LiDAR - Light Detection and Ranging
Table of Contents
- LiDAR applications overview
- What to consider when selecting a digitizer for LiDAR
- Commonly used products and features
Introduction to Light Detection and Ranging (LiDAR)
LiDAR is a remote sensing method used for geospatial measurements. Pulsed laser light emitted at surrounding objects/surfaces is partially reflected (backscattered) and recorded by a detector connected to a digitizer. Reflection flight time is measured by the digitizer and is used together with the speed of light to calculate object/surface distance. This is referred to as time-of-flight (ToF) measurements.
The systems are either airborne or terrestrial and are used for different applications as described below. Topographic LiDAR systems typically utilize either 1064 nm near-infrared (NIR) or 1550 nm short-wave infrared (SWIR) lasers whereas bathymetric systems additionally also use visible (VIS) 532 nm sources.
LiDAR applications overview
LiDAR supports a diverse set of applications, for example:
- 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 conditions 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 an 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 the quality of the returned signal. All are factors influencing the detection and data processing electronics needed.
What to consider when selecting a digitizer for LiDAR
- Flexible choice of vertical resolution, sampling rate, and channel count allows for perfect-fit solutions that help reduce system-level costs.
- Intra-board synchronization capability improves channel-count flexibility even further.
- High sampling rate simplifies peak detection in short returns and ultimately helps improve LiDAR range resolution.
- High vertical resolution (dynamic range) helps discriminate low-amplitude return pulses from noise hence improving target separation.
- Digitizer trigger, synch, and general-purpose input/output (GPIO) ports/signals enable interaction with the laser source, scan mirrors, inertial measurement unit (IMU), and GPS module (PPS synchronization) in order to simplify system-level design.
- Built-in trigger time (timestamp) readout simplifies flight time measurement.
- Flexible trigger modes enable data reduction without information loss using mechanisms such as level (threshold) triggering, leading- and trailing-edge window, trigger delay, and more.
- Acquired pulses are unipolar and therefore require a programmable DC offset to fully utilize the digitizer's input voltage range.
- Onboard real-time digital signal processing (DSP) via an open field-programmable gate array (FPGA) offers crucial benefits:
- Enables custom return pulse discrimination/characterization in real-time and with low latency.
- Multiple (all) channels can be processed in the same FPGA.
- Simplifies subsequent post-processing and/or data storage through real-time data reduction.
- Ranging algorithm processing examples could include Gaussian curve fitting, full width at half maximum (FWHM), deconvolution, waveform averaging (stacking), digital filtering, and more.
- FPGA firmware is field-upgradable so that new/improved features can be introduced in already operational LiDAR systems.
- High-speed data transfer to graphics processing units (GPUs) and/or disk storage can complement the onboard FPGA pre-processing. So-called peer-to-peer streaming is preferred as it minimizes the load on the host PC’s central processing unit (CPU) and memory.
- Industry-standard form factors simplify integration and ensure the availability of cost-efficient third-party products. The preferred form factor may differ depending on the LiDAR system type.
Commonly used products and features
SP Devices’ offers several waveform digitizers suitable for LIDAR applications:
- ADQ36 is a 12-bit digitizer primarily designed for LiDAR. It offers software-configurable two- or four-channel mode of operation with 5 or 2.5 GSPS sampling rates respectively. Two or more boards can be synchronized in order to increase system-level channel count, and it also features a large user-programmable Xilinx Kintex Ultrascale KU115 field-programmable gate array (FPGA).
Additional features include hardware trigger, timestamp, programmable DC offset, digital baseline drift correction, peer-to-peer streaming, and more. Overall this digitizer is ideal for advanced LiDARs such as multi-pulse systems.
- ADQ14DC-4C features 4 input channels with 14 bits vertical resolution and 1 GSPS sampling rate. 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 overvoltage condition. The signals are strong and the overvoltage protection circuitry often acts as a non-linear current sink. To avoid the non-linear condition and resulting large reflections during overvoltage, the first stage of the ADQ14DC is linear and resistive. This means that there is an attenuation of reflections even in an overvoltage scenario.
- ADQ7DC offer 14 bits vertical resolution and 10 GSPS sampling rate in single channel mode or 5 GSPS in dual channel mode. This unique combination of high resolution and sampling rate enables unrivaled distance accuracy with a maintained high dynamic range so that weak return signals can be discriminated from background noise. It also supports the same crucial features as listed for other digitizer models above.
The ADQ7DC is used in some of the most advanced LiDAR systems available on the market today.
LiDAR references and acknowledgements
Read more about LiDAR in "Full waveform hyperspectral LiDAR for terrestrial laser scanning".
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