Light Detection and Ranging, more commonly referred to as LiDAR, is a remote sensing system in the geospatial intelligence (GEOINT) arsenal that is used to map terrain in high-resolution detail.
LiDAR sensors work like giant laser range finders, scanning the environment with millions of individual laser beams, each of which returns a precise measurement of its range to the target.
Aircraft-mounted LiDAR sensors have become an indispensable tool for mapping purposes. Terrestrial LiDAR sensors are used by engineers to characterize the built environment from both fixed tripod-mounted and mobile vehicle-mounted configurations. LiDAR has even been deployed to space for earth observation science missions. The technology can be designed to measure everything from aerosols in the atmosphere, to interior walls, to high-resolution terrain data on the ground.
It’s all about selecting the right laser tuned to bounce off of particular things.
Once processed with location information, these millions of range readings provide a high-resolution, three-dimensional depiction of the environment. The output from a LiDAR is a “point cloud” of laser returns, each point containing an accurate 3D position.
LiDAR processing moves the data from raw sensor measurements (Level 0), to geo-registered point cloud (Level 3), then on to derived products like digital elevation models (Level 4).
Level 3 point clouds provide an exquisitely detailed view of the terrain, and have great utility for understanding features like vegetation and vertical obstructions. Level 4 digital elevation models (DEM) provide the surface for line-of-sight measurements and slope calculations, as well as a base for draping imagery and other geospatial features.
GPS and IMU
LiDAR’s ability to collect geospatially accurate data depends upon the quality of the Global Positioning System and Inertial Measurement Unit (GPS-IMU) that is attached to the sensor. In airborne and mobile collection, in particular, having a high quality GPS-IMU matters. It not only tells the location of the sensor, but also its exact orientation—pitch, yaw, roll, angle of view—which is critical to collect geospatially accurate measurements.
Which Kind of Laser?
Lasers are described in terms of wavelength on the electromagnetic spectrum. Several kinds of lasers are used in LiDAR sensors depending on the application. Near-infrared lasers are widely used for airborne and terrestrial collection. Green lasers can penetrate water to measure bathymetry. Ultraviolet lasers are used to measure the atmosphere.
Wide Area Mapping and More
A common application of LiDAR is wide area mapping from an airborne platform. In the United States and other developed nations, civilian agencies commonly use this collection method for urban areas, coastal areas, and flood plains. Typical collection is at one-meter “post spacing,” with an elevation reading roughly every three feet. In military applications, this has come to be known as “human scale High Resolution 3D (HR3D) data.” It is the minimum resolution that allows mission planners to make essential measurements for ground warfighters to successfully determine mobility and visibility over complex and urban terrain.
LiDAR can also be locked on a target using a gimbal, enabling a more dense collection of LiDAR points from every angle. This can be used to characterize a facility, including all of its facades at a higher resolution, or even to peek through foliage and observe underneath it. But lasers do not go through leaves; they peak through holes in the canopy. The more dense the canopy, the better it is to use a true foliage penetrator such as radar.
Newest LiDAR Technology
In recent years, a new breed of LiDAR has emerged, enabling collection from much higher altitudes and at higher resolutions. Traditional LiDAR is sometimes referred to as “linear mode,” where individual laser beams are used to measure range. The term “Geiger mode” is used to describe this new breed of LiDAR, where the sensors do not observe individual laser beam returns, but rather the returns of individual photons. This allows sensors to collect a much higher density of measurements with far less power from higher altitudes, and over larger geographies.
There are several different types of Geiger mode, including Flash, Avalanche Photo Diode (APD), and Photon Counting LiDARs. NASA, DARPA, NGA, the Air Force Research Lab, and the Army Geospatial Center have all experimented with this new breed of LiDAR, and operational experiments have been conducted with several different sensors and platforms.
This technology offers the possibility of someday collecting human-scale HR3D terrain data over massive geographic swaths from as far away as outer space.