Echo Width Research Articles

Enhancing LiDAR-UAS Derived Digital Terrain Models with Hierarchic Robust and Volume-Based Filtering Approaches for Precision Topographic Mapping

Airborne Laser Scanning (ALS) point cloud classification in ground and non-ground points can be accurately performed using various algorithms, which rely on a range of information, including signal analysis, intensity, amplitude, echo width, and return number, often focusing on the last return. With its high point density and the vast majority of points (approximately 99%) measured with the first return, filtering LiDAR-UAS data proves to be a more challenging task when compared to ALS point clouds. Various algorithms have been proposed in the scientific literature to differentiate ground points from non-ground points. Each of these algorithms has advantages and disadvantages, depending on the specific terrain characteristics. The aim of this research is to obtain an enhanced Digital Terrain Model (DTM) based on LiDAR-UAS data and to qualitatively and quantitatively compare three filtering approaches, i.e., hierarchical robust, volume-based, and cloth simulation, on a complex terrain study area. For this purpose, two flights over a residential area of about 7.2 ha were taken at 60 m and 100 m, with a DJI Matrice 300 RTK UAS, equipped with a Geosun GS-130X LiDAR sensor. The vertical and horizontal accuracy of the LiDAR-UAS point cloud, obtained via PPK trajectory processing, was tested using Check Points (ChPs) and manually extracted features. A combined approach for ground point classification is proposed, using the results from a hierarchic robust filter and applying an 80% slope condition for the volume-based filtering result. The proposed method has the advantage of representing with accuracy man-made structures and sudden slope changes, improving the overall accuracy of the DTMs by 40% with respect to the hierarchical robust filtering algorithm in the case of a 60 m flight height and by 28% in the case of a 100 m flight height when validated against 985 ChPs.

Digital elevation models of the sea-ice surface from airborne laser scanning during MOSAiC

Airborne laser scanners (ALS) are used to map the sea-ice surface at sub-meter resolution. We conducted 64 flights over the Arctic sea ice between September 2019 and September 2020 during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition to measure sea-ice surface elevation. The flights ranged from repeated, local-scale 5 × 5 km2 floe grid surveys to regional-scale transects more than 100 km long. We provide data at different processing levels: geolocated elevation point clouds and gridded segments of elevation and freeboard with a spatial resolution of 0.5 m. The latter product is corrected for atmospheric backscatter, sea-ice drift, and offset in elevation due to degraded INS/GNSS solutions > 85° N. For floe grid surveys, all data are combined to merged two-dimensional elevation maps. Other provided parameters include laser reflectance and echo width. The presented data offer a unique possibility to study the temporal evolution, spatial distribution, and variability of the snow and sea-ice surface and their properties in addition to validating satellite products.

Scattering of Laguerre–Gaussian Beam by a Topological Insulator Cylinder Coated with Metamaterials

A theoretical investigation is presented to study the orbital angular momentum (OAM) beam scattering from a topological insulator (TI) cylinder coated with metamaterials. To calculate the electromagnetic (EM) response of the scattered field, the field equations are modified using Laguerre–Gaussian (LG) potential and plane wave scattering. The response of the metamaterial coated geometry has been reported for Double positive (DPS), Double negative (DNG), Epsilon negative (ENG), and Mu negative (MNG) coating layers, respectively. The normalized bistatic echo widths for the TI cylinder also change by changing the topological charge ( $$l$$ ), the radius of the metamaterial coated layer $$(b)$$ , source frequency $$(f)$$ , and beam waist radius $$({w}_{0})$$ . Our result for $${LG}_{00}$$ beam exactly tones with plane wave scattering as a reference. Numerical results in the form of bistatic echo widths have been calculated. Besides, the prementioned parameters, the influence of OAM mode index on the bistatic echo width is also analyzed. It can be seen that OAM mode number plays major role in controlling the scattering amplitude and intensity.

Influence of phenology on waveform features in deciduous and coniferous trees in airborne LiDAR

Information on forest structure is vital for sustainable forest management. Currently, airborne LiDAR remote sensing has been well established as an effective tool to characterize the structure of canopies and forest inventory variables. Radiometry and geometry are highly intertwined in LiDAR remote sensing of forest vegetation and phenology influences the geometric-optical properties of deciduous and evergreen trees causing seasonal variation in LiDAR observations. This variation may be considered as a nuisance or exploited in for example tree species identification. Airborne LiDAR data are also influenced by sensor functioning, acquisition settings, scan geometry and the atmosphere. Reliable estimation of subtle phenological effects calls for data in which the impact of the external factors is minimal. We experimented with such data and explored LIDAR waveforms (WFs) in boreal trees in winter, early summer and late summer. Our objectives were to i) assess the match of the multitemporal LiDAR data for observing true changes in vegetation; ii) quantify the influence of phenology in deciduous and evergreen trees; iii) study the effect of varying scan zenith angle (SZA) and canopy age on WF features in different phenostates; iv) assess the temporal feature correlation in individual living and dead standing trees. A WF-recording pulsed LiDAR sensor unit operating at the wavelength of 1550 nm was used in repeated acquisitions. WF attributes such as energy, peak amplitude and echo width were derived for each pulse and were localized vertically to crown, understory and ground components. Silver and downy birch, black alder, European aspen, Siberian larch, Scots pine, Norway spruce and dead standing spruce formed our strata. Results showed that phenology caused more variation in WF features of deciduous trees compared to evergreen conifers. Deciduous trees displayed substantial between-species variation that was linked with differences in branching pattern, leaf orientation and bark reflectance. Pine displayed a possible winter-early summer anomaly in canopy backscattering that may be linked with changes in foliage clumping or with the role of stamens in early summer trees. Trees displayed positive temporal correlation in WF features and correlations were the strongest in evergreen and deciduous conifers and decreased with time. SZA had minor influence on WF features whereas age exercised a strong effect on many features with parallel variation between species and phenostates. Structural changes following death, i.e. ‘aging’ changed the geometric WF features of dead standing trees. Our results provide new insights for enhancing tree species identification by using WF LiDAR and for LiDAR time-series analysis of vegetation.

Airborne small-footprint full-waveform LiDAR data for urban land cover classification

Airborne small-footprint full-waveform LiDAR data have a unique ability to characterize the landscape because it contains rich horizontal and vertical information. However, a few studies have fully explored its role in distinguishing different objects in the urban area. In this study, we examined the efficacy of small-footprint full-waveform LiDAR data on urban land cover classification. The study area is located in a suburban area in Beijing, China. Eight land cover classes were included: impervious ground, bare soil, grass, crop, tree, low building, high building, and water. We first decomposed waveform LiDAR data, from which a set of features were extracted. These features were related to amplitude, echo width, mixed ratio, height, symmetry, and vertical distribution. Then, we used a random forest classifier to evaluate the importance of these features and conduct the urban land cover classification. Finally, we assessed the classification accuracy based on a confusion matrix. Results showed that Afirst was the most important feature for urban land cover classification, and the other seven features, namely, ωfirst, HEavg, nHEavg, RAω, SYMS, Srise, and ωRf_fl, also played important roles in classification. The random forest classifier yielded an overall classification accuracy of 94.7%, which was higher than those from previous LiDAR-derived classifications. The results indicated that full-waveform LiDAR data could be used for high-precision urban land cover classification, and the proposed features could help improve the classification accuracy.

Study and comparison of pulse compression techniques with three types of excitation for high-temperature EMAT detection

To address the low conversion efficiency and high ultrasonic attenuation in the detection of hot castings and forgings, the pulse compression technique (PCT) is an important and efficient method for improving the signal-to-noise ratio (SNR) and range resolution of ultrasonic echoes. A finite element model was established for the process of detection using a spiral coil electromagnetic acoustic transducer (EMAT) with three types of excitation, these being chirp signals, Barkercoded signals and combined Barker-coded signals, and the PCT processions were also provided. The SNR and range resolution of the PCTs with three types of excitation were compared using simulations and verified by experiments. The self-developed and water-cooled EMAT with three PCTs was used to measure a high-temperature forging and the pulse compression effect was also compared. The results indicate that the PCT with Barker-coded excitation has the highest SNR and the PCT with chirp excitation and combined Barker-coded excitation processes a higher range resolution. When the tested forging is at 710°C and compared with the traditional tone-burst excitation, the SNR from the PCT with combined Barker-coded excitation can be improved by 8.5 dB and the ultrasonic echo width can be reduced by 100%.

Analysis of wave scattering from 2D curved metasurfaces using Floquet and Fourier series expansions

An efficient technique for calculating the scattering from curved metasurfaces using the extinction theorem in conjunction with the Floquet and Fourier series expansions is presented. Here, we treat the two-dimensional metasurfaces that have transversal polarizabilities with no variation along the y-axis. The boundary conditions at the metasurface are given by the generalized sheet transition conditions (GSTCs) whose susceptibilities are given in an arbitrary local coordinate system. First, we use the extinction theorem to provide integral equations of the scattering problem. The integral equations involve the Green's functions, tangential electric and magnetic fields and their normal derivatives in regions above and below the metasurface. Then, we employ the Floquet theorem that gives us the analytical periodic Green's functions of each region. Next, we employ the Fourier theorem to expand the tangential fields in terms of unknown Fourier coefficients. The GSTCs and the integral equations provide equations to be solved for the unknowns. The method can calculate scattering from both periodic and non-periodic metasurfaces. The technique is used to analyse different applied problems such as carpet cloaking, illusion, and radar echo width reduction. The method is fast and accurate and can efficiently treat metasurfaces with electrically large curved geometries with dimensions as large as 120 times the wavelength.

Rigorous electromagnetic scattering solution of a PEMC strip placed at interface of dielectric-chiral media using Kobayashi Potential

ABSTRACT In this paper the rigorous solution of transverse electric (TE) and transverse magnetic (TM) plane wave scattering from a Perfect Electromagnetic Conductor (PEMC) strip located at the planar interface of two half spaces (dielectric and chiral medium) is presented by applying Kobayashi Potential (KP) method. Initially, the reflected field in dielectric and also transmitted Beltrami fields in chiral medium are divided into left and right handed fields. Then, contribution of the strip in terms of Bessel eigenfunctions is added as a perturbation to the dielectric and the chiral medium fields. Then, by applying the boundary conditions, utilizing the Weber–Schafheitlin (WS) discontinuous integrals and the Jacobi's polynomials, governing equations of infinite summations with unknown coefficients are derived. The infinite summations are effectively truncated with high numerical accuracy. Next, by solving the matrix equations, the unknown coefficients are calculated. Besides validating the method with the convergence analysis, an asymptotically comparative analysis with other investigations is also provided. Finally, the study of Echo Width (EW) for interesting parameters are depicted. The method is extremely accurate and is applicable to both narrow and wide strips. Also the presented formulations are validated by some methods (KP, physical optic and method of moment).

Combining single photon and multispectral airborne laser scanning for land cover classification

Feasibility of using single photon lidar (SPL) data for land cover classification was studied for the first time. Data from the Espoonlahti suburban study area in Finland were acquired with the SPL100 sensor from a flying altitude of 3600 m above ground level and classified into six classes using an object-based, random forest classification approach. The overall accuracy was 66% compared with a set of reference points. The results were good for buildings and trees, but there was significant confusion between ground-level land cover classes. The possibility to enhance classification accuracy by combining SPL data with multispectral intensity information from airborne laser scanning (ALS) was also tested and led to an overall accuracy of 86%. The multispectral intensity image was obtained by combining intensity data from two channels of the Optech Titan multispectral ALS sensor (infrared and near-infrared) with the SPL intensity (green). To allow for comparisons between SPL and conventional linear-mode ALS, single-channel data from the Optech Titan (near-infrared) were also applied for the same classification task. In this case, the overall accuracy was 83%. An important factor affecting the lower quality of the SPL results was the rough appearance of the intensity images created from the SPL data. Despite high point density, small features such as narrow roads were often difficult to distinguish in the intensity images. This relates to the SPL technique and preprocessing operations applied to the data. Considering future use of new ALS datasets, the different benefits of different systems need to be taken into account. A clear benefit of the SPL technique is its high efficiency in data acquisition. SPL is seemingly fit for deriving high objects that are distinguishable by using geometric rather than intensity information. The classification accuracy of buildings and trees was comparable in each classification test, although the flying altitude of the SPL100 was five times that of the Optech Titan. We propose that raw echo waveforms would be provided together with point cloud data to allow users to create their own intensity and echo width features usable for classification purposes. Combining multispectral intensity with SPL, in reference to multispectral single photon lidar, could result in a technique that is effective for mapping large areas and will also allow for automated classification of ground-level objects in a more reliable manner.

Characteristics of electromagnetic wave transmission and reflection from isotropic plasma coated circular nihility cylinder

Theoretical investigation has been carried out to study the scattering and transmission of electromagnetic energy from isotropic plasma coated circular nihility cylinder. The physical modeling of the problem is done in frame work of classical scattering theory. All the fields are expanded in terms of cylindrical vector wave functions (CVWFs). The analytical as well as numerical solution for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations is presented. The unknown scattering coefficients are computed by implementing the boundary conditions at each interface (free space/plasma and plasma/nihility). The influence of plasma parameters i.e., plasma density (ND) and effective collision frequency (v) on the bistatic echo width is analyzed and it is concluded that the plasma coating can be used to control and tune the scattering amplitude. Further it is also concluded that for the case ω > ωp the scattering amplitude is much less than for the case ω < ωp. Under special conditions, the results of present work compared with the published literature and good agreement is found.

Scattering due to uniaxial anisotropic chiral medium-coated PEC cylinder with conducting helix loading

ABSTRACT A perfect electrically conducting (PEC) cylinder (having a circular cross-section) encapsulated with uniaxial anisotropic chiral medium was studied emphasizing the features of scattering. A conducting sheath helix structure having a varying orientation with respect to the optical axis (or pitch angle) was assumed to be at the outermost surface. This allows alterations in scattering patterns. The echo width of the structure was evaluated under the assumption of parallel polarized oblique incidence of waves. The effect on scattering was evaluated under varying chirality and permittivity of the coating medium along with the variations in the helix pitch angle. The obtained results are compared with those achieved by replacing the chiral coating with a simple anisotropic kind of medium (without chirality).

Important LiDAR metrics for discriminating forest tree species in Central Europe

Numerous airborne LiDAR-derived metrics have been proposed for classifying tree species. Yet an in-depth ecological and biological understanding of the significance of these metrics for tree species mapping remains largely unexplored. In this paper, we evaluated the performance of 37 frequently used LiDAR metrics derived under leaf-on and leaf-off conditions, respectively, for discriminating six different tree species in a natural forest in Germany. We firstly assessed the correlation between these metrics. Then we applied a Random Forest algorithm to classify the tree species and evaluated the importance of the LiDAR metrics. Finally, we identified the most important LiDAR metrics and tested their robustness and transferability. Our results indicated that about 60% of LiDAR metrics were highly correlated to each other (|r| > 0.7). There was no statistically significant difference in tree species mapping accuracy between the use of leaf-on and leaf-off LiDAR metrics. However, combining leaf-on and leaf-off LiDAR metrics significantly increased the overall accuracy from 58.2% (leaf-on) and 62.0% (leaf-off) to 66.5% as well as the kappa coefficient from 0.47 (leaf-on) and 0.51 (leaf-off) to 0.58. Radiometric features, especially intensity related metrics, provided more consistent and significant contributions than geometric features for tree species discrimination. Specifically, the mean intensity of first-or-single returns as well as the mean value of echo width were identified as the most robust LiDAR metrics for tree species discrimination. These results indicate that metrics derived from airborne LiDAR data, especially radiometric metrics, can aid in discriminating tree species in a mixed temperate forest, and represent candidate metrics for tree species classification and monitoring in Central Europe.

High-range resolution spectral analysis of precipitation through range imaging of the Chung-Li VHF radar

Abstract. Multi-frequency range imaging (RIM) has been operated in the Chung-Li very high-frequency (VHF) radar, located on the campus of National Central University, Taiwan, since 2008. RIM processes the echo signals with a group of closely spaced transmitting frequencies through appropriate inversion methods to obtain high-resolution distribution of echo power in the range direction. This is beneficial to the investigation of the small-scale structure embedded in dynamic atmosphere. Five transmitting frequencies were employed in the radar experiment for observation of the precipitating atmosphere during the period between 21 and 23 August 2013. Using the Capon and Fourier methods, the radar echoes were synthesized to retrieve the temporal signals at a smaller range step than the original range resolution defined by the pulse width, and such retrieved temporal signals were then processed in the Doppler frequency domain to identify the atmosphere and precipitation echoes. An analysis called conditional averaging was further executed for echo power, Doppler velocity, and spectral width to verify the potential capabilities of the retrieval processing in resolving small-scale precipitation and atmosphere structures. Point-by-point correction of range delay combined with compensation of range-weighting function effect has been performed during the retrieval of temporal signals to improve the continuity of power spectra at gate boundaries, making the small-scale structures in the power spectra more natural and reasonable. We examined stratiform and convective precipitation and demonstrated their different structured characteristics by means of the Capon-processed results. The new element in this study is the implementation of RIM on spectral analysis, especially for precipitation echoes.

Scattering response of featured uniaxial anisotropic chiral medium-coated twisted PEC cylinder

Scattering of electromagnetic waves from a perfectly electrical conducting (PEC) cylinder of circular cross-section, coated with uniaxial non-magnetic chiral anisotropic medium, was discussed. A conducting sheath helix structure with adjustable pitch angle exists over the outer surface of coated cylinder, which is interfaced with the free-space. Under the uniform parallel polarized plane wave with fixed oblique incidence angle, the echo widths were determined emphasizing the other relevant scattering-related parameters (e.g. scattering/extinction efficiency etc.), taking into account different pitch angle values (of the sheath helix) and the kinds of coating mediums. The results revealed that the coating of negative uniaxial chiral medium causes enhancement in scattering efficiency (of the scatterer). Also, the scattering efficiency was found to be the same as the extinction efficiency, thereby yielding vanishing absorption of the medium. The obtained results were also compared with the situation of vanishing chirality of coating medium over the PEC cylinder; the echo width and scattering efficiency both decrease in this case.

Effects of admittance and gyrotropy on the scattering due to chiro-ferrite medium coated microstructured PMC cylinder

The paper deals with scattering of electromagnetic (EM) waves by perfectly magnetic conducting (PMC) cylinder coated with chiro-ferrite medium under the assumption of oblique angle of incidence wave with perpendicular polarization (transverse electric; TE). An on-demand (in respect of orientation) kind of conducting sheath helix structure is assumed to exist at the outer surface of cylinder. The effects of sheath helix orientation, along with the material parameters, such as chirality admittance and gyrotropy, on the echo width as well as the magnitude and phase of the electric and magnetic fields are investigated. Control on the anisotropic property remains greatly useful in obtaining the required optical response from the scatterer — the feature which would find fabulous sensing related applications.

Waveform width of a satellite laser altimeter illuminating on the sea surface.

The echo width is one of the most essential parameters of waveforms, which can be used to evaluate the ranging performance of a laser altimeter or retrieve wind speeds when laser pulses illuminate on the sea surface. The classical expression of waveform widths is not suitable for a current satellite laser altimeter because only the waves on the sea with smaller wavelengths can be covered by a satellite laser altimeter due to its small footprint. A new expression is derived by dividing the broadening effect of echo widths into the wind wave with small wavelengths and the swell cause with large wavelengths. The Geoscience Laser Altimeter System (GLAS) and National Centers for Environmental Prediction (NCEP) data are used to verify the new expression. Based on the new echo width expression, the ranging performance is evaluated when a laser is illuminating on the ocean surface. It indicates that a laser altimeter performs a better ranging accuracy (better than 5cm) than when it is evaluated using the classical expression (worse than 6cm) at moderate and strong winds.

Echo width of plasma coated topological insulator cylinder

Echo width of plasma coated topological insulator cylinder

Retrieval of higher order statistical moments from full-waveform LiDAR data for tree species classification

Tree inventories, inter alia, need information on the tree species. Previous studies aimed at distinguishing tree species based on three dimensional tree structure metrics derived from airborne laser scanning (ALS) point clouds or based on features from full-waveform (FW) laser scanning data provided by today's sensors. Classifications based on FW features mainly use echo amplitude, pulse energy (hereafter referred to as energy) and width, which are typically retrieved by waveform decomposition, often performed using the symmetric Gaussian distribution function.However, for forested areas, the symmetry of the echo shape is potentially modified by multiple scattering and the distribution of scattering elements (e.g. leaves). We assess the potential of processing full-waveform ALS data such that the third and fourth statistical moments, i.e. the echo skewness and the echo kurtosis, can be retrieved in addition to the amplitude, energy and FWHM. We propose a waveform decomposition approach using the skew normal distribution (SND) function, which enables the modelling of skewed echoes. We investigate the difference of tree-crown aggregated SND derived FW features between seven tree species (969 individual trees) in a temperate mixed forest with the aim of detecting the most descriptive echo features. The such derived FW features are tested for species classification.The results reveal that the largest differences across the tree species are in the mean energy of the first echoes (15 out of 21 species pairs show differences), followed by the mean amplitude of the first echoes and the mean skewness of all echoes originating from a single crown (14 out of 21 species pairs show differences against each other for both features).The differentiation of coniferous and deciduous trees benefits from the features derived from the SND decomposition compared to the use of echo amplitude only (0.39 vs. 0.61 in Cohen's κ). As the classification accuracy of the three dominant tree species within the test site only shows a small increase (0.20 vs. 0.26 in Cohen's κ) by adding FW features, we propose the use of such features in combination with features from multispectral data for this purpose.The SND decomposition is comparable to the Gaussian decomposition regarding the decomposition accuracy (RMSE=4.45 vs. RMSE=3.50) and computational cost. Hence, we propose the default use of the SND decomposition, as the SND is a more flexible function, allowing for the modelling of normally distributed echoes, as well as the fitting of skewed echoes, while no limitations regarding the direction of the skewness are introduced. We attribute the difficulties in the tree species classification to the relatively wide ranges of the crown aggregated features within one species, which for some features results in a considerable overlap of the feature ranges across the species.

Scattering of waves by chiral medium-coated dielectric elliptical/circular cylinders with PEC sheath helix loading

The paper reports scattering of plane waves by elliptical/circular homogenous, isotropic, and non-magnetic dielectric cylinders coated with chiral medium and surrounded by a perfectly electrical conducting sheath helix structure. The effect of pitch angle on the echo width under the assumption of parallel polarization is discussed, and the results are compared taking into account dielectric cylinders of different dimensions. The introduction of negative refraction in chiral mediums, and its effects on both the elliptical- and circular-shaped scattering objects are evaluated. The results indicate significant impact of the used chiral medium coating as well as the embedded sheath helix structure to alter the bi-scattering echo width of the object.

Scattering characteristics of homogeneous magnetized ferrite coated PEMC cylinder

The scattering of plane waves from a magnetized ferrite coated perfect electromagnetic conductor (PEMC) cylinder is analytically formulated in the present paper. The transvers electric (TE) and the transvers magnetic (TM) polarizations cases of the incident electromagnetic waves are formulated separately and analyzed numerically. The analysis and formulations presented in this paper are generalized for the cases of the homogeneous magnetized ferrite coated perfect electromagnetic conductor cylinders (PEC, PMC, or PEMC). At different values of applied dc bias magnetic field and saturation magnetization both Co and Cross-Polarized components of Radar Cross Section (RCS) are calculated by using scattering coefficients and plotted. Changes in monostatic echo width of both the Co and Cross-Polarized fields caused by variations in the applied dc magnetic field, frequency, and saturation magnetization are investigated. Comparison of the obtained results of this paper with already published literature displays the accuracy of the presented results and formulations.