Performance Of Lidar System Research Articles

Quality assessment of aerosol lidars at 1064 nm in the framework of the MEMO campaign

Abstract. Aerosol lidar networks can play an important role in revealing structural characteristics of the atmospheric boundary layer, the urban heat island effect, and the spatial distribution of aerosols, especially in relation to the monitoring of atmospheric pollution in megacities. To fulfill the need of the monitoring and numerical forecasting of atmospheric pollution, an aerosol lidar network is proposed by the China Meteorological Administration which serves as an important part of the “MegaCity Experiment on Integrated Meteorological Observation in China” (MEMO). To ensure a high standard of data quality and traceability of measurement error, an inter-comparison campaign, dedicated to the quality assessment of lidar systems from different institutes and manufacturers, was designed and performed at Beijing Southern Suburb Observatory in September 2021. Six Mie–Rayleigh lidar systems at 1064 nm were involved in this campaign. The strategies for lidar self-evaluations and inter-comparisons were predefined. A lidar system at 1064 nm, which was developed by the Atmospheric Remote Sensing group at Wuhan University, was selected as the reference lidar system after passing all strict self-evaluation quality checks. The reference lidar system serves as the cornerstone for evaluating the performance of other lidar systems. After using the Rayleigh fit and signal-to-noise evaluation self-tests for each individual lidar system as a fast check of the data quality, the range-corrected signal and backscatter coefficient obtained from all the lidar systems were inter-compared with a reference lidar system. In the end, the lidar systems passed the quality control/assurance, ensuring that the standard deviation of range-corrected signal could be controlled within 5 % at 500–2000 m and 10 % at 2000–5000 m. For the derived aerosol backscatter coefficients, standard deviations can be controlled within 10 % at 500–2000 and 2000–5000 m. The quality assurance strategy lays down a solid basis for atmospheric lidar at near-infrared wavelengths and will be applied in Chinese lidar network development.

Polarised full-waveform warning LIDAR with dust backscattering suppression

LIDAR, when used under adverse weather conditions such as fog and dust, suffers from attenuation and backscattering. In particular, backscattering creates spurious echo peaks in the received waveforms at incorrected ranges and leads to false alarms. As a solution, this paper proposes polarised full-waveform warning LIDAR for dust backscattering echo suppression, intended for applications under dusty environments. To this end, first, a theoretical model of the polarised LIDAR is developed based on the LIDAR equation and depolarisation characteristics of dust backscattering and target reflection. Thereafter, the depolarisation characteristics are experimentally evaluated under an artificially generated dusty environment; the results reveal that the target reflection features a strong depolarisation characteristic, whereas dust backscattering has a weak one. When the LIDAR emits linearly polarised light, a significant orthogonal-polarised component exists owing to the target reflection, and dust backscattering induces a mainly parallel-polarised component. On interposing an orthogonal-polarisation analyser in the receiving optical path, the parallel-polarised component from the dust backscattering and target reflection is blocked, whereas the orthogonal-polarised component is allowed to pass; this helps improve the contrast of the target reflection to dust backscattering. Finally, based on the theoretical model of the polarised LIDAR and experimental results, an orthogonal-polarised LIDAR prototype is designed and fabricated. Using this prototype, target detection experiments in a dusty environment are performed; the corresponding experimental results indicate that the orthogonal-polarised LIDAR effectively suppresses dust backscattering echoes and that the contrast of the target reflection to dust backscattering is improved by 4.78–7.12 dB. Utilization of the polarised full-waveform warning LIDAR can significantly reduce the false alarm rate and improves the LIDAR system performance when used under dusty environment conditions, in applications such as smart mining and autonomous driving.

Analysis of optimal detection range performance of LiDAR systems applying coaxial optics

In this study, the optimal detection range performance of a LiDAR system employing coaxial optics is analyzed. On analyzing laser beam diameter, beam divergence angle, light-receiving area, and receiving light intensity, the optimal detection range was exhibited when the collimator focal length was 6.5 mm, the laser-beam divergence angle was 1.23 mrad, and the hole of the receiving mirror was 1.4 mm. Furthermore, the maximum deviation of the optical scan angles with respect to the mechanical angles of the polygon mirror and the MEMS mirror were analyzed as -0.99° and +4.54° in the x-axis and y-axis directions, respectively.

Reconstruction and Synthesis of Lidar Point Clouds of Spray

Lidars are commonly used on autonomous vehicles, but their performance can be significantly affected by adverse weather. A number of studies have been devoted to analyzing and improving lidars’ performance in rain, fog, and snow. Yet, relatively little attention has been paid to road spray which occurs when vehicles travel on wet surfaces at high speed. Road spray produced by the vehicles causes false positive point measurements in lidar scans. To evaluate the performance of lidar perception systems or train learning-based perception models, a large amount of data collected in various weather conditions are needed. Unfortunately, collecting spray data is challenging due to the requirements for certain weather conditions (e.g., heavy rain), and vehicles with high speed. In this letter, we propose the first data-driven method combined with simulation to reconstruct and synthesize spray data. The proposed pipeline can be applied to data augmentation by adding spray effects to existing lidar data collected under good weather conditions. We compare the performance of vehicle detection models trained with and without augmented data. The model trained with augmented data achieve significant performance improvement given real-world spray-affected point cloud data.

An Accuracy Assessment of Snow Depth Measurements in Agro-Forested Environments by UAV Lidar

This study assesses the performance of UAV lidar system in measuring high-resolution snow depths in agro-forested landscapes in southern Québec, Canada. We used manmade, mobile ground control points in summer and winter surveys to assess the absolute vertical accuracy of the point cloud. Relative accuracy was determined by a repeat flight over one survey block. Estimated absolute and relative errors were within the expected accuracy of the lidar (~5 and ~7 cm, respectively). The validation of lidar-derived snow depths with ground-based measurements showed a good agreement, however with higher uncertainties observed in forested areas compared with open areas. A strip alignment procedure was used to attempt the correction of misalignment between overlapping flight strips. However, the significant improvement of inter-strip relative accuracy brought by this technique was at the cost of the absolute accuracy of the entire point cloud. This phenomenon was further confirmed by the degraded performance of the strip-aligned snow depths compared with ground-based measurements. This study shows that boresight calibrated point clouds without strip alignment are deemed to be adequate to provide centimeter-level accurate snow depth maps with UAV lidar. Moreover, this study provides some of the earliest snow depth mapping results in agro-forested landscapes based on UAV lidar.

Processing Strategy and Comparative Performance of Different Mobile LiDAR System Grades for Bridge Monitoring: A Case Study.

Collecting precise as-built data is essential for tracking construction progress. Three-dimensional models generated from such data capture the as-is conditions of the structures, providing valuable information for monitoring existing infrastructure over time. As-built data can be acquired using a wide range of remote sensing technologies, among which mobile LiDAR is gaining increasing attention due to its ability to collect high-resolution data over a relatively large area in a short time. The quality of mobile LiDAR data depends not only on the grade of onboard LiDAR scanners but also on the accuracy of direct georeferencing information and system calibration. Consequently, millimeter-level accuracy is difficult to achieve. In this study, the performance of mapping-grade and surveying-grade mobile LiDAR systems for bridge monitoring is evaluated against static laser scanners. Field surveys were conducted over a concrete bridge where grinding was required to achieve desired smoothness. A semi-automated, feature-based fine registration strategy is proposed to compensate for the impact of georeferencing and system calibration errors on mobile LiDAR data. Bridge deck thickness is evaluated using surface segments to minimize the impact of inherent noise in the point cloud. The results show that the two grades of mobile LiDAR delivered thickness estimates that are in agreement with those derived from static laser scanning in the 1 cm range. The mobile LiDAR data acquisition took roughly five minutes without having a significant impact on traffic, while the static laser scanning required more than three hours.

Spatial and temporal domain filtering for underwater lidar.

Combined spatial and temporal processing techniques are presented to enhance optical ranging in underwater environments. The performance of underwater light detection and ranging (lidar) is often limited by scattering. Previous work has demonstrated that both hybrid lidar-radar, which temporally modulates the amplitude of light, and optical spatial coherence filtering, which spatially modulates the phase of light, have independently reduced the effects of scattering, improving performance. The combined performance of the processing methods is investigated, and experimental results demonstrate that the combined filtering improves the performance of underwater lidar systems beyond what either method provides independently.

Prevention of laser damage precursors in spaceborne ultraviolet antireflection coatings.

In order to enhance the reliability and performance of space-based Lidar systems, it is desirable to increase the damage resistance of ultraviolet antireflective coatings. For laser pulses with nanosecond pulse duration, laser-induced damage is known to be triggered by nano-sized defects embedded in the optical coating. In this work, we demonstrate the mitigation of damage precursors during the manufacturing of ion-beam sputtered (IBS) coatings using two approaches: ion bombardment with a secondary ion source and laser irradiation with a nanosecond-pulsed laser. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in large-area raster scans.

Implementation of a violet Scheimpflug lidar system for atmospheric aerosol studies.

A violet Scheimpflug lidar system employing a 1-W 407-nm multimode laser diode is developed for remote sensing of atmospheric aerosols. The laser beam of the laser diode that is transmitted into atmosphere has been investigated in detail and a cylindrical lens pair is designed to improve the geometrical transmission efficiency. A measurement scheme with automatic exposure as well as a generalized signal processing method are established to optimize the signal-to-noise ratio of lidar signals. The performance of the violet Scheimpflug lidar system has been evaluated during a six-day continuous measurement campaign on a near horizontal path. The maximum measurement distance can reach up to 7 km in sunny clean weathers and to about 2 km during haze with an aerosol extinction coefficient of about 0.9 km. The aerosol extinction coefficient retrieved by the Fernald method is promising and shows good correlation with particle concentrations measured by a local national pollution monitoring station. This work promotes the development of all-time Scheimpflug lidar systems operating at other wavelengths or multiple wavelengths for various atmospheric applications.

Novel illumination strategy for lidar enabled by update in the laser product standards

In this article, the authors propose a novel illumination strategy for light detection and ranging (lidar) systems emitting in the retinal hazard region (400 nm ≤ λ < 1400 nm), which allows to improve lidar performance. This illumination strategy was enabled by the 2014 update from edition 2 to edition 3 of the international laser product safety standard IEC 60825-1 [Safety of Laser Products—Part 1: Equipment Classification and Requirements, 3rd ed. (IEC, Geneva, 2014)] regarding the exposure to multiple pulses. First, the authors introduce critical parameters for lidar performance. Second, the authors summarize the limitations of power emission and state how the standard has changed. Third, the authors highlight how edition 2 of the laser safety standard used to limit some of these parameters. To conclude, the authors present the novel illumination strategy and explain how it allows to improve the performance of lidar systems.

Fully integrated free-running InGaAs/InP single-photon detector for accurate lidar applications.

We present a fully integrated InGaAs/InP negative feedback avalanche diode (NFAD) based free-running single-photon detector (SPD) designed for accurate lidar applications. A free-piston Stirling cooler is used to cool down the NFAD with a large temperature range, and an active hold-off circuit implemented in a field programmable gate array is applied to further suppress the afterpulsing contribution. The key parameters of the free-running SPD including photon detection efficiency (PDE), dark count rate (DCR), afterpulse probability, and maximum count rate (MCR) are dedicatedly optimized for lidar application in practice. We then perform a field experiment using a Mie lidar system with 20 kHz pulse repetition frequency to compare the performance between the free-running InGaAs/InP SPD and a commercial superconducting nanowire single-photon detector (SNSPD). Our detector exhibits good performance with 1.6 Mcps MCR (0.6 μs hold-off time), 10% PDE, 950 cps DCR, and 18% afterpulse probability over 50 μs period. Such performance is worse than the SNSPD with 60% PDE and 300 cps DCR. However, after performing a specific algorithm that we have developed for afterpulse and count rate corrections, the lidar system performance in terms of range-corrected signal (Pr2) distribution using our SPD agrees very well with the result using the SNSPD, with only a relative error of ∼2%. Due to the advantages of low-cost and small size of InGaAs/InP NFADs, such detector provides a practical solution for accurate lidar applications.

End-to-End Simulation for a Forest-Dedicated Full-Waveform Lidar Onboard a Satellite Initialized from Airborne Ultraviolet Lidar Experiments

In order to study forests at the global scale, a detailed link budget for a lidar system onboard satellite is presented. It is based on an original approach coupling airborne lidar observations and an end-to-end simulator. The simulator is initialized by airborne lidar measurements performed over temperate and tropical forests on the French territory, representing a wide range of forests ecosystems. Considering two complementary wavelengths of 355 and 1064 nm, the end-to-end simulator computes the performance of spaceborne lidar systems for different orbits. The analysis is based on forest structural (tree top height, quadratic mean canopy height) and optical (forest optical thickness) parameters. Although an ultraviolet lidar appears to be a good candidate for airborne measurements, our results show that the limited energy is not favorable for spaceborne missions with such a wavelength. A near infrared wavelength at 1064 nm is preferable, requiring ~100 mJ laser emitted energy, which is in agreement with current and future spaceborne missions involving a lidar. We find that the signal-to-noise ratio at the ground level to extract both the structural and optical parameters of forests must be larger than 10. Hence, considering the presence of clouds and aerosols in the atmosphere and assuming a stationary forest, a good detection probability of 99% can be reached when 4 or 5 satellite revisits are considered for a lidar system onboard the ISS or ICESat, respectively. This concerns ~90% of forest covers observed from the lidar, which have an optical thickness less than 3.

Fiber laser-based scanning lidar for space rendezvous and docking

Lidar systems have played an important role in space rendezvous and docking (RVD). A new type of scanning lidar is developed using a high-repetition-rate pulsed fiber laser and a position detector. It will be a candidate for autonomous space RVD between two spacecrafts. The lidar can search and track cooperative targets in a large region without artificial guidance. The lidar's operational range spans from 18m to 20km, and the relative angle between two aircrafts can be measured with high accuracy. A novel fiber laser with tunable pulse energy and repetition rate is developed to meet the wide dynamic detection range of the lidar. This paper presents the lidar system's composition, performance, and experimental results in detail.

Scattering of a general partially coherent beam from a diffuse target in atmospheric turbulence

The second-order and fourth-order statistical moments of the speckle field from a diffuse target in atmospheric turbulence are studied which have great influence on the performance of lidar systems. By expanding a general rotationally symmetric beam as a sum of Gaussian—Schell model (GSM) beams, the mean intensity of the general beam propagating over a distance in an atmospheric turbulence is formulated. Expressions for the degree of coherence (DOC) and the normalized intensity variance of the scattered field of a general beam from a rough surface in turbulence are derived based on the extended Huygens—Fresnel principle. The general expressions reduce to the well-known forms for a GSM beam. Another example for the general beam used in this paper is the collimated flat-topped beam. The results of both kinds of beams show that the intensity profile on the target plane is a key factor affecting the statistical characteristics of the speckle field. A larger beam spot on the target plane induces a smaller coherence length and a smaller normalized intensity variance of the received field. As turbulence gets stronger, the coherence length becomes smaller, and the normalized intensity variance firstly increases and declines to unity finally.

Wind velocity retrieval and field widening techniques of fringe-imaging Mach-Zehnder interferometer for Doppler lidar

Compared with the traditional Doppler frequency discriminator Fabry-Perot interferometer, Mach-Zehnder interferometer (MZI) has the advantages in high transmittance, linear parallel fringes instead of circular rings and wide field of view. An incoherent Doppler wind lidar system is demonstrated based on a fringe-imaging MZI. The mathematical model of wind velocity retrieval is derived, and the field widening of MZI is also used to optimize the performance of lidar system. The ideal fringe pattern outputs are obtained by numerical simulation, and the SineSqr function fitting method is proposed to determine the high-precision displacement of fringes after a Doppler shift. Furthermore, the field widening of MZI can be realized by field compensation which reduces the effect of the incidence angle on optical path difference. The results show that the wind velocity error obtained by SineSqr function fitting method is less than 0.45 m·s-1 at the line-of-sight wind velocity in a range from -100 m·s-1 to 100 m·s-1, which overcomes the instability of wind velocity retrieval by the gravity method. A full-angle field of view of 1° for MZI frequency discriminator without significant performance reduction is also achieved. These supplied discussion of the application techniques for Doppler lidar based on MZI fringe technique would promote the practical development of Doppler lidar for wind velocity measurement of the middle and upper atmosphere.

Investigation of spherical aberration effects on coherent lidar performance

In this paper we demonstrate experimentally the performance of a monostatic coherent lidar system under the influence of phase aberrations, especially the typically predominant spherical aberration (SA). The performance is evaluated by probing the spatial weighting function of the lidar system with different telescope configurations using a hard target. It is experimentally and numerically proven that the SA has a significant impact on lidar antenna efficiency and optimal beam truncation ratio. Furthermore, we demonstrate that both effective probing range and spatial resolution of the system are substantially influenced by SA and beam truncation.

Lidar Strip Adjustment with Automatically Reconstructed Roof Shapes

AbstractIn recent years, the performance of lidar systems has steadily improved due to an increase in the achievable point density. The main error sources affecting the quality of lidar‐derived secondary products such as digital terrain models (DTMs) and digital surface models (DSMs) are the result of systematic errors coming from insufficient boresight calibration. The effects of boresight misalignments are visually recognised as discrepancies of the laser point clouds at selected objects (such as building roofs) in overlapping areas of neighbouring lidar strips. In this work, a strip adjustment method is presented that corrects the systematic errors of the laser point cloud using five strip parameters (three shifts plus two angular corrections for heading and roll). The strip adjustment procedure uses 2D and 3D tie elements and takes advantage of a new 3D measurement technique applied to building roofs. The strip adjustment method is applied to three different datasets, consisting of conventional first/last pulse and full‐waveform data. The results show that systematic discrepancies are typical for datasets that have already been processed by the flight companies. These errors can be reduced by more than 70% by applying the new strip adjustment approach. Preliminary investigations indicate that a higher laser point density and the inclusion of intensity and pulse width within full‐waveform lidar data lead to better results.

Backscatter 2-$\mu\hbox{m}$ Lidar Validation for Atmospheric $\hbox{CO}_{2}$ Differential Absorption Lidar Applications

A 2-μm backscatter lidar system has been developed by utilizing tunable pulsed laser and infrared phototransistor for the transmitter and the receiver, respectively. To validate the system, the 2-μm atmospheric backscatter profiles were compared to profiles obtained at 1 and 0.5 μm using avalanche photodiode and photomultiplier tube, respectively. Consequently, a methodology is proposed to compare the performance of different lidar systems operating at different wavelengths through various detection technologies. The methodology is based on extracting the system equivalent detectivity and comparing it to that of the detectors, as well as the ideal background detectivity. Besides, the 2-μm system capability for atmospheric CO2 temporal profiling using the differential absorption lidar (DIAL) technique was demonstrated. This was achieved by tuning the laser at slightly different wavelengths around the CO2 R22 absorption line in the 2.05-μm band. CO2 temporal profiles were also compared to in situ measurements. Preliminary results indicated average mixing ratios close to 390 ppm in the atmospheric boundary layer with 3.0% precision. The development of this system is an initial step for developing a high-resolution, high-precision direct-detection atmospheric CO2 DIAL system. A successful development of this system would be a valuable tool in obtaining and validating global atmospheric CO2 measurements.

An eight month test campaign of the Qinetiq ZephIR system: Preliminary results

The Beatrice Windfarm Demonstrator project is a major part of the European Union sponsored DOWNVInD programme (Distant Offshore Wind farms with No Visual Impact iN Demonstrator) that will install two multi megawatt wind turbines (of type REpower 5M) in the Moray Firth Scotland, adjacent to the Beatrice Alpha oil platform, owned and operated by Talisman Energy. As part of the project, a programme has been designed and instigated over the last eight months to thoroughly examine the performance and capabilities of the QinetiQ ZephIR LiDAR system as well as to establish its suitability for being the primary wind data acquisition instrument on the Beatrice project. This paper represents a first look at the results of this programme.

Three Dimensional Monitoring of Stack Plume Dynamics by a Scanning Mie Lidar System as a Plume Watchdog Station

A scanning lidar system was developed to watch the nighttime diffusion process of plume from a smokestack of a large incinerator located around 3 km from the system. Observed data sets were visualized as three dimensional images in which could be seen the diffusion pattern from any direction, and this made it easy to investigate the exhaust dynamics. Observation results showed that the original plume extended at least 1.6 km, where there was a residential area, from the smokestack increasing in diameter to about 500 m. High density aerosols originating from the smokestack were measured in that area even at midnight. The lidar system performance as a plume watchdog station was discussed from the standpoint of publichealth-related plume monitoring.