LiDAR - Light Detection and Ranging
Table of Contents
Introduction to the Basics of Light Detection and Ranging (LiDAR)
LiDAR or laser-based detection and range finding is a broad class of remote sensing technologies supporting a wide diversity of two- and three-dimensional spatial metrologies.
Like Radar but using laser illumination rather than radio for ranging, the technique is widely deployed. Today even contemporary consumer products such as ‘tape-free’ tape measures use LiDAR, it is even found in the iPad Pro to support augmented reality applications.
It is a technology widely researched for use in the automotive sector as a key safety enabler for autonomous driving. But beyond this, LiDAR is a well-established technique applied across many industrial, scientific, and engineering contexts. It is increasingly important for developing digital twins of cities and infrastructures as well as aiding the industry 4.0 revolution.
It is within the industrial and scientific sectors where there is an increasing need for high performance data acquisition system, designed in convenient form factors that enable LiDAR developers to maximize the operational envelope of their products; enhancing quality, speed and economics of their designs.
LiDAR Applications Overview
LiDAR supports a diverse set of applications, the following (non-exhaustive list) highlights just five variants:
Topographic LiDAR is used to scan terrain, wherein the laser pulses scanning the earth’s surface can provide precise surveys of various characteristics of the scanned area. The rise, fall and elevation of features such as tree cover, solid built structures, and native geomorphology – even vegetation density are all potentially uncovered by a laser scan.
Bathymetric LiDAR is used to scan bodies of water, primarily along shorelines or waterways and is often combined with topographic surveys. A bathymetric sensor comprises many of the same base components of topographic systems but exploit a special short wavelength green laser illuminator able to penetrate the body of water. Data processing needs may differ for bathymetry, however, when combined with topographic data, these units can illicit shorelines and elevations in considerable detail aiding coastal engineering, hydrography, and marine science
Differential Absorption LiDAR (DIAL) enables the measurement of gas concentrations in the atmosphere, and specifically, is used to monitor ozone levels or particulate pollution. DIAL systems may be either ground-based or airborne. DIAL exploits tunable lasers sources to produce two wavelengths of pulses that record light intensity from the peak of gas absorption line, and another obtained from a low-absorption region.
Wind movement LiDAR is designed to ease wind analysis which is a naturally challenging given the speed of directional changes. Advanced doppler systems provide 360-degree monitoring of wind condition and help understand turbulence, wind speed and wind shear dynamics derived from the complex datasets arising.
Raman LiDAR is a terrestrial system used for detecting and measuring the levels of water vapor and key aerosols within the atmosphere. Conventional LiDARs derive data from the backscatter signal amplitude (or intensity) from reflected laser pulses. Raman LiDAR goes further and detects characteristic molecular level shifts in the backscatter profile caused by atomic-level interplay of incident light causing characteristic Raman frequency shifting (RFS). The Raman inelastic scattering profiles can characterize specific molecules.
These applications, whilst using broadly similar detection methods differ in terms of the wavelength and power of laser illumination pulse, the optical system and quality of the returned signal. All are factors influencing the detection and data processing electronics needed. The rest of this short note is concerned with the basic operating principles and the physics of LiDAR.
SP Devices’ offers several waveform digitizers suitable for LIDAR applications:
- ADQ36 is a 12-bit digitizer with software-configurable two- or four-channel mode of operation that offers 5 or 2.5 GSPS sampling rates respectively. It also features a large user-programmable Xilinx Kintex Ultrascale KU115 field-programmable gate array (FPGA).
- ADQ14DC-4C offers 4 channels at 1 GSPS with 14 bits resolution. The high vertical resolution allows for accurate images during underwater surveys. The four channels are available for multiple detectors connected to lasers with different wavelengths. The ADQ14 also allows for high throughput to the PC for fast scanning capabilities. Input impedance matching is also important, especially in an over voltage condition. The signals are strong and the over voltage protection circuitry often acts as a non-linear current sink. To avoid the non-linear condition and resulting large reflections during over voltage, the first stage of the ADQ14DC is linear and resistive. This means that there is an attenuation of reflections even in an over voltage scenario.
- ADQ7DC offers 1 or 2 channels with 10 or 5 GSPS per channel and with 14 bits vertical resolution.
LiDAR Basic Principles
The principles of LiDAR are like Radar. However, rather than using radio waves, substantially higher frequency photonic energy - usually derived from a laser, provides target illumination. This offers an immediate benefit as the much shorter wavelength of light offers superior measurement precision. Like Radar, LiDAR uses the time of flight (ToF) principle to gain range information. LiDAR measures the photonic TOF in free space. The time delay, all be it short (measured often in mere microseconds), between outgoing and detected signal provides precise range information. Results can be sub-millimeter accurate depending on the specific attributes of the LiDAR system. The block diagram of a generic LiDAR detection system is illustrated in fig.1.
Arguably, the laser source is the most critical element as it establishes the LiDAR operating envelope. At its core, LiDAR uses laser light pulses to illuminate a segment of free space. A sensitive detector transduces scattered and reflected light. Laser wavelengths typically range from the visible at 532 nm up to 1550 nm shortwave IR (SWIR) and beyond. The illumination wavelength will determine the free space attenuation of the LiDAR pulse as well as affecting the likely strength of returns. This factor is even more critical with water penetrating bathymetry
Enabling 3D scanning requires a mechanical steering system using a movable mirror. More recently, vendors employ electrically scanned, solid state, 2D VCSEL matrices. The resulting LiDAR data output is known as a point cloud and is illustrated by the cityscape point cloud of figure 2.
LiDAR sometimes uses CW (continuous wave) modulation to extract velocity information from objects in motion. Furthermore, LiDAR may produce multiple returns from a single outgoing pulse. This is especially the case in densely forested (or built-up) areas as reflections occur from the tree canopy, branches, and the ground (see fig. 2) providing detailed object elevation data.
Basic physics of LIDAR
LiDAR surveys are invariably performed from mobile aerial platforms, so accurate positional information is necessary for precision measurement. Positional geospatial data comes from GPS/Galileo satellites combined with inertial data from an onboard inertial measurement unit (IMU). The IMU delivers a 3D coordinate set for the airborne platform (e.g. a drone or plane). Once a 3D positional reference is defined, the LiDAR system can return precision 3D spatial information.
In the aerial survey scenario shown an illuminating laser pulse travels through air towards the ground and back to the airborne detector. Here the outgoing pulse is reflected from three locations from the vegetation canopy before the final ground return is derived. The duration of this journey Δt, is twice the distance traveled. The received signal arising from a single illuminating pulse is pictured in the intensity/time curve.
Laser time-of-flight in free space is approximately 3.3 ns/m. Water slows this to 4.4 ns/m. The spatial resolution of a given LiDAR can be determined by considering object illumination with short laser pulses. A ten nano-second pulse, traveling in free-space, will have a resolution of three metres (10/3.3 m). Reduce this to one nano-second and the enhanced resolution rises to ~ 30 cm (10 x better). It may seem obvious that better resolution demands shorter laser pulses. Alas, there are system limits to pulse length setting given a fixed detector dynamic range. Tradeoffs arise for maximum measurement range and spatial precision, coupled to limited laser output power for eye safe operation.
Pulse repetition rate sets the data acquisition rate and importantly, the rate at which a survey of a given unit space can be performed. A typical aerial survey is illustrated in fig. x. In this example, four signal returns are shown. The last being the ground return. The time-of-flight round trip is denoted as Δt. In topographic surveys, altitude typically varies between 500-2000 m AGL (above ground-level) so each pulse’s total ToF ranges between 3.3 to 13.2 µs. Of the total ToF segment, the duration of the active LiDAR returns of the four back returns shown, assuming an initial reflection from a tree’s canopy at eighty metres height will last approximately 24 nano-seconds (80 m/3.3 ns).
Bathymetric surveying over water requires the use of a shorter wavelength, green laser to counter the increased light attenuation encountered in water. This sets the maximum penetration depth to typically around 50 m, though this varies based on water clarity (and turbidity). Beyond that, Sonar provides enhanced depth sounding.
One of the primary challenges for maritime bathymetry is in discriminating the tiny second (bottom) returns (fig. 3 green trace). The challenge being that this return is subject to variable excess attenuation because of the water’s turbulence. Consequently, this return is hard to extract from background noise. This threshold becomes a key system design factor.
This short summary has shown how photonic time-of-flight measurements can be used to accurately measure topographic and bathymetric data as well as aid atmospheric science with systems such as Raman LiDAR.
To learn more about how optronics establishes the important electrical parameters necessary to select an appropriate data acquisition system, check out the other LiDAR technical resources at Teledyne SP Devices.
Read more about LiDAR in "Full waveform hyperspectral LiDAR for terrestrial laser scanning".