Slant Range Research Articles

Sperm whale acoustic abundance and dive behaviour in the western North Atlantic

Sperm whales are an ideal species to study using passive acoustic technology because they spend the majority of their time underwater and produce echolocation clicks almost continuously while foraging. Passive acoustic line transect data collected between June and August 2016 were used to estimate a depth-corrected acoustic abundance and study the dive behaviour of foraging sperm whales in the western North Atlantic Ocean. 2D localizations (n = 699) were truncated at a slant range of 6500 m and combined with the multipath arrivals of surface reflected echoes to calculate 3D localizations (n = 274). Distance sampling using depth-corrected perpendicular distances resulted in a 10.5% change in the acoustic abundance estimate (2199 whales, CV = 14.6%) compared to uncorrected slant ranges (1969 whales, CV = 14.1%), and a detection function that was a better fit for the data. Sperm whales exhibited multiple foraging strategies, with bottom phases occurring at depths of 400–800, 800–1200, or > 1200 m, accounting for an average 39.2, 49.5, or 44.9% of the total recorded dive time, respectively. These results suggest that estimating 3D localizations using acoustic line transect data improves acoustic abundance estimation and can be used to answer population level questions about foraging ecology.

Elevation Change of CookE2 Subglacial Lake in East Antarctica Observed by DInSAR and Time-Segmented PSInSAR

In this study, elevation change and surface morphology of CookE2, one of the most active subglacial lakes in East Antarctica, were analyzed by using Differential Interferometric Synthetic Aperture Radar (DInSAR) and a newly adapted Time-Segmented Persistent Scatterer Interferometric Synthetic Aperture Radar (TS-PSInSAR) techniques. Firstly, several DInSAR pairs were used to study the surface morphology of the subglacial lake during the rapid discharge event in 2007 and the subsequent recharge in 2010 by using ALOS PALSAR data and the continuous recharge from 2018 to 2020 by using Sentinel-1 SAR data. For time-series observation from 2018 to 2020, however, simple integration of DInSAR deviates largely from the satellite altimeter data because errors from the horizontal flow of the surrounding ice field or atmospheric phase accumulate. Conventional PSInSAR deviates from the altimeter data if the LOS displacement exceeds 300 mm, i.e., approximately 1/4 of the slant range resolution of the Sentinel-1 SAR in Interferometric Wide-swath (IW) mode, during the time window. Therefore, a series of Time-Segmented PSInSAR with a 4-month time window could accurately distinguish 1.10 ± 0.01 m/year of highly linear (R2 = 0.99) surface rise rate of CookE2 and 0.63 m/year of horizontal deformation rate of the surrounding ice field from 2018 to 2020.

A Method for the Interpretation of Sonar Data Recorded during Autonomous Underwater Vehicle Missions

Abstract Image acquisition from autonomous underwater vehicles (AUVs) is useful for mapping objects on the seabed. However, there are few studies on the interpretation of data collected with side-scan sonar during autonomous underwater vehicle missions. By recording the seabed with 3D multibeam sonar, a large number of survey points can be obtained. The collected data are processed using applications based on remote sensing image processing. The data collected during AUV missions (or other sonar carriers) needs to be pre-processed to reach the proper effectiveness level. This process includes corrections of signal amplification (Time Varying Gain, or TVG) and geometric distortions of sonar images (Slant Range Corrections). It should be mentioned that, when carrying out the interpretation process for structures on the sea floor, sonar users need to understand the process of visualising seabed projections and depressions, as well as the resolution limitations of the sonar sensors.

Multicriteria Accuracy Assessment of Digital Elevation Models (DEMs) Produced by Airborne P-Band Polarimetric SAR Tomography in Tropical Rainforests

The penetration capability of P-band radar waves through dense vegetation, along with the ability of tomography to separate the contributions of different layers in a vertical reflectivity profile, make P-band radar tomography a promising tool for digital terrain modeling in forested areas, specifically in dense tropical forests under which terrain topography remains poorly known. This paper aims to assess the overall quality of a digital terrain model (DTM) produced using tomographic processing of airborne P-band SAR imagery acquired during the TropiSAR campaign in French Guiana. Many quality descriptors are used to evaluate the quality of this DTM. Position and slope accuracies are computed based on a lidar DTM considered as the reference, and the impact of several parameters on these accuracies is studied, namely, slope, slope orientation, off-nadir angle and local incidence angle. The realism of the landforms is also studied according to geomorphological criteria. The results of this multicriteria accuracy assessment show the high potential of P-band SAR tomography in depicting the topography under forests, despite the intrinsic limitations related to the slant range geometry: the absolute elevation error is around 2 m; the slope is overestimated with an error of about 16°, mainly due to a processing artifact for which easy and direct solutions exist. Indeed, this error is equal to about 3° in flat artifact-free areas. These errors vary depending on the acquisition parameters and the local topography. The shapes are globally well preserved. These results are also discussed in the frame of the upcoming BIOMASS mission developed by the European Space Agency (ESA) and expected to be launched in 2024.

A novel SLC to GEO image generation approach and automatic control points conversion from GEO to SLC domain for SAR data quality evaluation and analysis

This paper presents a novel and efficient approach for the generation of georeferenced (GEO) image from Single Look Complex (SLC) image and a procedure for automatic conversion of control points from GEO to SLC image. Using the proposed approach, the control points in one image can be transformed to the corresponding control points in other image with subpixel accuracy. Automatic transformation of control points from GEO to SLC domain is an essential requirement in many of the Geometric Data Quality Evaluation (GDQE) and Radiometric Data Quality Evaluation (RDQE) activities. One of the main reason for control point transformation is that it is difficult to identify control points in SLC image due to its orientation, distortion and geometry of slant range SAR data. The proposed approach is also made operational in ISRO’s Data Quality Evaluation (DQE) software and is extensively applied for GDQE/RDQE evaluations on SAR data products.

A Novel Imaging Scheme of Squint Multichannel SAR: First Result of GF-3 Satellite

The azimuth multichannel synthetic aperture radar (MC-SAR) is an effective means of earth observation due to its high-resolution and wide-swath (HRWS) imaging capability. Compared with the traditional MC-SAR, the two-dimensional (2D) spectrum of the squint multichannel synthetic aperture radar (SMC-SAR) is skewed, leading to the severe coupling between azimuth signals and range signals. Therefore, the traditional imaging algorithms are inapplicable to SMC-SAR. Currently, the existing imaging algorithms for SMC-SAR need interpolation or sub-aperture processing, which results in low efficiency. This paper proposes a novel imaging scheme for spaceborne SMC-SAR to handle this issue. With the squint angle known, the Doppler centroid can be compensated for by the spectrum shift function. Additionally, the frequency correlation method (FCM) is utilized to estimate the channel phase imbalance. After that, the azimuth multichannel data are reconstructed by the filter banks to eliminate the Doppler spectrum aliasing. Finally, the data are imaged based on the modified equivalent slant range model (MESRM). Both the simulation and Gaofen-3 (GF-3) data experiments demonstrate the effectiveness and efficiency of the proposed method.

Extended Detection of Shallow Water Gas Seeps From Multibeam Echosounder Water Column Data

Multibeam echosounder water column data provides a three-dimensional image of features between the water surface and the seafloor. Although this swath of acoustic data can be collected over a wide range of angles, most of the data, at least beyond the range to the first seafloor return, is contaminated by noise created by receiver array sidelobe interference. As a result, the water column data beyond the minimum slant range commonly is excluded from analysis. This paper demonstrates a method to consistently filter and extract targets comprising a gas seep feature across the multibeam swath, including targets within the areas dominated by receiver array sidelobe interference. For each sample range, data are filtered based on the mean plus a certain number (k) of standard deviations of the sample values along that range. The filtering is coupled with a morphological classification to retain only targets of interest while excluding background data and noise. Data were collected over a shallow water artificial gas seep using two different flow rates and at three different vessel speeds. Using the proposed method, 119 of 124 test seeps were identified correctly. Seep targets were identified at all angles across the water column fan up to beam pointing angles of 55°, with 19 of 23 seeps being correctly identified at angles greater than 50°. This method demonstrates that features can be extracted and geolocated in the sidelobe noise when the interference is appropriately filtered. These results will improve the areal extent of multibeam surveys and increase the utility of acoustic data in capturing information on water column targets directly above the seafloor.

Passive Inverted Ultra-Short Baseline Positioning for a Disc-Shaped Autonomous Underwater Vehicle: Design and Field Experiments

Underwater positioning is critical to autonomous underwater vehicles (AUVs) for navigation and geo-referencing. The rapid attenuation of the electromagnetic wave in the underwater environment prevents the use of traditional positioning methods such as the Global Positioning System, whereupon acoustic methods like ultra-short baseline (USBL) positioning systems play an important role in AUV navigation. However, the high cost and complexity of classical USBL systems have stifled the democratization of these technologies, which leads to a new method called passive inverted ultra-short baseline (piUSBL) positioning. In a typical piUSBL system, a single beacon is placed at a reference point, periodically broadcasting a positioning signal. A passive USBL receiver, time-synchronized to the beacon, is mounted on an AUV to get one-way travel-time (OWTT) slant range and azimuth estimates. The passive nature of the receiver means the system is inexpensive, low-power, and lightweight. Particularly, the omnidirectional broadcasted signals offer a feasible solution for concurrent multi-AUV navigation. This letter demonstrates a full-stack design and development of a piUSBL positioning system, and presents evaluations of the accuracy and reliability of the system through a series of experiments. More significantly, a successful sea trial of a disc-shaped AUV outfitted with our piUSBL was conducted in the South China Sea.

An Improved Independent Parameter Decomposition Method for Gaofen-3 Surveying and Mapping Calibration

The Gaofen-3 (GF-3) satellite can provide digital elevation model (DEM) data from its interferogram outputs. However, the accuracy of these data cannot be ensured without applying a surveying and mapping (SAM) calibration process, thus necessitating geometric and interferometric calibration technologies. In this paper, we propose an independent parameter decomposition (IPD) method to conduct SAM calibration on GF-3 data and generate high-accuracy DEMs. We resolved the geometric parameters to improve the location accuracy and resolved the interferometric parameters to improve the height accuracy. First, we established a geometric calibration model, analyzed the Range–Doppler (RD) model and resolved the initial imaging time error as well as the initial slant range error. Then, we established a three-dimensional reconstruction (TDR) model to analyze the height error sources. Finally, the interferometric phase error and baseline vector error were precisely estimated to ensure the vertical accuracy of the interferometric results by establishing the interferometric calibration model. We then used the GF-3 interferometric data derived on the same orbit in a north–south distribution to conduct the calibration experiment. The results show that the plane positioning accuracy was 5.09 m following geometric calibration, that the vertical accuracy of the interferometric results was 4.18 m following interferometric calibration and that the average absolute elevation accuracy of the derived DEM product was better than 3.09 m when using the GF-3 SAR data, thus confirming the correctness and effectiveness of the proposed GF-3 IPD calibration method. These results provide a technical basis for SAM calibration using GF-3 interferograms at the 1:50,000 scale in China.

An Adjusted Frequency-Domain Algorithm for Arc Array Bistatic SAR Data with One-Moving Transmitter.

Arc array synthetic aperture radar (AA-SAR), which can observe the scene in all directions, breaks through the single view of traditional SAR. However, the concealment of AA-SAR is poor. To mitigate this, arc array bistatic SAR (AA-BiSAR) with the moving transmitter is proposed, it has the advantages of good concealment and can expand the imaging scene, and improve the flexibility of the system. The imaging geometry including the signal model is established, and a range frequency-domain algorithm based on keystone transform (KT) is proposed in this paper. In the first step, the slant range equation is approximated by Taylor series expansion to compensate for the residual phase caused by the transmitter motion. In the second step, the range cell migration between the range and azimuth is eliminated through the KT method in the range frequency-domain. In the third step, using the data after range cell migration correction in step 2, an azimuth pulse compression is performed to obtain the focused image. In addition, the spatial resolution of the AA-BiSAR system is analyzed in detail. Finally, three simulation results verify the effectiveness of the proposed algorithm and the change in the spatial resolution.

A Tensor Generalized Weighted Linear Predictor for FDA-MIMO Radar Parameter Estimation

Radar parameter estimation in terms of its range, angle and velocity plays a crucial role in many applications. Multiple-input multiple-output (MIMO) radar with frequency diverse array (FDA) is capable of resolving targets in one directional beam with different ranges and velocities (Doppler shifts). Prevalent methods obtain target parameters in a sequential manner to get rid of exhausted multi-dimensional search, but they suffer from accumulated estimation errors. To tackle this issue, a tensor generalized weighted linear predictor (TGWLP) is devised for FDA-MIMO radar parameter estimation, where the parameters are estimated in a parallel manner. Tensor modeling of multidimensional FDA-MIMO radar signal is developed, so that the joint parameter estimation is casted into multiple-pulse-group version of three-dimensional (3D) HR problem associated to the mixed Swerling model. Pulse-group diversity is exploited to obtain precise velocity estimation. In the presence of targets with some identical parameters, the final estimations of unambiguous slant range, conic angle, and radial velocity of a moving target can be easily obtained after the parallel frequency estimations. Besides, all the parameter pairing is automatically achieved, which is free of the extra burden from pairing process. Furthermore, the identifiability of the proposed joint estimator is analyzed. Finally, theoretical analysis and simulations are included to demonstrate that the proposed approach can achieve improved performance compared to the existing methods.

Real-time processing pipeline for automatic streak detection in astronomical images implemented in a multi-GPU system

Abstract Detecting and tracking objects in low Earth orbit is an increasingly important task. Telescope observations contribute to its accomplishment, and telescope imagers produce a large amount of data for this task. Thus, it is convenient to use fast computer-aided processes to analyze it. Telescopes tracking at the sidereal rate usually detect these objects in their imagers as streaks, their lengths depending on the exposure time and the slant range to the object. We have developed a processing pipeline to automatically detect streaks in astronomical images in real time (i.e., faster than the images are produced) by a graphics processing unit parallel processing system. After the detection stage, streak photometric information is obtained, and object candidate identification is provided through matches with a two-line element set database. The system has been tested on a large set of images, consisting of two hours of observation time, from the Tomo-e Gozen camera of the 105 cm Schmidt telescope at Kiso Observatory in Japan. Streaks were automatically detected in approximately 0.5% of the images. The process detected streaks down to a minimum apparent magnitude of +11.3 and matched the streaks with objects from the space-track catalog in 78% of the cases. We believe that this processing pipeline can be instrumental in detecting new objects and tracking existing ones when processing speed is important, for instance, when a short handover time is required between follow-up observation stations, or when there is a large number of images to process. This study will contribute to consolidating optical observations as an effective way to control and alleviate the space debris problem.

Frequency‐domain multireceiver synthetic aperture sonar imagery with Chebyshev polynomials

Based on Chebyshev polynomials, the two-way slant range is first approximated by the fourth-order polynomial approximation. With the series reversion method, the point target reference spectrum (PTRS) which is further used for the development of imaging algorithm in this letter is obtained. The presented method processes the echoed signal corresponding to each transmitter/receiver pair. And then a high-resolution synthetic aperture sonar (SAS) image is obtained by coherently superposing all coarse-resolution images. Comparing the peak sidelobe ratio (PSLR) and integration sidelobe ratio (ISLR), the performance of the presented method is superior to that of the traditional method. Simulations are further validated by the presented method.

A Novel DEM Block Adjustment Method for Spaceborne InSAR Using Constraint Slices.

The lack and uneven distribution of Ground Control Points (GCPs) will lead to the deterioration of Digital Elevation Model (DEM) block adjustment results in the bistatic Interferometric Synthetic Aperture Radar (InSAR) system. Given this issue, we first explain the relationship between the stability of adjustment parameters and the GCP distribution pattern theoretically using matrix perturbation theory. Second, we put forward the Constraint Slices (CSs) concept and first introduce CSs into the adjustment optimization model as constraint conditions rather than actual values as GCPs. Finally, we propose a novel DEM block adjustment method for spaceborne InSAR using CSs based on an optimization model with nonlinear constraints. The simulated experiment shows the instability of the conventional method and validates the proposed method under different parallel baseline errors. Four groups of real experiments were carried out according to the size of the uncontrolled area using twelve Co-registered Single-look Slant–range Complex (CoSSC) datasets for Henan Province, China. The adjustment results verified by the ICESat-2 ATL08 data demonstrate that the performance of the proposed method is better than the conventional method in the uncontrolled area; the corresponding improvements in adjustment accuracies compared with the conventional method are 0.13 m, 1.02 m, 2.12 m, and 8.18 m, respectively. At the same time, the proposed method can enhance the height consistency in overlapping areas, which is vital for seamless DEM production.

Geolocation accuracy improvement for NovaSAR-1 imagery acquired through TLE orbit

NovaSAR-1 is a small S-band Synthetic Aperture Radar (SAR) mission mainly for land and maritime applications. Since SAR has side-looking geometry, thus slant and ground range SAR products have geometric distortions. To correct these geometric distortions, orthorectification needs to be done using SAR Rigorous Sensor Model (RSM) algorithm and Digital Elevation Model (DEM). NovaSAR-1 provides the satellite state vectors either through a Global Positioning System (GPS) or through a Two-Line Element (TLE) orbit source. It is observed that, after orthorectification, it shows poor geolocation accuracy (200 m to 9000 m) for the TLE orbit source. In this paper, geolocation accuracy improvement has been done for NovaSAR-1 ground range imagery acquired through TLE orbit source by generating orthorectified products using SAR Rigorous Sensor Model (RSM) algorithm and Ground control point (GCP). After orthorectification, geolocation accuracy of the NovaSAR-1 imagery acquired through TLE orbit source is reduced to less than 10 m.

Theoretical Feasibility Analysis of Fast Back-Projection Algorithm for Moon-Based SAR in Time Domain

Nowadays, the Earth observation based on the Moon has attracted attention from many researchers and relevant departments. There also exists a considerable amount of interest in monitoring large-scale and long-term geoscience phenomena using the Moon-based SAR (MBS). However, the Earth’s observation from MBS has long transmission time, and the relative motion of MBS with its Earth ground target (EGT) is much different to the space-borne SAR, the above reasons indicate that the traditional stop-and-go model is no longer suitable for MBS in frequency domain imaging. Here a dual-path separate calculation method for single pulse is presented in this paper for a better match of a real scenario, and then the slant range is fitted to a high-order polynomial series. The MBS’s location, the synthetic aperture time and other factors have effects on length of the dual- path and fit bias. Without thoroughly investigated phase de-correlation processing in frequency domain, and to avoid computational costs in traditional back-projection (BP) algorithm, the paper first proposes a fast back-projection (FBP) algorithm in time domain for MBS, a platform that has long transmission time and long synthetic aperture time. In the FBP algorithm, the original method, that projected echo on all pixels in the imaging area, is changed to projected echo on a centerline instead. A suitable interpolation for points on the centerline is adopted to reduce the projected error; the synthetic aperture length and imaging area are also divided into subsections to reduce computation cost. The formula indicates that the range error could be control once the product of sub-imaging area’s length and sub-aperture’s length stay constant. Through the theoretical analysis, the detailed range difference mainly at apogee, perigee, ascending, and descending nodes indicate the necessity to separately calculate the dual-path for MBS’s single pulse transmission in Earth-Moon motion, with real ephemeris been adopted; then, the high-order polynomial fitting will better describe the motion trajectory. Lastly, the FBP algorithm proposed is simulated in a specific scenario under acceptable resolution, and the result shows its feasibility for image compression.

3-D Millimeter-Wave Helical Imaging

This article proposes a low-cost three-dimensional (3-D) millimeter-wave (MMW) holographic imaging system using helical scanning with multiple receivers to achieve a fast continuous scanning over a large two-dimensional (2-D) cylindrical surface. First, the system geometry and its imaging process based on the back-projection algorithm (BPA) are presented. The corresponding imaging point spread function (PSF) and resolutions are analyzed accordingly. To reduce the computational cost significantly, a novel 3-D helical imaging algorithm is then proposed based on the piecewise constant Doppler (PCD) principle. The slant range difference resulting from the helical scanning can be compensated jointly along angular and vertical directions. The proposed imaging is prototyped using the AWR1843 radar sensor from Texas Instruments (TIs) and a moving platform composed of step motors and a micro-controller unit (MCU). The digital imaging process and the number of the required complex multiplications are also discussed in detail. Finally, simulation and experimental results are provided to validate the accuracy and efficiency of the proposed imaging system.

Range Gate Pull-Off Mainlobe Jamming Suppression Approach with FDA-MIMO Radar: Theoretical Formalism and Numerical Study

With the development of an electronic interference technique, the self-defense jammer can generate mainlobe jamming using the range gate pull-off (RGPO) strategy, which brings serious performance degradation of target tracking for the ground-based warning radar. In this paper, a RGPO mainlobe jamming suppression approach is proposed, with a frequency diverse array using multiple-input multiple-output (FDA-MIMO) radar. The RGPO mainlobe jamming differs from the true target in slant range, thus it is possible to identify the true target from the RGPO mainlobe jammings by exploiting the transmit beampattern diversity of FDA-MIMO radar. A RGPO mainlobe jamming suppression approach is devised by using joint transmit–receive beamforming for a group of range sectors. The jamming suppression performance is studied, in consideration of practical time-delay of RGPO jamming. Simulation examples are provided to verify the effectiveness of the proposed approach.

Accurate modeling and coherent integration method for weak maneuvering target with oblique movement

Prolonging the integration time is a widely used method to enhance the detection performance for weak targets, whereas range migration (RM) and Doppler frequency migration (DFM) might occur when the target moves with high speed and high maneuverability. Although various algorithms have been proposed to solve the migration problem and realize coherent integration, they are based on the assumption that the target is moving radially relative to radar, i.e., the instantaneous slant range from radar to target is expressed as a finite order polynomial with respect to slow time. However, the target in the practical scene is probably moving obliquely relative to radar, thus modeling error will exist between the polynomial range expression and the target’s actual instantaneous slant range from radar. We focus on the accurate modeling and coherent integration problems for the weak maneuvering target with oblique movement, where the oblique movement model (OMM) of target echo is established. Subsequently, the OMM-generalized Radon–Fourier transform method is proposed to coherently accumulate the target’s energy via correcting the complex RM and DFM. Both simulation and real data processed results have been provided to demonstrate the effectiveness of the proposed method.

A Novel Imaging Algorithm for High-Resolution Wide-Swath Space-Borne SAR Based on a Spatial-Variant Equivalent Squint Range Model

High-Resolution Wide-Swath (HRWS) is an important development direction of space-borne Synthetic Aperture Radar (SAR). The two-dimensional spatial variation of the Doppler parameters is the most significant characteristic of the sliding spotlight space-borne SAR system under the requirements of HRWS. Therefore, the compensation of the two-dimensional spatial variation is the most challenging problem faced in the imaging of HRWS situations. The compensatory approach is then proposed to address this problem in this paper. The spatial distribution of the Doppler parameters for the HRWS space-borne SAR data in the sliding spotlight working mode is firstly analyzed, based on which a Spatial-Variant Equivalent Slant Range Model (SV-ESRM) is put forward to accurately formulate the range history for the distributed target. By introducing an azimuth-varying term, the SV-ESRM can precisely describe the range history for not only central targets but also marginal targets, which is more adaptive to the HRWS space-borne SAR requirements. Based on the SV-ESRM, a Modified Hybrid Correlation Algorithm (MHCA) for HRWS space-borne SAR imaging is derived to focus the full-scene data on one single imaging processing. A Doppler phase perturbation incorporated with the sub-aperture method is firstly performed to eliminate the azimuth variation of the Doppler parameters and remove the Doppler spectrum aliasing. Then, an advanced hybrid correlation is employed to achieve the precise differential Range Cell Migration (RCM) correction and Doppler phase compensation. A range phase perturbation method is also utilized to eliminate the range profile defocusing caused by range-azimuth coupling for marginal targets. Finally, a de-rotation processing is performed to remove the azimuth aliasing and the residual azimuth-variance and obtain the precisely focused SAR image. Simulation shows that the SAR echoes for a 20 km × 20 km scene with a 0.25 m resolution in both the range and azimuth directions could be focused precisely via one single imaging processing, which validates the feasibility of the proposed algorithm.