Echo Waveform Research Articles

Semi-analytical Monte Carlo simulation of underwater target echoes from a small UAV-based oceanic lidar

Small unmanned airborne oceanic lidars are becoming increasingly popular for ocean surveys. They typically weigh less than 10 kg and can be mounted on commonly used commercial drones, making them easier to use than traditional large oceanic lidars, which typically weigh up to 80 kg and can only be carried on manned aircraft. Unmanned airborne oceanic lidars typically fly at lower altitudes and use higher laser emission frequencies, making them suitable for detecting underwater targets. However, most of the existing LiDAR underwater echo simulation models are designed for flat terrain and assume that the target fills the LiDAR receiving field of view, and there are few studies on the echoes of suspended targets smaller than the field of view range. Therefore, we combined ray tracing and semi-analytical Monte Carlo to model the laser echo energy of the underwater target. The study investigated the energy change of the target echo in the direction of deviation from the laser outgoing direction, and the impact of absorption and scattering coefficients on it. The difference in echo energy between different fields of view for extended and non-extended targets and the effect of water quality on it were explored. The echo waveforms of the target under different tilt angles were simulated, and it was found that the target echo pattern will be tilted with the change of tilt angle and offset position.

Ultrasound enabled simultaneous measurement of coating wear depth and lubricant film thickness in a sliding bearing

Sliding bearings, serving as critical supporting components in rotating machinery, often work by riding on an ultra-thin oil film. The rupture of this oil film can cause wear of the soft coating on the bearing and even lead to catastrophic failures. This paper introduces a novel ultrasonic reflection-based approach to simultaneously measure both lubricant film thickness and coating wear depth. Through an analysis of the ultrasonic wave propagation in a worn bearing, it is observed that the echo waveform remains independent of wear depth but correlates with changes in the time of flight back to the transducer. Given this, the captured time or phase shift between the reflected and incident waves, together with the reduction in the wave amplitude, enables a simultaneous calculation of both oil film thickness and coating wear depth. The efficacy of the proposed method is validated through acoustic simulations and experiments with varied coating roughness.

3D point cloud reconstruction for array GM-APD lidar based on echo waveform decomposition

Array GM-APD lidar only reconstructs one response distance per pixel, resulting in 3D point clouds sparse. This paper proposes a 3D point cloud reconstruction algorithm using echo waveforms directly. First, combining empirical mode decomposition and the array GM-APD lidar echo waveform characteristics to extract multiple target response distances per pixel. Then, assuming per-pixel target surface rotation to resolve the shared azimuth and polar angles of multiple response distances. Calculate the rotation angles using the decomposed distance difference and by minimizing the plane fitting error. Finally, reconstruct the 3D point cloud by rotation translation transformation. Simulations and experiments demonstrate that the proposed algorithm reduces the Chamfer Distance of the reconstructed 3D point cloud by over 10 % compared to the original. The reconstructed 3D point cloud of buildings and vehicles shows lower sparsity and stepwise, and the reconstructed 3D point cloud for each target in the multiple-target scene is unaffected by others.

Echo characteristics of pulsed lasers in non-uniform smoke environments

This work establishes a pulsed laser backscattering echo signal model based on an improved semi-analytical Monte Carlo method. The developed model is applied in non-uniform smoke environments to mitigate the smoke interference of laser fuzes for ground proximity detection. The model considers variations of the photon step size according to the concentration of the smoke environment, and the computational speed is accelerated by implementing an improved semi-analytical reception method. The proposed echo signal model is employed to investigate the effects of smoke-related parameters and laser detection system parameters on the backscattering echo waveform of smoke. Finally, the model is validated based on experiments. The results show that the intensity of the smoke backscattering echo and the emission pulse width approximately conform to a logarithmic relationship. Specifically, the echo intensity is positively correlated with the emission pulse width, and the ratio of the time between the rising edge and the falling edge of the echo signal is positively correlated with the pulse width. Meanwhile, the intensity of the smoke backscattering echo and the distance between the transmitter and receiver approximately adopt an exponential relationship. Herein, we describe the characteristics of a laser backscattering echo in a non-uniform smoke environment. The results can guide future research regarding laser fuze detection methods and strategies relevant for ground targets in smoke environments.

Mitigation method of acoustic doppler velocity measurement bias

A Doppler velocity log (DVL) is used to estimate ship’s velocity by processing seafloor echo. The velocity measurement bias (i.e., long term accuracy) is often regarded as an important technical specification to quantify the accuracy of DVL. The bias describes the accuracy of the instrument after averaging over a sufficiently long time, such that the velocity variance approaches zero. The goal of this paper is to obtain an effective method to mitigate the bias in velocity measurement based on analyzing the DVL seafloor echo characteristics. First, a DVL bottom echo model is constructed based on highlight model, which can focus on the statistical characteristics of the echo waveform information. Then the algorithm for retrieving the echo highlight information is presented. The parameters of the highlight model based on experimental data of DVL echo are analyzed. The intrinsic connection between the highlight model and the point-based scattering model is discussed. Finally, the method of velocity measurement to mitigate the bias according to the relationship between the statistical characteristics of the highlights and bias is put forward. Simulation results show that the proposed method significantly mitigates the bias.

Echoes of massless scalar field induced from hairy Schwarzschild black hole

A hairy Schwarzschild black hole describes the deformation of Schwarzschild black hole with an exponent correction due to the introducing of additional sources. Inspired by the novel feature that the hairy Schwarzschild black hole can have double photon spheres for certain parameters space, in this paper we study the echo signals of a massless scalar field from this hairy Schwarzschild black hole by calculating its time evolution. We mainly analyze how the hairy parameters and scalar field's angular momentum affect the echo waveform of the perturbing scalar field. Additionally, we roughly evaluate the time delay of echoes. We find that the hairy parameters have significant influences on the echo signals.

A high-precision fusion bathymetry of multi-channel waveform curvature for bathymetric LiDAR systems

Airborne LiDAR bathymetry (ALB) system is an attractive and efficient method for nearshore bathymetry and underwater topography mapping. To ensure and improve the measurement accuracy and reliability in various water environments, different receivers using a segmented field of view (FOV) have been designed and are implemented in ALB. These are used to obtain various echo data of multiple channels and perform bathymetry at various water depths. However, in echo waveform processing, detailed information about the water body described by the different echo waveforms is lacking, making it difficult to overcome severe waveform superposition in extremely shallow areas or to separate the weak echo of the water bottom from the noise in extremely deep areas. Therefore, we employed a novel method of multi-channel waveform fusion bathymetry (MWFB) to achieve information complementarity, gain multi-channel waveforms, and detect the robustness of water and bottom echo signals. This method uses an adaptive signal waveform extraction of a multichannel fusion mechanism to eliminate systematic and random noise, and the waveform curvature is used to reconstruct the energy–waveform curves. Iterative decomposition based on waveform curvature and energy curves was conducted to improve the correctness of waveform decomposition and the reliability of waveform components. Furthermore, the water surface and bottom peaks were detected based on the multichannel waveform curvature, which enabled acquisition of the accurate temporal positions of the peaks and achieve high-accuracy bathymetry. According to the experimental results and a comparison with reference datasets, the highest bathymetric accuracy of the MWFB method reached an RMSE of 0.22. For the different study areas, the bathymetric accuracy of the slope reached 95%, 92%, and 97% at Ganquan Island, Lingyang Reef, and Bei Island, respectively. Furthermore, the bathymetric point numbers were effectively increased and bathymetric accuracy was also improved in shallow and deep water, which illustrates the superior bathymetric performance of the MWFB method and its ability to provide a high-efficiency waveform dataset through multi-channel data fusion. The novel LiDAR bathymetric method proposed in this study can effectively achieve high-accuracy nearshore bathymetry and seafloor topographical mapping.

Measurement of ultrasonic pulse arrival time by constructing a signal model to determine its propagation velocity

The paper considers several methods of measuring the arrival time of ultrasonic pulses. A method for determining the pulse arrival time based on the construction of a signal model with an adaptive dictionary and the search for the minimum of the target function by the quantum swarm intelligence method is proposed. The results of numerical and modeling experiments on measuring the propagation velocity of ultrasonic waves in various samples are presented. It is shown that the proposed method of determining the time of pulse arrival is more resistant to distortion of the echo waveform arising due to frequency-dependent attenuation in the material of the control object.

The effects of range and echo-phase on range resolution in bottlenose dolphins (Tursiops truncatus) performing a successive comparison taska).

Echolocating bats and dolphins use biosonar to determine target range, but differences in range discrimination thresholds have been reported for the two species. Whether these differences represent a true difference in their sensory system capability is unknown. Here, the dolphin's range discrimination threshold as a function of absolute range and echo-phase was investigated. Using phantom echoes, the dolphins were trained to echo-inspect two simulated targets and indicate the closer target by pressing a paddle. One target was presented at a time, requiring the dolphin to hold the initial range in memory as they compared it to the second target. Range was simulated by manipulating echo-delay while the received echo levels, relative to the dolphins' clicks, were held constant. Range discrimination thresholds were determined at seven different ranges from 1.75 to 20 m. In contrast to bats, range discrimination thresholds increased from 4 to 75 cm, across the entire ranges tested. To investigate the acoustic features used more directly, discrimination thresholds were determined when the echo was given a random phase shift (±180°). Results for the constant-phase versus the random-phase echo were quantitatively similar, suggesting that dolphins used the envelope of the echo waveform to determine the difference in range.

Modelling and Characterization of LiDAR Echo Waveforms in Fog with Experiment Validations

Modelling and Characterization of LiDAR Echo Waveforms in Fog with Experiment Validations

Simulation Test of The Aerodynamic Environment of A Missile-Borne Pulsed Laser Forward Detection System at High Flight Speed

When a missile-borne pulsed laser forward detection system flies at supersonic speed, the laser beam will be distorted by the uneven outflow field, resulting in a significant reduction in ranging accuracy. In this paper, the impact of high flight speed on a pulsed laser detection system is studied. First, a new ray tracing method with adaptive step size adjustment is proposed, which greatly improves the computational efficiency. Second, the aerodynamic environment of a munition flying at high speed is simulated by an intermittent transonic and supersonic wind tunnel to obtain the schlieren data of the flow field at various Mach numbers. The schlieren data present a shock wave structure similar to that of the simulation. In addition, the variation patterns of the pulsed laser echo waveform of the model under different aerodynamic conditions are studied to evaluate the detectability and operational stability of the laser detection system under static conditions. The test results match the simulation results well, and the two offer relatively consistent shock wave structures, which verifies the correctness and effectiveness of the flow field simulation model. The test echo waveforms are in good agreement with the simulated echo waveforms; the relative errors between the peak values of test and simulated echo waveforms at various Mach numbers do not exceed 20%, and the correlation coefficients between the test and simulated echo waveforms all exceed 0.7, indicating high correlations between the two.

Calculation method and mathematical model of projectile echo power of active laser detection target

AbstractTo meet the requirements for parameter testing within the projectile test system, a computational model for calculating the echo power of the projectile passing through the detection screen based on the principle of active laser detection target is established in this paper. On the basis of the fundamental formula of echo energy in laser detection, and by combining the energy distribution function of the detection screen, the light scattering cross‐section formula specific to the projectile, and the illuminated area formula when the projectile intersects the detection screen, the formula for calculating the echo power of projectile is derived and analyzed. Furthermore, taking into account the parameters of the laser detection target, a computational model is constructed to calculate the voltage amplitude associated with the echo power of the projectile. Combining the atmospheric transmission optical characteristics, distance variations, and changes in the projectile's reflection attitude in the test environment, we conducted a simulation analysis to assess the accuracy of the established calculation model. Additionally, experimental analysis of the echo power calculation model was performed. In both simulation and experimental analyses, the trends in the impact of atmospheric transmission optical characteristics, distance conditions, and changes in projectile attitude upon the calculation of projectile echo power were consistent. Moreover, the width of the projectile echo waveform obtained in experiments was equal to the calculated projectile duration time. These research findings can serve as a reference basis for the development of theories in laser detection target.

Target edge extraction for array single-photon lidar based on echo waveform characteristics

The target edge contains the shape and structure information of the target. Accurate extraction of target edges is the basis of target recognition and location. The number of pixels and signal-to-noise ratio severely limit the edge extraction capability of single-photon lidar. In this work, a target edge extraction algorithm for array single-photon lidar is proposed based on the distribution characteristics of the target edge echo waveform. The algorithm does not need to image the target or rely on the fixed direction edge detection template. The target edge is divided into the inside and outside edges based on the established target edge echo signal model of array single-photon lidar. Single and double Gaussian function fitting is used to extract the outside edge whose echo waveform is a double pulse distribution. The feature vector composed of skewness, FWHM, and total detection probability of the echo waveform is weighted K-means clustered to extract the inside edge whose echo waveform is a single pulse distribution. Screening misjudged pixels based on the Jensen-Shannon divergence between echo waveforms of each pixel to improve the edge extraction accuracy. Simulation and experimental results demonstrate that the proposed algorithm can extract target edges more completely and accurately than traditional edge extraction methods. The minimum inside edge depth of the target that the proposed algorithm can extract is cτ/2, where τ is the standard deviation of the emitted Gaussian laser pulse. This research has the potential to improve the target detection and recognition capabilities of array single-photon lidar.

Waveform design for resonance signature of fighter‐class target

AbstractThe authors investigate the feasibility of forming a composite Gaussian waveform in the high‐frequency region for jetfighter class targets with a high potential to combat the noise effect and the late‐time signature onset ambiguity when estimating the target resonance signature. The excitation waveform comprises a composite of modulated carriers with fractional bandwidth and amplitude adapted to lower specular content while enhancing the radar target resonance content in the radar return, subsequently increasing the likelihood of extracting the resonance parameters more accurately. The validation of improvement in the resonance signature requires finding the error between the excitation frequencies and the extractable resonant frequencies in the return echo and, second, the similarity or resemblance between the original echo waveform and the reconstructed waveform by the estimated resonance set. The results presented the robustness rate in instantaneous, average, and overall fashion. In conclusion, a fractional bandwidth between 0.3 and 0.4 would significantly improve the signature overall robustness with a circular then slant scattering polarisation set outperforming the horizontal and vertical directions. Also, excluding the signal's high and low energy portions further enhance the accuracy of the estimated resonance mode set.

A Calibration Method for Large-Footprint Full-Waveform Airborne Laser Altimeter without a Calibration Field

The geometrical measurement precision of laser spots is affected by the deviation between the parameters of the laser altimeter and the laboratory measurement results, and the inversion accuracy of surface object height is also limited. The measurement parameters and the load state can be obtained by calibration of the laser altimeter system. Usually, ground detectors are deployed to calibrate the measurement parameters of the laser altimeter, including the divergence angle and the energy distribution of the laser beam. A calibration method for a laser footprint spot without a calibration field was proposed in this paper, focused on the airborne large-footprint laser altimeter system. The geometric parameters of the laser spot were calibrated through the laser echo waveforms of a specific terrain. The experimental results show that geometric calibration of the large-footprint laser altimeter can be achieved in the area of the step surface. The divergence angle of the laser beams obtained from the six experimental areas is 4.604 ± 0.359 mRad, and the consistency of the energy distribution from each laser spot reaches 92.67%. A new method of on-orbit calibration and verification is provided for the satellite laser altimeter system.

Analysis and Simulation of Space-Based LM-APD 3D Imaging

The linear mode avalanche photodiode (LM-APD) array has the capability of real-time 3D imaging for moving targets, which is a promising 3D imaging means in space. The main system parameters of the LM-APD array 3D imaging system, the characteristics of the space target itself, and the relative positional relationship between them will affect the 3D imaging results at the same time, and there is a need for an appropriate simulation method to describe the space target point cloud acquired by the LM-APD array 3D imaging system under different conditions. We propose a simulation method for the 3D imaging of space targets with LM-APD arrays, which takes the characteristics of the space targets and the relative position into consideration, and build a link from the laser to the receiving system to simulate the echo waveform of each pixel in the LM-APD array. The experiment results under different conditions show that the proposed simulation method can accurately describe the imaging results of the LM-APD array 3D imaging system for space targets with different shapes, materials, and motion states, which providing theoretical and data support for the design of LM-APD array 3D imaging systems.

Pulse Lidar imaging algorithm based on adaptive triangle window-width centroid discrimination

The pulse light detecting and ranging (Lidar) can obtain rich environmental information of the target, and the centroid algorithm can obtain the distance and intensity information of the target from the echo pulse. However, there are problems of sensitivity to noise and fixed window width. Therefore, this paper proposes an adaptive triangle window-width centroid algorithm (ATWC) to improve the ranging accuracy and robustness of Lidar. This algorithm combines the characteristics of the steep front and slow back of the pulse echo waveform. The centroid calculation point is selected by an isosceles triangle envelope in the time domain and a non-isosceles triangle envelope in the intensity domain with a longer front and a shorter back. Further combined with intensity weighting, the distance and intensity values of ATWC are obtained. On the one hand, the simulation verifies the influence of window width selection on the accuracy of the centroid algorithm. On the other hand, real environment comparison experiments are conducted with traditional waveform centroid (TWC), peak ranging (PR), and adaptive threshold methods (ATM) under a pulse Lidar system constructed by our method, and our method achieves significant improvements. At different distances, when the peak signal-to-noise ratio (PSNR) is above 20 dB, the ranging error of ATWC is below 0.3 ns, which is much better than the compared algorithms. When the target is at a distance of 25 m, which is a weak echo signal environment with a PSNR of 15.6 dB, our algorithm can still achieve a ranging error of 0.56 ns, and the detection success rate can still reach 88.8%. In addition, our method can obtain a clearer intensity image, which further demonstrates the research potential and practical utility of the proposed method.

Laser echo waveform modulation modelling from lateral structure using a mathematical formula

ABSTRACT The relationship between the axial structures of three-dimensional (3-D) targets and the signal of the remote-sensing techniques, such as the modulated echo waveform of the full-waveform light detection and ranging (FW-LiDAR), has been well established. However, the relationship between the lateral structures on modulated echo waveform has not been exploited in detail. For FW-LiDAR, the peak intensity, rather than the shape and width, of the echo waveform reflects lateral structural information. Using four typical two-dimensional shapes to approach the lateral structures, a mathematical formula bridging the peak intensity and the lateral structures is derived. Based upon the echo waveform simulated using the formula, modulations of the lateral structural information on the peak intensity are examined to establish a database of target properties, including shape, size and position. The physics regarding peak intensity dependence on target position is used for facile shape recognition. The modulation of the lateral shapes of ground and aerial targets on the FW-LiDAR can be assessed extensively following the procedure demonstrated in this paper. Owing to the conciseness, the peak intensity formula enables the retrieval of lateral structural information by inversion, promoting the applications of FW-LiDAR in domains including topological mapping, ecological monitoring and space debris detection and removal.

Effects of absolute range and echo phase on range discrimination in bottlenose dolphins (Tursiops truncatus)

Echo-delay (range) resolution is a critical feature of animal biosonar. In 1973, Jim Simmons was the first scientist to test bat range discrimination thresholds at a variety of absolute ranges. He used a two alternative-force-choice paradigm with two phantom echo generators (PEGs) and compared the results to the bats’ performance with physical targets. In the present study, methods similar to Simmons’ 1973 study were used to test two dolphins’ range discrimination thresholds using a two channel PEG system. Range discrimination thresholds were tested at seven different absolute ranges between 1.75 and 20 m. Ranges were simulated by manipulating echo-delay while relative echo level was held constant for all parameters. To examine potential effects of echo phase, measurements were repeated at a 7 m range while randomizing echo phase. For both subjects, range discrimination thresholds increased exponentially with absolute range. Randomizing echo phase did not significantly change thresholds, suggesting that dolphins use the envelope of the echo waveform to determine range versus the fine structure.

Classification of Terrestrial Lidar Data Directly From Digitized Echo Waveforms

Information derived from full-waveform (FW) data collected by FW laser scanning systems has already been shown to be relevant for point cloud analysis tasks. Relevant waveform attributes to populate the corresponding point’s feature vector are typically provided through a post-processing FW analysis (FWA) technique based on fitting the echo waveform with a parametric function describing the shape and location of the echo pulse in the waveform. Samples of the digitized echo are the primary source for any waveform analysis using parametric functions. On the other hand, for some FW laser scanning systems, describing the complex system response model using a simple parametric function seems challenging or impractical. Earlier studies have shown the potential of waveform’s digital samples as relevant waveform attributes, for point cloud classification. The main goal of this study is to extend earlier experiments on direct exploitation of returned waveform signals collected by a FW terrestrial laser scanning (TLS) system in a built environment for point cloud classification, to multi-return waveform signals. Furthermore, the classification performance on feature vectors containing calibrated waveform attributes, derived from a waveform processing approach performed in real-time by the FW TLS system, is evaluated on multiple-echo waveforms and compared with the classification performance derived from the proposed FW data classification technique. Classification performance derived through the proposed technique demonstrates high information content of raw digitized waveform samples. Results show that feature vectors containing samples of digitized echoes carry more information about physical properties of the target than those containing calibrated waveform attributes.