and pointing direction of the laser for each measurement. The second one is the distance, which can be measured through different approaches.
One of them consists in employing a pulsed laser and measure the time of flight, that is, the time it takes the light to reach a surface and return to the source, therefore:
D = c · ΔT/2
where D is the distance to the object, c is the speed of light and ΔT is the time of flight. This system is only limited by the requirement of a return signal, meaning powerful lasers have to be employed when distances of several kilometers have to be measured.
Another approach for calculating distances is based on measuring the phase. In this case, an amplitude modulated continuous waveform laser (AMCW) is used. The phase shift between the incident and returning light is employed to deduce the distance:
D = c/2 · ΔΦ/(2·π·fM)
where D and c are again the distance to the object and the speed of light, respectively; ΔΦ is the phase shift and fM is the modulation frequency of the amplitude of the signal. The main disadvantage of this approach is that the range that can be measured without ambiguity is short, typically around 100 m.
A third approach would be the frequency modulated continuous wave (FMCW), where the emitted instantaneous optical frequency is periodically modified, usually by varying the power applied to the source. A typical depth resolution for the first two approaches is 1 cm, while in the last case is 0.1 cm.
There are two main types of LiDAR: airborne and terrestrial. Airborne LiDARs are usually installed in an aircraft or a helicopter pointing downwards towards the ground and normally they measure an angular range of 180º. Among airborne LiDARs there are two subtypes: topographic and bathymetric. Topographic LiDARs are employed in forestry, urban planning or landscape ecology to examine the surface while bathymetric LiDARs are used for measuring the elevation and depth of water in coastlines, shores or banks.
On the other hand, terrestrial LiDARs can be 1-D or 2-D, covering 180º or 360º in this last case. They are utilized to characterize in great detail facilities and infrastructures and for creating 3D models of places. There are two basic subtypes: mobile or static, depending if the scanning system is mounted on a moving vehicle or on a fixed tripod. Mobile LiDARs are used in roads, highways, railways; while static LiDARs are utilized in engineering, archaeology or mining.
LiDAR systems have gained popularity in the last years due to its presence in the autonomous vehicle. LiDAR enables to map the environment surrounding the car faster and more precisely than the sonar (sound navigation and ranging) or the radar (radio detection and ranging, based on the use of microwaves). LiDAR is considered the “eyes” of the autonomous vehicle, being responsible for avoiding hitting pedestrians, obstacles or other vehicles. However, LiDAR technology still has pending issues such as its operation under fog, rain or snow (the detection is degraded by the absorption and scattering caused by water droplets) or the need to further increase its resolution to detect small obstacles in the road, so some manufacturers prefer a vision-based system with cameras for the same purpose.
In conclusion, LiDAR technology allows to obtain an image of the surrounding environment through the use of light in many different disciplines such as agriculture, archaeology, robotics, forestry or autonomous vehicles, and it is expected that its utilization will rise exponentially in the near future.
Written by J.J. Imas
Bibliography
[3] Harrap, R.; publication, M.L.-N.; 2010, An overview of LIDAR: collection to application.
[5] What Lidar Is and Why It’s Important for Autonomous Vehicles