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The East African Rift System (EARS) is an active continental rift zone that hosts a diverse range of processes that result in recent and ongoing crustal deformation. To analyze these processes, we used Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) to complement the sparse GNSS velocity field with high resolution measurements of line-of-sight (LOS) deformation between 2015 and 2022. A broad survey approach was employed to detect new signals within the central section of the EARS, and to establish a baseline for future studies focused on tectonic, volcanic, and/or hydrologic deformation and change spanning geodetic time scales. With this regional approach, we were able to resolve signals varying over time-spans of a few weeks to several years that have magnitudes greater than ∼1 cm/yr. Our resulting deformation field shows a diverse range of signals related to the processes listed above as well as other unknown sources. This includes up to 8.5 cm/yr of groundwater-related subsidence in Nairobi, up to 70 cm of LOS displacement during the eruption of Nyiragongo in 2021, and steady uplift in the Manyara basin with a rate of 2.8 cm/yr. We update previously published InSAR measurements of Mount Suswa volcano, and the geothermal fields in Olkaria. We also show LOS change ranging between 2 and 5 mm/yr over a distances of approximately 200 km spanning the Kenya Rift with the greatest differential rates occurring south of Lake Turkana. More generally, our results highlight the ambiguity of relying solely on the sparse network of GNSS for studies such as those characterizing tectonic motion and rift opening. Such studies are likely missing many important signals and/or includes sites that are contaminated with signals unrelated to the target process. • InSAR complements the GNSS velocity field in the EARS, revealing high-resolution LOS deformation between 2015 and 2022. • Identified a diverse range of volcanic, tectonic, and anthropogenic deformation signals. • Illuminated the limitations of sparse GNSS networks for comprehensive deformation analysis. [ABSTRACT FROM AUTHOR]

Synthetic aperture radar (SAR) which acquires planar images at any time of the day and under any weather conditions is an important surveillance sensor in modern hi-tech war. For airborne SAR the image formation algorithms are ideally analyzed with the presumption that the aircraft moves in a straight line and at constant speed. However, the aircraft is affected by kinds of factors such as airstreams and its own instability, so the motion error between real aviation and ideal aviation exists, which can seriously blur the SAR image. So the motion compensation must be executed. In this paper a novel method for resolving turbulent aircraft motion is proposed based on SAR raw data. This method is very valid to conquer the turbulent aircraft motion, also the SAR image can be focused. The effectiveness of the proposed method is demonstrated by measured data imaging experiment. Meanwhile the method introduced in this paper causes only low computational load. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to monitor the postural activity of sows in farrowing pen using a millimeter-wave radar imaging system. Three-dimensional images of the scene are obtained from a 77 GHz Multiple-Input Multiple Output radar and the mechanical scanning of the radar beam. We show that the processing of radar images based on the Cell-Averaging Constant False Alarm Rate allows detecting the standing and lying postures of sows despite the electromagnetic clutter. The experimental study reported here is performed from the monitoring of 16 sows from the Large White breed for a total of 46 h. 3D positions of the sow are recorded and heatmaps of the sow locomotor activity are proposed. With few training radar data, we perform a classification of standing and laying postures with a mean balanced accuracy greater than 90%. The radar-based technique applied here in a cluttered environment inside farrowing pens may be extended to monitor pigs under other housing condition. • Postural activity of sows is monitored by using a millimeter-wave radar imaging system. • Sows are detected inside farrowing pens in presence of their piglets. • Millimeter-wave radar imaging system records heatmaps of the sow locomotor activity. • Millimeter-wave radar imaging system detects standing and lying postures. [ABSTRACT FROM AUTHOR]

This paper addresses an inverse scattering problem in a near-zone setting under a multi-monostatic multi-frequency configuration. The main goal is to design a sampling scheme for collecting the data to be convenient and efficient. Accordingly, the considered approach to achieve this goal is a non-uniform sampling strategy which functions based on approximation of the point-spread function via a migration-like inversion algorithm. This scheme benefits from a convolutional scattering operator which can capture the most significant singular values of the continuous operator. However, this scheme leads to a complicated measurement grid due to the influence of different frequencies. To overcome this problem, we exploit an iterative optimal strategy, which optimizes the sampling grid with respect to frequency samples and spatial points. This solution results in an efficient, simple and convenient grid in which the data points are distributed uniformly. We will support the theoretical findings by numerical results. [ABSTRACT FROM AUTHOR]

Understanding the morphology and internal layering of large aeolian dune fields through radar observations can provide unique insights into the climatic and geophysical conditions that led to their formation. In this study, we perform a large-scale characterization of the morphology and internal layering of linear dunes in hyper-arid areas on Earth, through utilizing multiple complementary radar datasets (SIR-C imaging, SRTM interferometry-derived elevations and radar sounding or Ground Penetrating Radar (GPR)). Linear dune fields in the Egyptian desert are of special interest, due to their significance as planetary analogs to dunes on Mars and Saturn's largest moon, Titan. Satellite radar imagery and elevation data of the region show significant variance in the geomorphology of different dune fields in Egypt. In addition, GPR probing of the first few meters suggests different inner settings in the layering of dunes of different ages in eastern and western Egypt, reflecting different paleo-climatic regimes that led to their formation. Furthermore, our radiometric analysis suggests that dunes with different inner layering arrangement also exhibit different radar backscatter returns as a function of their heights. For relatively younger dunes with a homogeneous inner structure, like the ones in eastern Egypt in the Qattaniya dune field, we observe that sigma0 does not change as a function of the dune height. For relatively older dunes in western Egypt like the Great Sand Sea (Northern (Siwa) and Southern dune fields), we observed a linear correlation between sigma0 and the dune height. Thus, surface properties of dunes like morphology and backscatter variation with height are related to inner characteristics like arrangement of internal layering, relative ages and can be used to infer their depositional history. Linear dunes discovered in the equatorial regions of Titan by the Cassini-Huygens mission are morphologically very similar to these linear dune fields in the Egyptian Sahara. Hence, assessing the variability of morphology and radar backscatter properties of Titan's dunes as a function of their heights can help constrain the ambiguities associated with their internal structure and formation history and provide insights into Titan's paleo-wind regimes. [ABSTRACT FROM AUTHOR]

This paper deals with the forward-looking imaging problem under high-speed platform for the airborne single-channel forward-looking radar (ASFLR). Biphase-based synthetic bandwidth azimuth modulation imaging radar (Bi-SBAMIR) can be used to increase the azimuth resolution. However, the performance degrades significantly when the velocity of the platform is high. To solve this problem, a more generalized SBAMIR framework based on linear frequency modulation (LFM) waveform (LFM-SBAMIR) is proposed. The LFM waveform is more Doppler tolerant than the biphase waveform, which makes it a good choice for high speed platform. In LFM-SBAMIR, the synthetic bandwidth of the received signal has two parts: the modulation bandwidth and the Doppler bandwidth. Therefore, large synthetic bandwidth can be achieved by increasing the modulation bandwidth even in the case of small Doppler bandwidth, and high azimuth resolution can be achieved after matched filtering. Moreover, it is firstly proved that the azimuth resolution is proportional to the time bandwidth product (TBP) of the modulated LFM waveform. Finally, the azimuth resolution performance under different TBPs, SNRs and velocities are analyzed. The performance assessment conducted by resorting to simulated data, also in comparison to the previously proposed Bi-SBAMIR method, has confirmed the effectiveness of the newly proposed algorithm in high-speed platform scenarios. [ABSTRACT FROM AUTHOR]

Multiple-input multiple-output (MIMO) radar with sparse planar arrays is expected to provide one snapshot imaging of complex motion targets at low hardware costs. In recent years, compressive sensing (CS) has been applied to MIMO radar imaging with limited antenna arrays. However, CS has to convert the matrix into a vector, which results in a large measurement matrix and a demanding amount of storage. Matrix completion (MC) is a new theory of recovering the entire data with partial observations. Compared with CS, MC is more suitable for matrix operations and avoids the basis mismatch problem. Due to the potential advantages, we introduce MC to MIMO radar imaging using a sparse planar array. Despite the success of MC in other fields, direct application in MIMO radar imaging leads to several problems, such as strict restrictions of the low-rank property and uniform random samples in MC. To solve these problems, a structured MC(StMC)-based method is proposed. With the method, MIMO radar imaging via a sparse planar array is converted to complete a low-rank structured matrix, which can be easily solved as a semi-definite program problem. Experimental results confirm the validity of the proposed method. [ABSTRACT FROM AUTHOR]

An efficient airborne single-channel forward-looking radar (ASFLR) imaging framework to improve the azimuth resolution performance with synthetic bandwidth azimuth modulation imaging radar (SBAMIR) is proposed. During the framework, we incorporate azimuth modulation to the transmitted signal with a coded sequence in azimuth direction, and derive the SBAMIR imaging model for the first time. In SBAMIR, the synthetic bandwidth of the received echo is not only determined by the Doppler bandwidth, but also by the modulation bandwidth. Even though the Doppler bandwidth has little contribution to the synthetic bandwidth in ASFLR, the modulation bandwidth can be utilized to increase the synthetic bandwidth. Accordingly, a high azimuth resolution can be achieved after azimuth matched filtering. Achievable azimuth resolution and performance to SNR are analyzed later. Simulation results are given to verify the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Sparse signal recovery is attractive in compressed sensing (CS). Based on the smoothed ℓ 0 norm (SL0) algorithm, we have developed an error-tolerant regularized SL0 (ReSL0) algorithm, which has the same computational advantages as the SL0 algorithm while having better immunity against inaccuracy caused by noise or model mismatch. The performance of the ReSL0 is evaluated with simulated data. In addition, we have extended the ReSL0 to the matrix form (MReSL0), which is more suitable for dealing with matrix form signals and also has good resilience against inaccuracy. Finally we apply the ReSL0 and MReSL0 to joint CS-based radar imaging and phase error correction. Experimental results from both simulated and real data demonstrate that the proposed algorithms provide remarkable performance improvements in inaccurate scenarios (such as noisy data and mismatched settings) compared with the SL0 algorithm. [ABSTRACT FROM AUTHOR]

This study develops the preprocessing theory for time-reversal multiple signal classification (TR-MUSIC) imaging from the one-dimensional (time-domain) gating proposed in earlier papers to a two-dimensional one. Although the one-dimensional gating technique for TR-MUSIC mitigates the effect of noise as well as reduces the required number of antenna elements, the resulting image is subject to focusing errors when there are many targets in the gated region. In this paper, we propose a two-dimensional (angle-and-time-domain) gating technique based on the fast Fourier transform (FFT) and spatial FFT processing. By using these transforms and gating iteratively, we can reduce the number of targets to be detected at one time. The TR-MUSIC technique for the gated data provides more reliable images and precisely estimated coordinates compared with the previous work. [ABSTRACT FROM AUTHOR]

In this paper, we describe a new digital receiver developed for a 1.3-GHz range imaging atmospheric radar. The digital receiver comprises a general-purpose software-defined radio receiver referred to as the Universal Software Radio Peripheral 2 (USRP2) and a commercial personal computer (PC). The receiver is designed to collect received signals at an intermediate frequency (IF) of 130MHz with a sample rate of 10MSs−1. The USRP2 digitizes IF received signals, produces IQ time series, and then transfers the IQ time series to the PC through Gigabit Ethernet. The PC receives the IQ time series, performs range sampling, carries out filtering in the range direction, decodes the phase-modulated received signals, integrates the received signals in time, and finally saves the processed data to the hard disk drive (HDD). Because only sequential data transfer from the USRP2 to the PC is available, the range sampling is triggered by transmitted pulses leaked to the receiver. For range imaging, the digital receiver performs real-time signal processing for each of the time series collected at different frequencies. Further, the receiver is able to decode phase-modulated oversampled signals. Because the program code for real-time signal processing is written in a popular programming language (C++) and widely used libraries, the signal processing is easy to implement, reconfigure, and reuse. From radar experiments using a 1-μ s subpulse width and 1-MHz frequency span (i.e., 2-MHz frequency bandwidth), we demonstrate that range imaging in combination with oversampling, which was implemented for the first time by the digital receiver, is able to resolve the fine-scale structure of turbulence with a vertical scale as small as 100m or finer. [ABSTRACT FROM AUTHOR]

The Middle Atmosphere Alomar Radar System (MAARSY) on the Norwegian island of Andøya is a 53.5MHz monostatic radar with an active phased array antenna. The total array consists of 433 3-element linearly polarized Yagi antennas and can be configured to receive with multiple antenna sections (currently up to 16 complex receiving channels). In order to exploit its multiple-receiver capability for improving the space-time ambiguities of atmospheric/ionospheric targets, the phase difference between receiving channels has to be measured with good precision. Such phases are intrinsic to the system and are due to different cable lengths, pointing positions, filters, attenuators, amplifiers, antenna impedances, etc. In this work, we have operated MAARSY in a radio passive mode to observe the strong radio signals of Cassiopeia A and Cygnus A sources and calibrate the receiving system. By using the so-called fringe-stopping method, we have been able to calibrate the 16 complex channels, including the smaller antenna module that can be used, i.e., an Hexagon consisting of 7 Yagi antennas. The measured phases have been obtained with a mean standard deviation of ~5 °. We have tested the validity of such phases using meteor-head echoes with different configurations and pointing directions. Given that the procedure is easy to implement, it should be used in a routine manner either to corroborate the stability of the system or to measure new phases after upgrades or repairs. [ABSTRACT FROM AUTHOR]

[Display omitted] • A more robust beamformer than the reference implementation of DAS and DMAS. • The algorithm complexity is of a similar order to DAS. • The DMAS-D4 achieves higher contrast and finer resolution. • The improved performance is achieved with minimal additional computational load. • The iterative implementation makes it suitable even for large MIMO imaging volumes. Confocal microwave imaging (CMI) has the potential to be an alternative or complementary method for mammography in breast cancer screening. Data acquisition in CMI is very fast, safe, and comfortable for patients, but the reconstructed images are not always of high quality. High level of artifacts and poor resolution cause impaired quality. In this work, a novel, fast and robust beamformer, DMAS-D4 is presented. Delay-multiply-and-sum-degree-4 (DMAS-D4) is a next generation beamformer that expands the two-dimensional summations in delay-multiply-and-sum (DMAS) to four-dimensional summations in which all possible fourth degree terms are summed to increase robustness and resolution. A computationally-efficient implementation is presented which mitigates the additional computation required by DMAS. Furthermore, the proposed method can run iteratively which makes the ability to update the result using new observations. An idealized simulation compares basic performance of the proposed method with two other beamformers: (1) DAS, which is the gold standard, and (2) DMAS, which has been reported as the best beamformer so far. The results demonstrate that the DMAS-D4 achieves higher contrast and finer resolution. The proposed method increased the signal-to-max-ratio (SMXR) from 3.1 dB using DMAS to 5.7 dB in the heterogeneous breast mimicking phantom which illustrates the higher tumor detectability using the proposed method. Similarly, the Signal-to-Clutter Ratio (SCR) is 23.9 dB higher using the proposed method. By exploiting DMAS-D4 these benefits and potentially enhancing clinical decisions are attainable by the same computational load order as DAS. These results may be useful to inform future microwave breast imaging beamformer design. [ABSTRACT FROM AUTHOR]

Abstract: In this letter, we propose a regularized iterative reweighted minimization approach (RIRMA) to accurately estimate properties (angle, range, and Doppler) of targets for multiple-input multiple-output (MIMO) radar. In RIRMA, a nonlinear model is reformulated as a linear model and a sequence of weighed minimization optimization problems is solved. In the optimization procedure, a closed-form solution of the optimization problem is obtained for next iteration. Numerical comparisons with existing algorithms show that RIRMA produces accurate angle–range target estimates, and even in the range–Doppler dimension, RIRMA can also achieve excellent Doppler estimate. Furthermore, numerical simulations show that the reduction of the q values decreases the average iteration numbers without loss of angular resolution and Doppler resolution. [Copyright &y& Elsevier]

• A fast super-resolution OAM-radar imaging has been proposed. • Imaging efficiency has been enhanced greatly. • The influence of non-main-lobe direction can be avoided effectively. • A signal processing method has been introduced for reconstructing the target. We have proposed a fast super-resolution (FSR) imaging based on the electromagnetic (EM) OAM (orbital angular momentum) radar. The proposed method only needs to transmit EM vortex waves twice to achieve super-resolution imaging, which greatly improves the imaging efficiency of OAM radar. The OAM modes of the transmitting vortex EM waves are opposite in the proposed method. Therefore, the proposed method does not need to use concentric uniform circular array (UCA) for controlling beams, and the complexity of the system can be simplified effectively. In this paper, the FSR imaging model has been deduced, and the FSR imaging algorithm based on pulse compression and FFT has been also proposed for reconstructing the targets. The simulation results demonstrate that our proposed method has the superior effectiveness and better performances. [ABSTRACT FROM AUTHOR]

Abstract: Recently, an L-statistics based method for the micro-Doppler effects removal has been proposed by the authors. Order statistics is performed on the spectrogram, while the rigid body signal synthesis is done by using the remaining STFT samples, after micro-Doppler removal. By the proposed method, the Fourier transform is recovered with a concentration close to the original one. However, during the procedure of the micro-Doppler removal, the STFT samples that correspond to the rigid body are retracted, as well. Consequently, in the reconstructed Fourier transform of the rigid body, we get one very highly concentrated pulse, as in the original Fourier transform, and a number of low-concentrated components, being spread around the peak. These low concentrated components are summed up by different random phases. In this paper, we propose a genetic algorithm for the estimation of the removed STFT samples corresponding to the rigid body. Each individual in the genetic algorithm contains possible estimation of the phases of the missing STFT samples, whereas fitness function forces individuals (combination of phases) for which a minimal energy of the side lobs is obtained. The individual with the highest fitness is considered as the final phases'' estimation of the missing STFT values, and then used for the reconstruction of the original Fourier transform. The amplitude of a STFT sample is estimated as median of the amplitudes of the remaining samples at the same frequency. Performance of the proposed genetic algorithm is illustrated by examples. [Copyright &y& Elsevier]

Abstract: One of the problems that radar imaging technique based on compressed sensing (CS) must confront is the relatively high computation complexity. The sparse representation model of stepped frequency radar echo is established and a 2D joint imaging method based on 2D-SL0 is proposed, which makes the best of the 2D separability of sparse dictionary and compressive measurement, thus has greatly improved efficiency. The performance of CS imaging methods, including SL0, 2D-SL0 under 2D joint model and iterative SL0, MSL0 under 2D decoupled model, are analyzed and compared theoretically. Experiment results verify the validity and superiority of the proposed method. [Copyright &y& Elsevier]

Abstract: Cassini synthetic aperture radar (SAR) images of Titan, the largest satellite of Saturn, reveal surface features with shapes ranging from quasi-circular to more complex ones, interpreted as liquid hydrocarbon deposits assembled in the form of lakes or seas. One of the major problems hampering the derivation of meaningful texture information from SAR imagery is the speckle noise. It overlays real structures and causes gray value variations even in homogeneous parts of the image. We propose a filtering technique which can be applied to obtain restored SAR images. Our technique is based on probabilistic methods and regards an image as a random element drawn from a prespecified set of possible images. The despeckle filter can be used as an intermediate step for the extraction of regions of interest, corresponding to structured units in a given area or distinct objects of interest, such as lake-like features on Titan. This tool can therefore be used, among other, to study seasonal surficial changes of Titan''s polar regions. In this study we also present a segmentation technique that allows us to separate the lakes from the local background. [Copyright &y& Elsevier]

Abstract: Compressive sensing (CS) techniques offer a framework for the detection and allocation of sparse signals with a reduced number of samples. Today, modern radar systems operate with high bandwidths—demanding high sample rates according to the Shannon–Nyquist theorem—and a huge number of single elements for phased array antennas. Often only a small amount of target parameters is the final output, arising the question, if CS could not be a good mean to reduce data size, complexity, weight, power consumption and costs of radar systems. There is only a small number of publications addressing the application of CS to radar, leaving several open questions. This paper addresses some aspects as a further step to CS-radar by presenting generic system architectures and implementation considerations. It is not the aim of this paper to investigate numerically efficient algorithms but to point to promising applications as well as arising problems. Three possible applications are considered: pulse compression, radar imaging, and air space surveillance with array antennas. Some simulation results are presented and enriched by the evaluation of real data acquired by an experimental radar system of Fraunhofer FHR. [Copyright &y& Elsevier]

The introduction of laser technologies into the field of art conservation and the formation of the associated professional society, LACONA, have paralleled similar events that took place in laser application to the entertainment, science, manufacturing, military ordnance, communication, and medical disciplines (to name a few). At the time of the establishment of LACONA, numerous conservation applications for lasers had been shown possible. During the early years of the society, many papers presented at this forum reported on projects that had developed some of these into practical conservation tools. In many cases, this involved producing models for the laser processes, advancing the associated laser technologies, or determining optimum laser parameters for particular circumstances. In recent years, maturing research reported here has focused on case histories, refinements and adaptations in technique, and the broadening of databases. Much remains to be accomplished along these veins and will continue to be reported in our future assemblies. However, the success of LACONA suggests a potential for a much broader role of service within the art-conservation community. Specifically, there are numerous new and emerging technologies that could be useful in conservation. Scientifically, they have a great deal of commonality with laser technology, except for their operation outside of the visible portion of the electromagnetic spectrum. A few of these are surface penetrating radar, monopulse radar, radar tomography, millimeter wave imaging, portable and free-induction nuclear magnetic resonance, and X-ray backscatter imaging. In addition, there are laser technologies such as photoacoustic spectroscopy and photodynamic chemistry that hold promise for conservation science, but have yet to be applied. Consequently, this may be a propitious time to consider opening up LACONA to incorporate such allied methodologies in order to encourage the continuing vitality and relevance of the organization to art conservation. [Copyright &y& Elsevier]

Observing Earth from the space is key for monitoring land-surface dynamics and probing near-surface geomorphic and geological features, including those obscured by sand sheets/dunes and human activity. The hyper-arid area of the Arabian Peninsula was selected for detecting and extracting obscure paleohydrological systems and modeling the spatio-temporal dynamics of sand dunes using synthetic aperture radar (SAR) and optical high-resolution remote sensing images. Interferometric synthetic aperture radar (InSAR) coherence change detection (CCD) using Sentinel-1 imagery was performed, revealing subtle changes in the surficial landforms of the study area. These images also discriminated between active and inactive geomorphic features. In addition, low-frequency L-band Advanced Land Observing Satellite/Phased Array type L-band Synthetic Aperture Radar (ALOS/PALSAR) and Shuttle Radar Topography Mission (SRTM) data revealed the details of paleochannels in the basins that drain into the Arabian Gulf. These ephemeral rivers are the source of sand that was later reshaped into dunes by aeolian processes. The results also revealed that the morphology and extension of dunes were clearly influenced by topography, slope, and wind velocity and direction, rather than human activities. Knowledge gained by exploring desert features is essential for planning settlements and development, and integrated approaches using a variety of remote sensing datasets are key for enhancing Earth observation across a range of spatial and temporal scales. • InSAR quantified the land surface changes in spatio-temporal scale. • L-band ALOS/PALSAR obscured the buried paleohydrologic systems. • High image resolution of Sentinel-1&2 and Google Earth well successfully defined the dynamic of sand dunes. [ABSTRACT FROM AUTHOR]