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The coherent properties of radar give rise to speckle noise in synthetic aperture radar (SAR) images. Speckle noise, mixed with valid information, directly affects information extraction in SAR images, especially the accuracy of persistent scatter point selection. Based on a detailed analysis of speckle noise characteristics, an innovative speckle noise reduction method combining independent vector analysis and an improved fast independent component analysis (FastICA) is proposed in this study. First, the principle of independent vector separation is followed to retain the maximum correlation of internal information in each channel of the SAR image. Then, a high-order Newton iterative scheme is constructed and added to the traditional FastICA algorithm to improve the speed and stability of iteration processing. Finally, the relaxation factor is introduced to relax the initial value requirement to minimize image distortion during speckle noise reduction. To verify the proposed algorithm, two groups of SAR images are selected from "Sandia National Laboratories" and Sentinel-1A. The proposed algorithm is compared with several other algorithms on speckle noise reduction efficiency. The experimental results showed that the proposed method could more effectively reduce speckle noise and retain edge features of SAR images, indicating that it had a potential to enhance image quality for the subsequent interpretation of SAR images. [ABSTRACT FROM AUTHOR]

In distributed radar, a coherent system often gains attention for its higher detection potential in contrast to its non-coherent counterpart. However, even for a distributed coherent radar, it is difficult to coherently accumulate local observations in the searching mode if target returns in local channels are decorrelated. In order to obtain the superiority of coherent processing while overcoming the real implementation difficulties of a coherent framework, this paper studies a distributed coherent detection algorithm for fusion detection. It is utilized in detecting a target during tracking while a target is searched for in a non-coherent manner. From historic observations on target tracking, relative phase delays in different channels are predicted by a phase lock loop and then used to compensate phases for observations in the current frame. Moreover, to enhance the detection performance of distributed radar during tracking, a switching rule between phase prediction-based coherent and non-coherent processing is proposed based on their detection performance. Numerical results indicate that the switching operation can improve the detection probability during tracking, and the non-coherent operation can still provide a moderate detection performance if the phase prediction is unreliable. [ABSTRACT FROM AUTHOR]

Recent advancements in photonic integrated circuits (PICs) have paved the way for a new era of frequency-agile coherent radar systems. Unlike traditional all-electronic RF radar techniques, fully photonic systems offer superior performance, overcoming bandwidth limitations and noise degradation when operating across S (2–4 GHz), X (8–12 GHz), and K-band (12–40 GHz) frequencies. They also exhibit excellent phase noise performance, even at frequencies exceeding 20 GHz. However, current state-of-the-art PICs still suffer from high processing losses in the optical domain, necessitating careful design of the electrical RF domain. This study delves into the critical challenges of designing RF front-ends for microwave photonic radars, including stability, noise minimization, and intermodulation distortion reduction. To demonstrate the feasibility of the proposed design, a functional prototype is constructed, achieving a total power gain of 107 dB (radar system at 10 GHz) while minimizing signal noise degradation. Furthermore, a comprehensive demonstration of the RF front-end, encompassing both optical RF signal generation and experimental measurements of a rotor blade's Doppler fingerprint with 0.5 Hz resolution, validates the proposed system's performance. [ABSTRACT FROM AUTHOR]

Coherent S-band radar is a remote sensing observation device with high spatial-temporal resolution and can be used to achieve deterministic sea wave reconstruction and prediction (DSWRP) technology. However, coherent S-band radar can observe nonlinear details of the sea surface due to its high resolution, which makes the propagation operator matrix an ill-conditioned overdetermined matrix. To solve this problem, this paper proposes a DSWRP scheme using condition number regularized least squares (CN-RLS) for coherent S-band radar. First, the space-time velocity information was obtained from the radar echo. Second, the CN-RLS method solved the phase-resolved model coefficients. Finally, the deterministic wave field was predicted according to the solved model coefficients. The proposed scheme was verified by simulation data and the real radar dataset observed by the coherent S-band wave-measuring radar onboard the ship XIANGYANGHONG-18 in the East China Sea in April 2024. The predicted wave elevation of the proposed method was compared with the wave elevation observed based on the X-band wave-measuring radar, and the root mean square error (RMSE) and correlation coefficient (CC) were 0.22 m and 0.76, respectively, which show that the proposed method could effectively implement the DSWRP technology. [ABSTRACT FROM AUTHOR]

Airborne radar forward-looking imaging holds significant promise for applications such as autonomous navigation, battlefield reconnaissance, and terrain mapping. However, traditional methods are hindered by complex system design, azimuth ambiguity, and low resolution. This paper introduces a distributed array collaborative, forward-looking imaging approach, where multiple aircraft with linear arrays fly in parallel to achieve coherent imaging. We analyze signal model characteristics and highlight the limitations of conventional algorithms. To address these issues, we propose a high-resolution imaging algorithm that combines an enhanced missing-data iterative adaptive approach with aperture interpolation technique (MIAA-AIT) for effective signal recovery in distributed arrays. Additionally, a novel reference range cell migration correction (reference RCMC) is employed for precise range–azimuth decoupling. The forward-looking algorithm effectively transforms distributed arrays into a virtual long-aperture array, enabling high-resolution, high signal-to-noise ratio imaging with a single snapshot. Simulations and real data tests demonstrate that our method not only improves resolution but also offers flexible array configurations and robust performance in practical applications. [ABSTRACT FROM AUTHOR]

This paper deals with the imaging problem from data collected by means of a microwave photonics-based distributed radar network. The radar network is leveraged on a centralized architecture, which is composed of one central unit (CU) and two transmitting and receiving dual-band remote radar peripherals (RPs), it is capable of collecting monostatic and multistatic phase-coherent data. The imaging is herein formulated as a linear inverse scattering problem and solved in a regularized way through the truncated singular value decomposition inversion scheme. Specifically, two different imaging schemes based on an incoherent fusion of the tomographic images or a fully coherent data processing are herein developed and compared. Experimental tests carried out in a port scenario for imaging both a stationary and a moving target are reported to validate the imaging approach. [ABSTRACT FROM AUTHOR]

A Low lAtitude long Range Ionospheric raDar (LARID), which was built recently at Dongfang (19.2°N, 108.8°E, magnetic latitude 13.9°), Hainan Is., China, is employed to study the ionospheric variability over a large region. The LARID consists of two high frequency (HF) coherent backscatter radars, looking toward east and west, respectively, with a field‐of‐view covering a wide longitude from Africa to Pacific. Here we report some results by the LARID, with main focus on its capabilities, advantages and limitations in monitoring ionospheric variability. The results show that the LARID can detect various ionospheric variations at low latitudes, including (a) sequentially generated equatorial plasma bubbles over a longitude span of ∼80°, (b) near‐range E region irregularities drifting across more than 1,000 km, (c) ionospheric background information retrieved from ground scatters, (d) traveling ionospheric disturbances, and (e) sporadic E structures reflecting HF radio waves at low elevation angles. It is demonstrated that the LARID provides an important tool for investigating different types of ionospheric variability over a broad region, especially over the Indian Ocean and west Pacific, and will contribute significantly to the regional ionospheric weather forecasting. Plain Language Summary: The ionosphere is almost always undergoing various variations. At low latitudes, there is a lack of single‐station instrument capable of continuously monitoring ionospheric variability over a large area ranging thousands of kilometers, especially over the oceans where ground‐based observations may suffer from data gaps. For the purpose of monitoring the low latitude ionospheric variability over a broad region, a Low lAtitude long Range Ionospheric raDar (LARID) was built recently at Hainan Is., China, under the support of Chinese Meridian Project phase II. This paper demonstrates how the LARID could perform over‐the‐horizon investigations on different kinds of ionospheric variability. Based on LARID backscatter echoes, equatorial bubbles were observed sequentially after sunset over more than 8,000 km in longitude, and E region irregularities were revealed to drift across more than 1,000 km zonally. Based on the LARID ground scatters with different echo patterns, we succeeded in retrieving the background plasma density, tracing traveling ionospheric disturbances, and characterizing Es layer reflecting high frequency radio waves. Discussions on the capabilities, advantages and limitations of LARID in monitoring the various kinds of low latitude ionospheric variability, and its possible applications in regional ionospheric weather are involved. Key Points: A Low lAtitude long Range Ionospheric raDar (LARID) is employed to investigate ionospheric variability over a large zonal regionLARID shows capabilities in studying ionospheric irregularity, traveling disturbance and background variation over a longitude span of ∼90°Both advantages and limitations of LARID in investigating different types of ionospheric variability, and possible applications are discussed [ABSTRACT FROM AUTHOR]

Led by the National Space Science Center of the Chinese Academy of Sciences, we have built a Chinese dual auroral radar network in northern China, which is called the CN‐DARN. The CN‐DARN consists of three pairs of high‐frequency coherent scattering radar facilities and is one of the key parts of the Chinese Meridian Project Phase II. It has been fully constructed and started trial operations at the end of 2023. The detection range of the radar network extends longitudinally over approximately 9 hr of local times and covers the middle to high latitudes of the entire Asia region above 40° ${}^{\circ}$. In this paper, we present the basic design of the CN‐DARN and its preliminary observations of ionospheric irregularities, subauroral polarization streams (SAPSs) and traveling ionospheric disturbances (TIDs). We also investigate its contribution to the ionospheric convection pattern of the Northern Hemisphere derived from Super Dual Auroral Radar Network (SuperDARN) observations. The results indicate that the CN‐DARN provides excellent measurements and better specifications of flows in the Asian sector, improving our understanding of the global‐scale ionospheric convection pattern in the Northern Hemisphere. These encouraging results lead us to believe that the CN‐DARN will play an important role in studies on the evolution of ionospheric irregularities, the characteristics and evolution of SAPSs, the propagation of TIDs, and global‐scale ionospheric convection dynamics. Plain Language Summary: The ionosphere is home to all charged particles in the Earths atmosphere, and it is also home to many satellites, including space stations. The density of the charged particles is uneven. There are different scales of electron density irregularities in the ionosphere and they can disrupt signals. To monitor the ionospheric environment of the mid‐ and high‐ latitude regions of Asia, three pairs of high‐frequency coherent scattering radars have been deployed in northern China. This network of radars is called the CN‐DARN, short for the Chinese Dual Auroral Radar Network. The CN‐DARN achieves large‐scale continuous detection of the movement of decameter‐scale ionospheric irregularities in the middle and high latitudes of the Asian sector. The detection range can reach 4,000 km from south to north and extends longitudinally over 9 hr of local time. We describe the basic design and parameters of the radar network and present the preliminary observation results obtained during trial operations. The observations results show that the CN‐DARN can monitor ionosphere disturbances effectively over this region; moreover, the observations can be combined with those in the Western Hemisphere to create a comprehensive picture of large‐scale features in geospace. Key Points: The Chinese Dual Auroral Radar Network (CN‐DARN) consists of three pairs of newly constructed radar facilities in northern ChinaThis paper details the hardware, key parameters and preliminary detection results of the CN‐DARNThe CN‐DARN achieves continuous detection of the mid‐ and high‐latitude ionosphere in Asia [ABSTRACT FROM AUTHOR]

In this study, the evolutional features and underlying driving mechanisms of the postmidnight‐to‐dawn equatorial plasma bubble (EPB) irregularities during the weak geomagnetic activity period on 13 November 2015 were investigated based on the multiple satellite and ground‐based observations. By using the coherent scatter radar operating at very high frequency at Fuke (19.5°N, 109.1°E, dip latitude 14.4°N), China, it was found that the freshly developed field‐aligned irregularities (FAIs) occurred within the radar's field of view around ∼04:37 LT and sustained for more than 40 min. The remarkable EPB‐related density and total electron content (TEC) depletions measured by the satellite and GNSS receivers were also observed, which indicates the persistence of EPB irregularities until ∼06 LT. The significant elevation of bottomside F‐layer's virtual height obtained by the Digisonde at Fuke as well as the upward vertical F‐layer plasma drifts derived from a nearby Digisonde at Sanya (18.4°N, 109.6°E, dip latitude 13.1°N) both imply the existence of strong eastward perturbation electric fields after local midnight. These findings suggest that the collective effects of eastward overshielding penetration electric field (PEF) resulted from the substorm onset and rapid northward turning of the interplanetary magnetic field (IMF) Bz, surpassed the role of westward undershielding prompt penetration electric field (PPEF) induced by the southward turning of IMF Bz. Thus, the former predominated in modulating the equatorial/low‐latitude zonal electric fields and raised the F‐layer considerably, which consequently boosted the growth of R‐T instability and created the favorable conditions for the postmidnight‐to‐dawn EPBs development. Key Points: Freshly developed field‐aligned irregularities last for 40 min within the radar's field‐of‐view and the equatorial plasma bubble (EPBs)‐related density depletions show the EPBs sustaining until ∼06 LTThe competition process among different perturbation electric fields for triggering the postmidnight‐to‐dawn EPB irregularities is examinedThe eastward overshielding penetration electric field resulted from substorm onset and interplanetary magnetic field Bz northward turning provide favorable conditions for EPB development [ABSTRACT FROM AUTHOR]

This paper addresses polarimetric adaptive coherent detection of radar targets embedded in sea clutter. Initially, radar clutter data across multiple polarimetric channels is modeled using a compound Gaussian framework featuring an unspecified speckle covariance matrix and lognormal texture distribution. Subsequently, three adaptive polarimetric coherent detectors are derived, employing parameter estimation and two-step versions of the generalized likelihood ratio test (GLRT): the complex parameter Rao and Wald tests. These detectors utilize both clutter texture distribution information and radar data's polarimetric aspects to enhance detection performance. Simulation experiments demonstrate the superiority of three proposed detectors over the competitors, and that they are sensitive to polarimetric channel parameters such as secondary data quantity, target or clutter speckle correlation, and signal-to-clutter ratio disparity. Additionally, the proposed detectors exhibit a near-constant false alarm rate relative to average clutter power and speckle covariance matrix. [ABSTRACT FROM AUTHOR]

The coherent mono-pulse instrumentation radar changes the pulse repetition frequency to resolve range and velocity ambiguities and improve detection rates. Owing to this change in the pulse repetition frequency, the time–Doppler map of the coherent mono-pulse instrumentation radar cannot be obtained from methods used by existing Doppler radars, such as FFT or SFTF. In this study, we propose a method to obtain a time–Doppler map by considering the pulse repetition frequency change and velocity ambiguity of a coherent mono-pulse instrumentation radar. The simulation results confirm that the movement of the target can be visualized on a time–Doppler map. [ABSTRACT FROM AUTHOR]

When charged particles are accelerated from Earth's magnetosphere and precipitate into the atmosphere, their impact with neutral gas creates the aurora. Structured electric fields drive the acceleration processes but they are also passed down to the ionosphere, meaning that turbulence can in part be embedded into the ionosphere rather than emerge through instability processes locally. Applying a point‐cloud analysis technique adapted from observational cosmology, we show how observed turbulence in the ionosphere matches electrical current signatures in the pulsating aurora in a series of conjunctions between space‐ and ground‐based instruments. We propose that the temporal spectrum of pulsations in the pulsating aurora is the driver of a clearly observed energy injection into the ionosphere's unstable bottomside. Precipitating electrons produce electric fields through charge deposition, and we observe wave characteristics that are present in this pattern. Next, the relative electron‐ion drifts excite the Farley‐Buneman instability, the distribution of whose waves are organized according to the local electric field. It is the temporal characteristics of chorus wave interactions in the magnetosphere that is imparted, via precipitating electrons, to the pulsating aurora, and so we propose that chorus wave interactions are capable of embedding turbulent structure into the ionosphere. This structure (now pressure gradients) dissipate energy in the E‐region through turbulent processes, observed by the icebear coherent scatter radar. Key Points: A study of multiple space‐ground conjunctions that occurred during a strong pulsating aurora eventThere were identical turbulent properties in in‐situ field‐structuring and in E‐region plasma turbulenceWe suggest that this structure is driven, or embedded, by electromagnetic waves, mediated by precipitating particles in the pulsating aurora [ABSTRACT FROM AUTHOR]

Observations of coherent scatter from patchy sporadic E layers in the subauroral zone made with a 30‐MHz coherent scatter radar imager are presented. The quasiperiodic (QP) echoes are similar to what has been observed at middle latitudes but with some differences. The echoes arise from bands of scatterers aligned mainly northwest to southeast and propagating to the southwest. A notable difference from observations at middle latitudes is the appearance of secondary irregularities or braids oriented obliquely to the primary bands and propagating mainly northward along them. We present a spectral simulation of the patchy layers that describes neutral atmospheric dynamics with the incompressible Navier Stokes equations and plasma dynamics with an extended MHD model. The simulation is initialized with turning shears in the form of an Ekman spiral. Ekman‐type instability deforms the sporadic E layer through compressible and incompressible motion. The layer ultimately exhibits both the QP bands and the braids, consequences mainly of primary and secondary neutral dynamic instability. Vorticity due to dynamic instability is an important source of structuring in the sporadic E layer. Key Points: Radar observations and theory of subauroral sporadic E layer irregularities are presentedCoherent scatter from the layers presents as primary bands of irreguarities with secondary, oblique, braided structureSimulations of neutral dynamic instability associated with turning shears in Es layers consistent with observations [ABSTRACT FROM AUTHOR]

This study focuses on the improvement of the tracking performance of space observation radar systems. Coherent radar systems usually provide an observation vector including four observables (range, range rate, azimuth, elevation) measured at a given epoch. Moreover, for some coherent radars, such as the TIRA system, it is possible to determine an additional parameter, the range rate, which is extracted from the collected complex amplitude. The tracking performance can be improved by increasing the observation parameter accuracy. The influence of each parameter on the overall tracking performance can be assessed by conducting a sensitivity analysis. The investigation shows that the best way to increase the accuracy of the state vector is by improving the angle estimates. The TIRA system is equipped with a tracking radar and an imaging radar. A powerful solution to improve the tracking performance is the realization of a multi-sensor data fusion between the two radars of the system. [ABSTRACT FROM AUTHOR]

Sporadic E (Es) layers are thin enhanced ionization patches. The electron density of Es layer is very high, up to 100 times that of the regular E layer. Therefore, Es layer has a significant effect on radio wave propagation. The formation mechanisms of Es layers is very complicated, so it needs multi-observations and simulations research. Combined observations using an ionosonde, very high frequency (VHF) coherent backscatter radar, VHF broadband backscatter radar, and meteor radar were performed. The height and irregular structure of mid-latitude Es layers observed by ionosonde, VHF coherent backscatter radar and VHF broadband backscatter radar were compared with the zonal and meridional winds observed by meteor radar, the role of wind shear on the formation of Es layer was confirmed, and examples were also found where wind shear theory could not explain and apply to the formation of Es layer. We simulated the formation of Es layers based on wind shear theory. The results of simulations and observations revealed other potential role for Es layer formation besides wind shear. By comparing the height of turbopause with that of Es layer, it was found that the variation of the altitudes of turbopause and Es layer was consistent over different latitude stations, and the effect of the turbopause on the formation of Es layer was proposed. The combined effects of eddy diffusion below and molecular ion collision above turbopause can prevent metal ions from drifting downward and promote their convergence to form thin plasma layers at the turbopause height. The turbopause accumulation effect is considered as a potentially important mechanism for the formation of Es layers besides wind shear theory. The study in the paper is helpful for analysis of physical origin and modeling of Es layer. [ABSTRACT FROM AUTHOR]

The eruption of the Hunga Tonga Hunga Ha'apai volcano on 15 January 2022 significantly impacted the lower and upper atmosphere globally. Using multi-instrument observations, we described disturbances from the sea surface to the ionosphere associated with atmospheric waves generated by the volcanic eruption. Perturbations were detected in atmospheric pressure, horizontal magnetic field, equatorial electrojet (EEJ), ionospheric plasma drifts, total electron content (TEC), mesospheric and lower thermospheric (MLT) neutral winds, and ionospheric virtual height measured at low magnetic latitudes in the western South American sector (mainly in Peru). The eastward Lamb wave propagation was observed at the Jicamarca Radio Observatory on the day of the eruption at 13:50 UT and on its way back from the antipodal point (westward) on the next day at 07:05 UT. Perturbations in the horizontal component of the magnetic field (indicative of EEJ variations) were detected between 12:00 and 22:00 UT. During the same period, GNSS-TEC measurements of traveling ionospheric disturbances (TIDs) coincided approximately with the arrival time of Lamb and tsunami waves. On the other hand, a large westward variation of MLT winds occurred near 18:00 UT over Peru. However, MLT perturbations due to possible westward waves from the antipode have not been identified. In addition, daytime vertical plasma drifts showed an unusual downward behavior between 12:00 and 16:00 UT, followed by an upward enhancement between 16:00 and 19:00 UT. Untypical daytime eastward zonal plasma drifts were observed when westward drifts were expected. Variations in the EEJ are highly correlated with perturbations in the vertical plasma drift exhibiting a counter-equatorial electrojet (CEEJ) between 12:00 and 16:00 UT. These observations of plasma drifts and EEJ are, so far, the only ground-based radar measurements of these parameters in the western South American region after the eruption. We attributed the ion drift and EEJ perturbations to large-scale thermospheric wind variations produced by the eruption, which altered the dynamo electric field in the Hall and Pedersen regions. These types of multiple and simultaneous observations can contribute to advancing our understanding of the ionospheric processes associated with natural hazard events and the interaction with lower atmospheric layers. [ABSTRACT FROM AUTHOR]

At present, the measurement of the near wave field of ships mostly relies on shipborne radar. The commonly used shipborne radar is incoherent and cannot obtain information on wave surface velocity. Therefore, the mathematical model of wave reconstruction is remarkably complex. As a new type of radar, coherent radar can obtain the radial velocity of the wave surface. Most wave surface reconstruction methods that use wave velocity are currently based on velocity potential. The difficulty of these methods lies in determining the initial value of the velocity integral. This paper proposes a wave surface reconstruction method based on an artificial boundary matrix. Numerical simulation data of regular and short-crest waves are used to verify the accuracy of this method. Simultaneously, the reconstruction stability under different wave velocity measurement errors is analyzed. The calculation results show that the proposed method can effectively realize the reconstruction of wave field. [ABSTRACT FROM AUTHOR]

• A moderate geomagnetic storm could trigger unseasonal post-sunset EPBs. • The periodic EPBs, with spacings of ∼ 650–870 km, extended to middle latitudes. • Enhanced F layer upward drifts alone may not be enough to trigger unseasonal EPBs. In the East and Southeast Asia, equatorial plasma bubbles (EPBs) mainly occur at post-sunset in equinoctial months. On 7–8 July 2022, a moderate geomagnetic storm occurred, with minimum Dst of −85 nT. This storm could trigger unseasonal post-sunset EPBs over the East/Southeast Asian sector. Observations from GNSS TEC/scintillation receivers, VHF coherent radar and ionosonde show that on 7 July, a few clusters of EPBs, which caused strong amplitude scintillations and significant TEC fluctuations, occurred over a wide latitude range, from low to middle latitudes up to ∼ 32°N. The EPB irregularities at middle latitudes drifted westward, with velocities of about 97 m/s. Before the generation of EPBs, apparent background ionospheric disturbances were detected. An abrupt rise of ionospheric F layer, with upward drifts of about 33 m/s was observed near sunset at low latitudes. It is suggested that the moderate geomagnetic storm was the main reason for the generation of unseasonal post-sunset EPBs. The prompt penetration of eastward electric field caused by the geomagnetic storm could lead to the abrupt rise of F layer near sunset and promote the growth of Rayleigh-Taylor instability. The background ionospheric disturbance could act as a seeding role for the unseasonal EPBs. [ABSTRACT FROM AUTHOR]

Interrupted sampling repeater jamming can produce numerous active false targets with high fidelity, which is a significant challenge for coherent pulse radar. This poses a threat to the effectiveness of anti‐jamming technology, such as time‐frequency analysis and frequency agility for conventional constant pulse repetition interval (PRI) radar, which will render the radar inoperable to detect and track the actual target. A random PRI and intra‐pulse frequency agile radar waveform with low interception properties is proposed, which can effectively restrict the acquisition of radar waveform parameters by the jammer, destroy the coherence of jamming signals, and capture the range‐dimensional centroid feature of the echo. Accordingly, a jamming identification method based on the scale constrained Gaussian mixture model and an entity radar target energe recovery algorithm based on the neighbourhood weighted maximum entropy are proposed for target detection. Theoretical analysis and simulation experiments demonstrate the effectiveness of the proposal. [ABSTRACT FROM AUTHOR]

Introduction. Conventional contact measurements of hydrographic parameters frequently fail to provide the necessary accuracy of data in the field of water area monitoring. This problem can be solved using coherent radars enabling direct measurements of surface current velocities.Aim. To establish the accuracy of surface current velocities measured by a Doppler radar using drifter data.Materials and methods. In June 2022, coastal operational oceanography studies were conducted at the hydrophysical test site of the Institute of Oceanology of the Russian Academy of Sciences in the Black Sea near Gelendzhik. Measurements were carried out using a coherent X-band radar installed on the Ashamba research vessel simultaneously with drifter experiments using Lagrangian drifters of the near-surface layer with an underwater 0.5 m sail. Coordinates were transmitted via mobile communication. The drifter data on the current velocity and direction were used to verify radar measurements. Measurements were taken onboard of the research vessel at a low speed and different distances from the shore, near the drifters. The tracks of the vessel and drifters were recorded simultaneously. Processing of the radar data involved obtaining Doppler spectra of signals to estimate the dynamic processes on the sea surface, including the current velocity.Results. Radial components of the near-surface current velocity were calculated. Then, the current velocity values obtained based on the drifter and radar data were compared.Conclusion. The present work makes a contribution to the advancement of methods for measuring surface currents from the board of a moving ship by Doppler radars. The obtained results confirm the suitability of the radar hardware and software and signal processing algorithms for measuring currents. The radar measurement data were found agree well with drifter data in the velocity range from 15 cm/s.

Coherent frequency-agile radar (FAR) has a low probability of intercept (LPI) and excellent performance of electronic counter-countermeasures (ECCM) and electromagnetic compatibility, which can improve radar cooperation and survivability in complex electromagnetic environments. However, due to the nonlinearity of radar carrier frequency and the limitation of the Doppler tolerance of high-resolution range cells, the undesirable blind-speed sidelobes are generated in the two-dimensional (2D) range–velocity plane after coherent integration (CI) using the traditional methods based on a matching filter, which may degrade the target detection performance. To solve this problem, an adaptive step-size sparsity adaptive matching pursuit (SAMP) algorithm combining off-grid correction (ASSAMP-OC) is proposed in this paper, which seeks to achieve a better trade-off between recovery efficiency and detection performance. Firstly, an adaptive iteration step size based on the Spearman correlation coefficients (SCCS) is devised, which solves the problem of the traditional SAMP algorithm being insensitive to the change in iteration step size when the residuals vary slightly, and improves the recovery speed. Secondly, the off-grid correction method by combining a regularized stagewise backtracking idea and gradient descent optimization (GDO) is adopted to improve the recovery accuracy and suppress the blind-speed sidelobe energy (BSSE), which helps to reduce CI gain loss and improve the target detection performance without the prior information of the sparsity lever. Finally, simulation and experimental results demonstrate the effectiveness and efficiency of the proposed method in terms of target detection probability, target signal energy ratio after recovery, and computational cost, compared to several existing methods. [ABSTRACT FROM AUTHOR]

Long‐time coherent integration (LTCI) is one of the most effective approaches to enhance radar detection ability of weak targets. The existing LTCI methods generally assume that the target motion satisfies polynomial models in range coordinates. However, it may be hard to provide precise description of common target motions, especially manoeuvring motions, using a polynomial with finite terms. In this study, LTCI for radar detection of weak manoeuvring targets is investigated. The range evolution rules over time are derived and employed to formulate the target signal for two common manoeuvring motions, constant acceleration (CA) and constant turn (CT) motions in Cartesian coordinates. Aiming at realising accurate energy accumulation and improving detection performance of CA and CT targets, two LTCI algorithms based on the derived range evolution model are proposed. By extracting target signal and compensating for phase fluctuation between samples in accordance with the accurate range evolution model, the coherent accumulation of target signal can be effectively accomplished, eliminating the complex range and Doppler frequency migration effects simultaneously. The use of accurate range evolution model guarantees effective energy integration regardless of target manoeuvrability and integration time as long as the target performs CA or CT motion. Moreover, more information of target kinematic characteristics such as target acceleration and turn rate, additional to range and Doppler, can be observed. Simulations are conducted to validate the effectiveness of the proposed algorithms. The cover image is based on the Research Article Long‐time coherent integration for radar detection of manoeuvring targets based on accurate range evolution model by Zeyu Xu et al., https://doi.org/10.1049/rsn2.12445. [ABSTRACT FROM AUTHOR]

Polarimetric Airborne Scientific INstrument, mark 2 (PASIN2) is a 150 MHz coherent pulsed radar with the purpose of deep ice sounding for bedrock, subglacial channels and ice‐water interface detection in Antarctica. It is designed and operated by the British Antarctic Survey from 2014. With multiple antennas, oriented along and across‐track, for transmission and reception, it enables polarimetric 3D estimation of the ice base with a single pass, reducing the gridding density of the survey paths. The off‐line data processing stream consists of channel calibration; 2D synthetic aperture radar (SAR) imaging based on back‐projection, for along‐track and range dimensions; and finally, a direction of arrival estimation (DoA) of the remaining across‐track angle, by modifying the non‐linear MUSIC algorithm. Calibration flights, during the Antarctic Summer campaigns in 16/17 and 19/20 seasons, assessed and validated the instrument and processing performances. Imaging flights over ice streams and ice shelves close to grounding lines demonstrate the 3D sensing capabilities. By resolving directional ambiguities and accounting for reflector across‐track location, the true ice thickness and bed elevation are obtained, thereby removing the error of the usual assumption of vertical DoA, that greatly influence the output of flow models of ice dynamics. [ABSTRACT FROM AUTHOR]

Using a Doppler radar to measure river surface velocity is a safe and effective technique. However, the measurement would be severely affected by undesired targets that enter the illuminated area of radar. The issue is worsened when measuring the surface velocities of wide rivers because undesired targets such as boats and ships are more likely to be present. The buoy boats fixed on the river surface and cargo ships sailing on the river would generate ground clutter and moving target interference, respectively. The clutter and interference can mask the signal produced by the Bragg scattering and seriously bias the extraction result of river surface velocity. This paper proposes two effective methods to remove ground clutter and moving target interference, respectively. One is an improved phase-based method that eliminates ground clutter after obtaining its boundaries through the phase in the frequency domain, and another is an improved constant false alarm rate (CFAR) detector that combines smallest-of selection logic and a multi-step deletion scheme to detect and remove interference in the time-Doppler spectrum. The experimental data measuring the surface velocity of the Yangtze River with a coherent S-band radar in July 2022 are used to verify the proposed methods. The results show that the proposed methods can effectively remove ground clutter and moving target interference, respectively. After clutter and interference cancellation, a more reasonable result of river surface velocity distribution can be extracted. Therefore, the methods proposed in this paper can be used to remove clutter and interference when extracting the surface velocity of rivers with numerous undesired targets. [ABSTRACT FROM AUTHOR]

Multiple-input multiple-output (MIMO) radars offer significant advantages over conventional standalone radars in terms of target detection and localization capabilities. However, to fully exploit their potential, such systems require excellent time and phase synchronization among the central unit and the radar heads. Only recently, thanks to microwave photonics (MWP) techniques, it has been possible to develop the first coherent MIMO radar demonstrators. In this paper, a simulation tool in MATLAB programming language is proposed to model coherent MIMO radars based on MWP techniques in maritime surveillance applications. Moreover, a novel approach for estimating the radar cross section of extended maritime targets is presented. The system performance in co-located and distributed MIMO configuration, as well as in single- and multi-band operations, is evaluated by means of relevant key performance indicators (KPIs). Simulations are carried out in two close-to-reality scenarios. In the first, a co-located MIMO radar is mounted on top of a patrolling vessel. In the second, a distributed MIMO radar is deployed inside a port. The proposed KPIs are analyzed in terms of both geometric and frequency diversities of the system, laying the foundations for general system optimization criteria valid in any given surveillance application. [ABSTRACT FROM AUTHOR]

Based on observations of the Jicamarca Unattended Long‐term investigations of the Ionosphere and Atmosphere (JULIA) radar, we perform statistical analysis on the occurrence of 3‐m‐scale E‐region irregularities and F‐region irregularities (FRI) in Jicamarca region of South America. The results show that there is a negative correlation between the occurrence rates of equatorial irregularities in E‐ and F‐region. When the equatorial spread F appears, the occurrence rate of irregularities in E‐layer decreases. By analyzing the ion zonal drift velocity detected by JULIA radar, it is proposed that the vertically polarized electric field in the bottom F‐layer could extend downward to E‐layer when FRI appears, leading to the weakening or even reversal of E‐layer electric field. As a result, the electron vertical drift velocity in the E‐layer decreases so as to suppress the growth rate of two‐stream instability, which cause a weakening or even disappearance of equatorial irregularities in E‐region. Plain Language Summary: For the equatorial region, the previous researches on E‐ and F‐region coupling are mainly focused on the relationship between valley region irregularities and F‐region irregularities (FRI). Up to now, the connection between the E‐region irregularities (ERI) existing in Equatorial Electrojet and FRI at night in the equatorial region has not been revealed and needs to be further studied and supplemented. Based on observations of the Jicamarca Unattended Long‐term investigations of the Ionosphere and Atmosphere radar from 2018 to 2022, we conduct a statistical study on the occurrence rate of 3 m‐scale irregularities in the E‐ and F‐layer at Jicamarca at night. The results show that there is a negative correlation between the occurrence rates of ERI and FRI. Based on the observation of ion drift, the vertical polarized electric field in the FRI is suggested to suppress the ERI. Key Points: Occurrence distributions of E‐region irregularities (ERI) and F‐region irregularities (FRI) in the equatorial ionosphere are obtainedThe occurrence rates of equatorial irregularities in E‐region and F‐region is negatively correlatedThe vertical polarized electric field in the FRI is suggested to suppress the ERI in the equatorial ionosphere [ABSTRACT FROM AUTHOR]

The Jiamusi (JME) radar is the first high‐frequency coherent scatter radar independently developed in China. In this study, we investigate the statistical characteristics of the Jiamusi radar scattering occurrence rate from the F‐region ionosphere between 40°N and 65°N geomagnetic latitude (MLAT) from March 2018 to November 2019. Then, the diurnal and seasonal variations in scattering echoes and their dependence on geomagnetic conditions are statistically investigated. It is shown that the local time of the peak scattering occurrence rate varies depending on the seasons, that is, approximately 20–22.5 magnetic local time (MLT) in summer, 17.5–20.5 MLT in equinox, and 16–17.5 MLT in winter, which is closely associated with the time of sunset. The occurrence rate also increases with the enhancement of the Kp index. To further understand the mechanism of these features, we simulate the distribution of the gradient drift instability (GDI) indicator ∇n·V→/n $\nabla n\cdot \overrightarrow{V}/n$ by using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). The analysis results indicate that the GDI may be one of the factors that contribute to these characteristic features. Key Points: Using the JME radar data to study the diurnal, seasonal, and Kp dependencies of mid‐latitude F‐region scattering occurrence rateAt night, there is a long and narrow area with an increasing scattering occurrence rate in the mid‐latitudeGradient drift instability may influence the distribution characteristics of the scattering occurrence rate of the Jiamusi radar observation [ABSTRACT FROM AUTHOR]

Distributed coherent aperture radar (DCAR) can realize high-precision long-range detection because of its excellent angular resolution. However, it is accompanied by a low search efficiency problem caused by the narrow main lobe and the angular ambiguity problem stemming from the high grating lobes. By simultaneously utilizing multiple grating lobes, we propose an effective search method and pertinent unambiguous signal processing algorithms to address the issue of using collocated DCAR for unambiguous target search. We investigate the beam migration of a grating lobe-irradiated target at various frequencies in order to carry out the multi-frequency coherent integration of the suggested search method in DCAR. In order to address the issue of using multiple beam lobes to simultaneously detect multiple targets, we also take into account the scenario of multiple targets and introduce the multiple hypothesis method to solve the problem. The DCAR search efficiency can be improved by tens of times through grating lobes exploitation, which makes use of DCAR to make the searches feasible. We also improved the performance of multi-frequency DOA ambiguity solving compared to existing algorithms with the sliding window clustering scheme. The viability of using grating lobes for the DCAR target searching and detection is shown in simulations. The experiments confirm the performance enhancement of multi-frequency coherent integration for parameter estimation. [ABSTRACT FROM AUTHOR]

We associate new data from icebear, a coherent scatter radar located in Saskatchewan, Canada, with scale‐dependent physics in the ionosphere. We subject the large‐scale icebear 3D echo patterns (treated as 2D point clouds) to a data analysis technique hitherto never applied to the ionosphere, a technique that is widely applied in cosmological red‐shift surveys to characterize the spatial clustering of galaxies. The technique results in a novel method to calculate the spatial power spectral density of the greater ionospheric irregularity field. We compare results from this method to in‐situ plasma density and magnetic field observations from the Swarm mission. We show that there is a remarkable similarity between echo clustering spectra in the E‐region and the field‐aligned current structuring spectrum observed in the F‐region: a clear and characteristic preferred scale (5 km) both in the E‐ and F‐region spectra. We discuss the possibility that this represents evidence of an energy injection into the ionospheric irregularity field via energetic particle precipitation, but offer alternative interpretations with wider connotations for the ionosphere‐magnetosphere system. These findings open new and promising avenues of research for the study of the location of ionospheric scatter echoes with 3D information. It constitutes a novel way to consider the pattern of ionospheric irregularities over wide fields of view when there is an abundance of radar echoes, which allows for the analysis of radar data as point clouds. Plain Language Summary: With the advent of precision radar data from the aurora containing detailed information about radar echo locations, we are in a position to apply statistical methods from cosmology to extract information about plasma turbulence in the ionosphere. This interdisciplinary paper presents some birds‐eye view discussions about the use of correlation functions in physics, showing how it can be used to understand physics on virtually all spatial scales in the observable universe. We present results pertinent to the ionosphere, shedding light on plasma turbulence and its relation to energetic particles in the aurora. Here, we use observations both from space, by the orbiting satellites in the European Space Agency's Swarm mission, and using an experimental auroral radar in Saskatchewan, Canada. Key Points: Clustering Statistics are used to extract large scale turbulence information from 3‐m structures in the E‐regionE‐region clustering spectra are remarkably similar to in‐situ spectra from field‐aligned currents in the F‐regionA preferential 5‐km scale is simultaneously observed in the E‐region and in the F‐region field‐aligned currents [ABSTRACT FROM AUTHOR]

This article deals with the analysis of backscattering at automatic target recognition (ATR) by ground-based radar located on rough terrain surfaces, using the properties of wave polarization. The purpose of the study is to examine and compare linear and circular polarized reflected waves, which can be described by decomposition theorems. Coherent decompositions (Pauli, Krogager, Cameron decomposition) are considered in the case of a rough terrain, for which the advantage of the Pauli decomposition has been shown. The article demonstrates an approach to the extraction of polarization signal backscattering data for two types of vehicles with different profiles. It is shown that the measurement results can be calibrated by a corner reflector that takes into account the properties of the ground surface, and further used for ATR based on supervised learning algorithms. The accuracy of object classification was 68.1% and 54.2% for the signal generated by linearly and elliptically polarized waves, respectively. Based on these results, we recommend using a linearly polarized wave as an object recognition mechanism. At the same time, any reflected depolarized wave significantly reshapes the structure due to the rotation of the object profile and the influence of a rough surface (vegetation fluctuations). This explains the low recognition accuracy in general. [ABSTRACT FROM AUTHOR]

Extracting meaningful attributes of radar scatterers from SAR images, PAZ in our case, facilitates a better understanding of SAR data and physical interpretation of deformation processes. The attribute categories and attribute extraction method are not yet thoroughly investigated. Therefore, this study recognizes three attribute categories: geometric, physical, and land-use attributes, and aims to design a new scheme to extract these attributes of every coherent radar scatterer. Specifically, we propose to obtain geometric information and its dynamics over time of the radar scatterers using time series InSAR (interferometric SAR) techniques, with SAR images in HH and VV separately. As all InSAR observations are relative in time and space, we convert the radar scatterers in HH and VV to a common reference system by applying a spatial reference alignment method. Regarding the physical attributes of the radar scatterers, we first employ a Random Forest classification method to categorize scatterers in terms of scattering mechanisms (including surface, low-, high-volume, and double bounce scattering), and then assign the scattering mechanism to every radar scatterer. We propose using a land-use product (i.e., TOP10NL data for our case) to create reliable labeled samples for training and validation. In addition, the radar scatterers can inherit land-use attributes from the TOP10NL data. We demonstrate this new scheme with 30 Spanish PAZ SAR images in HH and VV acquired between 2019 and 2021, covering an area in the province of Friesland, the Netherlands, and analyze the extracted attributes for data and deformation interpretation. [ABSTRACT FROM AUTHOR]

The target signal-to-noise ratio (SNR) can be notably improved by coherent-on-receive synthesis (CoRS) in distributed coherent aperture radar (DCAR). A core challenge of CoRS is to estimate the coherent parameters (CPs), including time, frequency, and phase, in order to cohere the multi-radar within DCAR. Conventional methods usually rely on the target's own information to estimate the CPs, which is not available in highly dynamic environments. Additionally, the CPs of different targets, especially the phase, are unequal in high-frequency systems. This means that we cannot directly use the CPs of one target to compensate for others. To address these issues, an adaptive CoRS method using the dominant scatterer is proposed for millimeter-wave (MMW) DCAR in this paper. The basic idea is to correct the CPs of the dominant scatterer to compensate for other targets. The novelty lies in the adaptive phase compensation based on the estimated CPs. This phase compensation depends on a series of discrete phase values, which are derived from the limit of synthesis loss within a given configuration. Hence, this method avoids the requirement of prior information or massive searches for the possible locations of other targets. Moreover, the dominant scatterer in this work is an unknown target with strong scattering points in radar detection scenarios, and we focus on analyzing its selection criteria. To validate the proposed method, a prototype system has been fabricated and evaluated through experiments. It is demonstrated that the multi-target can realize CoRS effectively, thus enhancing the target SNR. [ABSTRACT FROM AUTHOR]

The false‐target echo generation to Inverse Synthetic Aperture Radar (ISAR) is significant in jamming the enemy ISAR and promoting ISAR development. Generally, it requires false‐target echo coherent with the radar, real time and fine. However, conventional methods, such as digital image synthesizer (DIS), cannot meet those requirements. Moreover, existing methods do not consider the target's radial moving. To meet those demands, we propose an improved method in this study. We equivalently model echo formation as the synthesizer of two independent parts: (1) echo of remote target with radial moving and (2) echo of nearby extended target. In part one, accuracy is improved by utilising the Inner Pulse Motion (IPM) model and complexity is simplified by deducing it as a frequency offset modulation. In part two, the fine extended target echo is constructed by using convolution filtering whose resources consumption can be greatly reduced by separating it into an offline stage and a real‐time stage. Our method is verified by algorithm simulations and actual experiments. The results indicate that it can build the fine false‐target echo in real‐time and can adapt to the target's radial velocity, different resolution and size. Compared with the conventional DIS method, our method reduces the computational complexity significantly and has more comprehensive functions. [ABSTRACT FROM AUTHOR]

We consider a dual function radar communications network and propose a waveform diversity-based approach to embed both target and scheduling data in the transmitter pulses. The scheduling data of radar signal bandwidth, carrier frequency, and waveform along with the information on target detection, range, and Doppler are shared among the network radar nodes without a need for designated communication links. Scheduling and target information are encoded to bits and transmitted over the radar coherent processing interval using up- and down-chirps. We derive exact expressions of probability of bit error and false alarm rate for sharing scheduling and target information data between two radar nodes. The effect of different parameters and data sharing on the radar performance is analyzed and validated by computer simulations. Using the cross-ambiguity function, we show the capability of the up- and down- chirp sequence transmission scheme in reducing range sidelobes compared to the conventional transmission of only the up- (or down-) chirps. To overcome the limited data rate associated with only using two chirp waveforms, we consider orthogonal chirp division multiplexing and derive its CAF. We demonstrate the suitability of OCDM for high data rate radar operations by examining the mainlobe width and sidelobe levels of the zero-Doppler and zero-delay cuts of the CAF. [ABSTRACT FROM AUTHOR]

Substorms have been found to play an important role in ionospheric electrodynamics at low and equatorial latitudes. In this study, we have investigated the possible influence of substorm on the generation of equatorial spread F (ESF). Coherent backscatter radar (JULIA) and incoherent scatter radar measurements between 2000 and 2017 at the Jicamarca Radio Observatory (11.95° S, 76.87° W, ~0° dip lat) are used. In statistical analysis, 12,525 h of ESF measurements are considered and have been divided into two groups based on their local time: post-sunset (18–24 LT) and post-midnight (00–06 LT). The superposed epoch analysis shows that the substorm-caused disturbance on vertical plasma drift can last longer than 12 h, which further affects the occurrence rate of ESF during a substorm. The increase/decrease of ESF at the post-sunset/post-midnight sector with about a 1 h lag to the substorm commencement is attributed to the prompt penetration electric field (PPEF), while the disturbance dynamo electric field needs approximately 3.5 h to suppress/promote the post-sunset/post-midnight ESF after substorm. In addition, the absolute value of correlation coefficients between the AE index and the occurrence rate of ESF is the largest when a 3–3.5 h lag is considered, which implies that the effect of disturbance dynamo electric fields could be more significant than prompt penetration electric field for the generation (suppression) of post-midnight (post-sunset) ESF during a substorm. [ABSTRACT FROM AUTHOR]

Coherent scatter echoes from disintegrating meteors and from the unstable ionospheric E‐region can overlap considerably between 90 and 110 km altitudes. As the physical origin of plasma irregularities produced by meteor trails differs starkly from that of E‐region auroral irregularities, this has consequences for winds as well as electrodynamic studies, thereby introducing a need to distinguish between the two types of echoes. To that goal, we have developed a novel separation algorithm to automatically sort through arbitrarily large data sets in the region of overlap. This proves very useful when the 3D location of echoes is available. The algorithm uses a definition of crowding, or clustering, in both time and space and has been developed and tested with a comprehensive data set obtained from the recently built Canadian icebear 3D radar. We discuss the characteristics belonging to the two classes of echoes, and present statistical results about the location of each type of echo as a function of conditions. Our proposed algorithm can be applied to any coherent scatter echo data with high resolution 3D location information. Plain Language Summary: Radars measure turbulence in space plasma by bouncing a signal off of sharp plasma density gradients in the ionosphere. For decades, such coherent radars have been useful tools to investigate the ionosphere. However, when probing E‐region plasma, a coherent radar will pick up plasma produced by meteors as they enter Earth's atmosphere. We have developed an automatic algorithm to separate meteor trail echoes, as they are called, from space plasma measurements. We demonstrate the efficiency of this algorithm on the icebear 3D data set, which has an unprecedented resolution, and offers accurate 3D location data for space plasma turbulence. Key Points: Using new data from icebear 3D, we find that ionospheric turbulence overlap with meteor trail echoes between 90 and 110 km altitudeThe starkly different patterns of clustering in time and space of high latitude ionospheric turbulence and meteoric echoes are used to distinguish the echo typesThe spatial and temporal separation between observed meteor trail echoes is normally several orders of magnitude above that of ionospheric turbulence echoes at high latitudes [ABSTRACT FROM AUTHOR]

Multi-static SARs from LEO orbits allow the single-pass high-resolution imaging and detection of moving targets. A coherent MIMO approach requires the generation of multi-band, thus orthogonal, signals, the fusion of which increases the system resolution. Up to now the synchronization capability of SAR signals of different satellites is critical. Here, we propose the use of photonics to generate, receive and distribute the radar signals in a coherent multi-static SAR constellation. Photonics overcomes issues in the implementation of MIMO SAR, allowing for the flexible generation of multi-band signals and centralized generation in a primary satellite with coherent distribution to all the secondary satellites of the SAR signals over FSO links. The numerical analysis shows the proposed system has a NESZ < −29.6 dB, satisfying the SAR system requirements. An experimental proof of concept based on COTS, for both signal up- and down-conversion, is implemented to demonstrate the system functionality, showing performance similar to the simulations. The implementation of the proposed systems with integrated technologies could reduce the system SWaP and increase robustness to vibrations. A design based on the consolidated SOI platform with the transfer printing-based hybrid integration of InP semiconductor optical amplifiers is proposed. The amplifiers compensate for the losses of the passive SOI waveguides, decreasing the overall conversion loss. The polarization multiplexing of the modulated and unmodulated combs to be sent from (to) the primary to (from) the secondary satellite over the FSO links avoids complex space-consuming optical filters requiring several control signals. [ABSTRACT FROM AUTHOR]

For distributed satellite constellations, detection performance can be equivalently regarded as a single large satellite by the cooperative operation of multiple small satellites, which is a promising research topic of the Next-Generation Radar (NGR) system. However, dense grating lobes inevitably occur in the synthetic transmit pattern due to its distributed configuration, as a result of which the detection performance of dynamic coherent radar is seriously weakened. In this paper, a novel transmit beampattern optimization method for dynamic coherent radar based on a distributed satellite constellation is presented. Firstly, the effective coherent detection range interval is determined by several influence factors, i.e., coherent detection, far-field, and system link constraints. Then, we discuss the quantitative evaluation method for coherent integration in terms of synchronization error, beam pointing error, and high-speed motion characteristics and we allocate the corresponding terms in a reasonable way from the perspective of engineering. Finally, the space–time–frequency degrees of freedom (DOFs), which can be collected from satellite spacing, carrier frequencies, and platform motion characteristics, are utilized to realize a robust transmit beampattern with low sidelobe by invoking a genetic algorithm (GA). Simulation results validate the effectiveness of our theoretic analysis, and unambiguous coherent transmit beamforming with a satellite constellation of limited scale is accomplished. [ABSTRACT FROM AUTHOR]

Airborne distributed coherent aperture radar is of great significance for expanding the detection capability of the system. However, the extra observation dimension introduced by its sparse configuration also deteriorates the performance of traditional adaptive processing in a non-uniform environment. This paper focuses on moving target detection when the system works in a clutter–jamming-coexisting environment. In order to make full use of the specific low-rank structure to reduce the requirement for training data, this paper proposes a two-stage adaptive scheme that cancels jamming and clutter separately. The proposed suppression scheme first excludes the mainlobe jamming component from the training data based on the prior clutter subspace projection and performs intra-node clutter suppression. Then, the remaining jamming is jointly canceled based on the covariance obtained with its inter-pulse mixture model. Numerical examples show that this scheme can effectively reduce the blocking effect of main lobe jamming on high-speed targets but, due to the inaccuracy of the prior subspace, there is a certain additional loss of signal-to-noise ratio for near stationary targets. The simulation also shows that the proposed scheme is equally applicable to systems with a time-varying distributed geometry. [ABSTRACT FROM AUTHOR]

In the morning on 29 May 2017, the Sanya very‐high‐frequency Radar and the Hainan COherent Scatter Phased Array Radar (HCOPAR) at Fuke were cooperated with similar operating frequencies to investigate the dissipation of the daytime F‐region Field‐Aligned Irregularities (FAIs). The two radars are located on south and north sides of the Hainan island of China, and the distance between them is about 140 km. The Sanya VHF radar recorded the FAIs between 00:12 and 02:42 UT (07:12 and 09:42 LT), and the FAIs observed by the HCOPAR appeared more than 10 min later at lower altitudes and suffered severe dissipation. The echoes emerged in all the beams simultaneously, and their Doppler velocities were close to zero, implying that the daytime FAIs were traveling along the magnetic field lines downward and northward with almost no zonal velocity. The front and bottom edges of the irregularities decayed earlier, and the parts in the FAIs with steeper density gradient lasted longer. The dissipation mechanism is attributed primarily to the photoionization and diffusing across the geomagnetic field in different period and gradient of the irregularities. Plain Language Summary: Equatorial Plasma Bubbles, which cause Field‐Aligned Irregularities (FAIs), are widely accepted as a nighttime phenomenon. They usually disappear before sunrise and are rarely recorded by radars during daytime. Especially the dissipation process of the daytime FAIs is seldom reported. The Sanya very‐high‐frequency radar located on the south of the Hainan Island and the Hainan COherent Scatter Phased Array Radar located on the north observe the daytime FAIs successively. The observation results show that while the daytime FAIs were traveling from Sanya to Fuke, the dissipation process first occurred in the front and bottom edges as well as the regions of the irregularities with small density gradient. The dissipation mechanism for the fossil F‐region FAIs traveling along the magnetic field lines under the sun are discussed. This investigation results may deepen our understanding of the dissipation processes in the irregularity and their interaction with the ambient atmosphere. Key Points: Sanya very‐high‐frequency radar and Hainan COherent Scatter Phased Array Radar at the south and north sides of the Hainan island were cooperated to investigate the daytime irregularitiesDaytime irregularities drifted along magnetic field lines northward and downward without zonal movement, and suffered severe dissipationFront and bottom edges of the irregularities decayed earlier, and their parts with steeper density gradient persisted longer [ABSTRACT FROM AUTHOR]

• A self-trained classifier for ionospheric radar data is presented. • Its training is based on finding optimal hidden classes in the data. • The dataset consists from measured parameters and numerically simulated ones. • Found classes could be interpreted from physical point of view. In the paper an algorithm for automatic classification of signals received by decameter coherent scatter radars is described. The algorithm is self-trained on unlabeled dataset and is based on training optimal physics-based classification using clusterization results. The approach allows to find optimal (hidden) classification, based on physically interpretable parameters of the received data, both obtained during physics-based numerical simulation and measured experimentally. As a result we trained the classification artificial neural network and found 11 well-interpretable classes from physical point of view in the available data. Other found classes are not interpretable from physical point of view, demonstrating the importance of taking into account radiowave propagation for correct classification. The advantages of the method include the possibility of its automatic training: it does not require preliminary data analysis by an expert, and it is self-trained to separate available data into the classes, that can be interpreted from a physical point of view. [ABSTRACT FROM AUTHOR]

The NATO SPS G-5014 project has shown the possibility of using a holographic RADAR for the detection of anti-personnel mines. To use the RADAR on a robotic scanning system, it must be portable, light, easily integrated with mechanical handling systems and configurable in its operating parameters for optimal performance on different terrains. The novel contribution is to use software programmable electronics to optimize performance and to use a time reference to obtain synchronization between the RADAR samples and the position in space, in order to make it easy to integrate the RADAR on robotic platforms. To achieve these goals we used the Analog Devices "ADALM Pluto" device based on Software Defined Radio technology and a time server. We have obtained a portable system, configurable via software in all its operating parameters and easily integrated on robotic scanning platforms. The paper will show experiments performed on a simulated minefield. The electronics project reported in this work makes holographic RADARs portable and easily reconfigurable, therefore adaptable to different applications from subsurface soil investigations to applications in the field of non-destructive testings. [ABSTRACT FROM AUTHOR]

Objective: Microwave imaging has been investigated for medical applications such as stroke and breast imaging. Current systems typically rely on bench-top equipment to scan at a variety of antenna positions. For dynamic imaging of moving structures, such as the cardiovascular system, much higher imaging speeds are required than what has thus far been reported. Recent innovations in radar-on-chip technology allow for simultaneous high speed data collection at multiple antenna positions at a fraction of the cost of conventional microwave equipment, in a small and potentially portable system. The objective of the current work is to provide proof of concept of dynamic microwave imaging in the body, using radar-on-chip technology. Methods: Arrays of body-coupled antennas were used with nine simultaneously operated coherent ultra-wideband radar chips. Data were collected from the chest and thigh of a volunteer, with the objective of imaging the femoral artery and beating heart. In addition, data were collected from a phantom to validate system performance. Video data were constructed using beamforming. Results: The location of the femoral artery could successfully be resolved, and a distinct arterial pulse wave was discernable. Cardiac activity was imaged at locations corresponding to the heart, but image quality was insufficient to identify individual anatomical structures. Static and differential imaging of the femur bone proved unsuccessful. Conclusion: Using radar chip technology and an imaging approach, cardiovascular activity was detected in the body, demonstrating first steps towards biomedical dynamic microwave imaging. The current portable and modular system design was found unsuitable for static in-body imaging. Significance: This first proof of concept demonstrates that radar-on-chip could enable cardiovascular imaging in a low-cost, small and portable system. Such a system could make medical imaging more accessible, particularly in ambulatory or long-term monitoring settings. [ABSTRACT FROM AUTHOR]

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The last decade witnessed remarkable growth in the number of global road accidents. To minimize road accidents, transportation systems need to become more intelligent. Multiple detection of target vehicles under adverse weather conditions is one of the primary challenges of autonomous vehicles. Photonic radar sensors may become the promising technology to detect multiple targets to realize autonomous vehicles. In this work, high-speed photonic radar is designed to detect multiple targets by incorporating a cost-effective wavelength division multiplexing (WDM) scheme. Numerical simulations of the proposed WDM-based photonic radar is demonstrated in terms of received power and signal to noise (SNR) ratio. The performance of the proposed photonic radar is also investigated under diverse weather conditions, particularly low, medium, and thick fog. The proposed photonic radar demonstrated a significant range resolution of 7 cm when the target was placed at 80 m distance from the photonic radar sensor-equipped vehicle. In addition to this, traditional microwave radar is demonstrated to prove the effectiveness of the proposed photonic radar. [ABSTRACT FROM AUTHOR]

Radio altimeters are an important component of modern helicopter on-board systems. These devices currently involve the use of narrowband deterministic signals, that limits their potential technical characteristics. Given the significant breakthrough in the development of wideband and ultra-wideband radio electronics, it is promising to create on-board radio complexes capable of obtaining the necessary information using wideband stochastic signals. At the same time, when developing such complexes, it is important to use optimal synthesis methods for radio systems, which will allow optimal signal processing algorithms and potential accuracy parameters to be obtained. In this work, the algorithm to measure flight altitude for a helicopter or an unmanned aerial vehicle based on the processing of wideband and ultra-wideband pulsed stochastic signals is synthesized for the first time by the maximum-likelihood method. When formulating the problem, the mathematical model of the signal and observation is specified, and their statistical characteristics are investigated. The peculiarity of the synthesis task is the use of a noise pulse transmitter, which implements the function of an underlying surface illuminator, as well as considering the signal structure destruction during its radiation, propagation, and reflection. This signal shape destruction makes it impossible to synthesize a radar with internally coherent processing when working on one receiving antenna. In accordance with the synthesized algorithm, a simulation model of a pulsed radar with a stochastic probing signal has been developed and the results of its modeling are presented. [ABSTRACT FROM AUTHOR]

Synthetic Aperture Radar (SAR) Coherent Change Detection (CCD) allows for the detection of very small scene changes. This is particularly useful for reconnaissance and surveillance as small changes such as vehicle tracks can be identified. In some situations, it is desirable to rapidly collect repeat pass SAR images for use in change detection, and multistatic geometries may facilitate this. Such repeat passes may however have significant baselines, particularly for satellite‐based platforms, though CCD products are reliant on high coherence for good interpretability. This work investigates the sources and levels of incoherence associated with bistatic SAR imagery with increasing baselines using simulations and measured laboratory data. [ABSTRACT FROM AUTHOR]

Coherent backscatter radar observations made at the Jicamarca Radio Observatory (JRO) have contributed significantly to our understanding of equatorial F-region irregularities. Radar observations, however, have been made predominantly at the Very-High Frequency (VHF) band (50 MHz), which corresponds to measurements of 3-m field-aligned irregularities. The deployment of the 14-panel version of the Advanced Modular Incoherent Scatter Radar (AMISR-14) at Jicamarca provided an opportunity for observations of Ultra-High Frequency (UHF - 445 MHz) echoes which correspond to measurements of irregularities with 0.34 m scale sizes. Here, we present what we believe to be the first report describing the quiet-time climatology of sub-meter equatorial F-region irregularities derived from UHF radar measurements. The measurements were made between August 2021 and February 2023 using a 10-beam AMISR-14 mode that scanned the F-region in the magnetic equatorial plane. The results show how F-region sub-meter irregularities respond to variations in season and solar flux conditions. The results also confirm, experimentally, that the occurrence of UHF F-region echoes is controlled by the occurrence of equatorial spread F (ESF). Higher occurrence rates were observed during pre-midnight hours and during Equinox and December solstice. Reduced occurrence rates were observed during June solstice. The results show that an increase in solar flux was followed by an increase in the altitude where noticeable occurrence rates (≳ 10%) start and in the maximum altitude of these occurrence rates. The observations also show that occurrence rates lasted longer (in local time) during low solar flux conditions. Comparisons with collocated VHF radar observations showed that, despite differences in radar parameters, observation days, and the scale size (one order of magnitude) of the scattering irregularities, the two systems show similar climatological variations with only minor differences in the absolute occurrence rates. Finally, the analysis of the occurrence rates for different beams did not show substantial climatological variations over local (within a few 100s of km) zonal distances around JRO. We point out, however, that observations on a single day can show strong local variations in echo detection and intensity within the AMISR-14 field of view due to the intrinsic development and decay of ESF structures. • We report for the first time the climatology of nighttime F-region echoes observed by a UHF radar at the magnetic equator. • We present the response of the occurrence rate of UHF echoes to variations in season and solar flux that are related to ESF. • We show that the climatology of irregularities from AMISR-14 is similar to that observed by the collocated VHF JULIA radar. • Analysis of the 10-beam observations confirms that the ESF echo climatology does not change within local zonal distances. [ABSTRACT FROM AUTHOR]

This article presents detailed geometric analyses of ground clutter in bistatic passive airborne radars. Analytic closed-form solutions are derived for finding the intersection of iso-ranges and iso-velocities. This allows clutter bistatic coordinates to be easily converted to its Cartesian coordinates. Based on those solutions, a theoretical clutter map in the bistatic coordinates can be calculated assuming uniform clutter distribution on the Earth’s surface. This is done by first converting a resolution cell in the bistatic coordinates into a corresponding cell in the Cartesian coordinates. Next, the power distribution in the bistatic coordinates is calculated taking into account range dependency, incident angles, and the radiation patterns of the transmitter and the receiver. The aim of this analysis is to characterize clutter in terms of expected mean power map rather than in terms of statistical distribution. The obtained results provide insights into the expected characteristics of clutter, which can be helpful in designing signal processing algorithms for ground moving target indication (GMTI) and synthetic aperture radar (SAR). The theoretical clutter distribution on the range-velocity map is compared with real-life data acquired with a Digital Video Broadcasting–Terrestrial (DVB-T)-based passive radar and good agreement between theory and measurement is presented. [ABSTRACT FROM AUTHOR]

The holographic subsurface radar (HSR) has been a promising geophysical electromagnetic technique for detecting shallowly buried targets with high lateral resolution image. However, radar images are considerably interpreted by strong reflections from rough surface and inhomogeneity in media of interest. In this article, we focus on mitigating the clutter in HSR applications using a learning-based approach, which requires neither prior information regarding the penetrable medium characteristics nor analytic framework to describe the through-medium interference. The generative adversarial network (GAN) with attentive subspace projection is developed to remove the clutter and recover the target image. The subspaces containing target response are selected with the multihead attention preliminarily. Then, the generative network will further focus on the target regions, and the discriminative network will assess the generated results locally and globally. Experiments using real data were conducted to demonstrate the effectiveness of our approach. The visual and quantitative results show that the proposed approach achieves superior performance on removing clutter in HSR images compared with the state-of-the-art clutter mitigation approaches. [ABSTRACT FROM AUTHOR]