Search

In scenarios with constrained physical aperture sizes, aiming to enhance the resolution and accuracy of Direction of Arrival (DOA) estimation, this paper proposes a novel approach that integrates a moving synthetic virtual array with Sparse Bayesian Learning (SBL) for DOA estimation. Initially, a virtual array is constructed based on the motion characteristics of the target. Subsequently, the SBL method is employed to estimate the DOA of the target. Simulation experiments validate the effectiveness of this approach, demonstrating comparable DOA estimation performance to synthetic aperture methods with larger aperture sizes, even in situations with limited aperture expansion. Furthermore, under constant virtual aperture expansion, this method surpasses non‐SBL methods regarding DOA resolution. We explore the construction of a virtual array through the relative displacement induced by target motion. It further investigates the performance improvement of Direction of Arrival (DOA) estimation resolution using sparse Bayesian learning within the context of a virtual array. [ABSTRACT FROM AUTHOR]

A photonics-based leakage cancellation and echo signal de-chirping approach for frequency-modulated continuous-wave radar systems is proposed based on a dual-drive Mach–Zehnder modulator (DD-MZM). The de-chirp reference signal and the leakage cancellation reference signal are combined and applied to the upper arm of the DD-MZM, while the received signal, including the leakage signal and echo signals, is applied to the lower arm of the DD-MZM. When the amplitudes and delays of the leakage cancellation reference signal and the leakage signal are precisely matched, the leakage signal is canceled in the optical domain. The de-chirped signal is obtained after the leakage-free optical signal is detected in a photodetector. An experiment is performed. The cancellation depth of the leakage signal after de-chirping is around 23 dB when the center frequency and bandwidth of the linearly frequency-modulated signal are 11.5 and 2 GHz, respectively. When the leakage cancellation is not employed, the leakage will seriously affect the imaging results and distance measurement accuracy. When the leakage cancellation is enabled, the imaging results of multiple targets can be clearly distinguished, the distance measurement error of a moving target is significantly reduced to less than 10 cm, and a dynamic range increase of 21 dB is achieved. [ABSTRACT FROM AUTHOR]

This article proposes an accurate clutter synthesis method for a dynamic radar under various environmental conditions. Previous clutter synthesis methods mostly rely on empirical clutter models fitting the measurement results obtained from the limited conditions of observations. Inherently, these models have difficulties synthesizing clutter for unexplored environments and operational conditions. The method presented in this article overcomes this limitation by creating and summing the clutter patch by patch. The clutters of individual patches are synthesized by the clutter model corresponding to each patch's specific conditions, such as the grazing angle, ground texture, and wind velocity. It enables one to synthesize clutter signals under various observation scenarios accurately. For example, this work demonstrates the synthesis of clutter signals from heterogeneous textures for an aircraft radar on a mission. Also, the clutter signals for a monopulse radar are synthesized and included in the radar simulation to evaluate the performance of a detection algorithm. Potentially, this method can be utilized to design a robust radar algorithm against the clutter environment of interests. For a given condition of measurement, it is shown that the synthesized clutters agree well with the actual measured data in their statistical characteristics. The program is freely available through the open code. [ABSTRACT FROM AUTHOR]

We propose a deep learning-based approach to localizing a small moving object with a single millimeter-wave frequency-modulated continuous-wave (FMCW) radar. The main challenge that foils conventional localization techniques, such as 3-D fast Fourier transform (3-D-FFT), Pisarenko method, multiple signal classification (MUSIC), estimation of signal parameters via rotational invariance technique (ESPRIT), Capon’s method, and Burg’s method, is the low signal-to-noise ratio of the reflected signal from millimeter-sized objects. Our key idea is to combine useful but noisy features from classical transforms [e.g., fast Fourier transform (FFT)] with neural networks that can refine and interpret those features into range and angle estimates by training on a large dataset of examples. Importantly, our networks were designed to be translation-equivariant, which enables accurate predictions of unseen object locations and improves the range and azimuth root mean square error (RMSE) scores by 34%–46% and 41%–60%, respectively, over state-of-the-art approaches. This pilot study establishes a new baseline for small-object tracking using FMCW and can enable tracking of small animals, such as ants inside the colony for behavior studies. Our first FMCW-small-object dataset and the source code are publicly available on https://github.com/shikuzen/RA-CNN. [ABSTRACT FROM AUTHOR]

Limited access to available spectrum motivates new age radio frequency (RF) systems to co-exist and cooperate. In this paper, we consider a joint communication and radar system (labeled Comm-Radar), that is capable of receiving and decoding communication signals while simultaneously enabling radar functionalities. Traditionally, systems that leverage principles of spectrum convergence, deemed each functions to be detrimental to the others. This has led to successive interference cancellations strategies that effectively isolate each of the functionalities. Here, in the context of Comm-Radar processing structure, we show that ambient communication transmissions actually assist in radar estimation. The insight is that communications multipath reflects off radar targets (we denote these comm-target paths) in the environment, and can be processed at the receiver to extract extra information. In this work we characterize the fundamental performance limits of such a joint system.To demonstrate the radar performance gain achievable via the processing of extra comm-target paths, the closed-form Cramér-Rao Bound (CRB) for the estimation of direction, range and velocity of a target is derived, for both the general case, and practical radar and communication signalling schemes. Based on the CRB, joint outer bounds are obtained to characterize the trade off between estimation performance and communication rate. Finally, numerical results verify the advantages of utilizing the target reflections from the communication signal to bolster radar performance. [ABSTRACT FROM AUTHOR]

A novel matrix-pencil (MP)-based interference mitigation approach for frequency-modulated continuous-wave (FMCW) radars is proposed in this article. The interference-contaminated segment of the beat signal is first cut out, and then, the signal samples in the cutout region are reconstructed by modeling the beat signal as a sum of complex exponentials and using the MP method to estimate their parameters. The efficiency of the proposed approach for the interference with different parameters (i.e., interference duration, signal-to-noise ratio (SNR), and different target scenarios) is investigated by means of numerical simulations. The proposed interference mitigation approach is intensively verified on experimental data. Comparisons of the proposed approach with the zeroing and other beat-frequency interpolation techniques are presented. The results indicate the broad applicability and superiority of the proposed approach, especially in low SNR and long interference duration situations. [ABSTRACT FROM AUTHOR]

This article introduces a precise radar-based direction-of-arrival measurement system using a Viterbi decoder. Based on the frequency-modulated continuous-wave principle, the presented method employs both the frequency difference and the round-trip phase difference of the radio wave signal impinging at two widely spaced receiving antennas, thereby accomplishing highly accurate and absolute measurements of the direction-of-arrival. In order to resolve angular ambiguity, which is inherently caused by the $2 \pi $ ambiguity in phase measurement, the proposed Viterbi decoder determines the most likely sequence of direction-of-arrivals of the impinging radio wave signals. For proof of concept, experiments based on a 77 GHz radar unit equipped with one transmitting and two widely spaced receiving antennas were carried out. The measurement results show that angular ambiguity can be resolved by the presented Viterbi decoder since absolute, accurate, and repeatable direction-of-arrival estimates were obtained, even for receiver antenna spacings significantly larger than the signal wavelength. [ABSTRACT FROM AUTHOR]

FILtering by COHerence (FILCOH) is a novel technique for mitigating ground clutter echoes of weather radar, particularly those transmitting in multiple pulse repetition time (PRT) schemes. FILCOH takes advantage of the differences in correlation time for separating rain and ground echoes. In short, the longest time-lag coefficients of the autocorrelation function are used to estimate the ground clutter contribution which is then subtracted from the shortest lag coefficients before the extraction of the meteorological parameters using the conventional estimators. The initial results are obtained for uniform PRT data provided by the Degreane Horizon wind profiler radar. The analysis illustrates the behavior of FILCOH filtering and its high performance in such cases. The simulation results are then presented for 2- and 3-PRT pulse schemes using standard PRT ratios. For the 2-PRT scheme, FILCOH filters out the ground clutter echoes up to a clutter to signal ratio (CSR) of 65 dB for the received power and up to 45 and 60 dB for the radial velocity and spectrum width, respectively. For the 3-PRT scheme, ground clutter filtering is effective up to $CSR = 45$ dB for the received power and up to $CSR = 35$ dB for radial velocity and spectrum width. These results are confirmed by 3-PRT real data issued from the French X- and C-band radars. The observed performances are, however, a little less than those of the simulation since we observed a maximum 40-dB attenuation of the ground clutter echoes of the studied rain events. [ABSTRACT FROM AUTHOR]

Many modern phased-array radars (PARs) are multichannel systems that include multiple receivers for data acquisition. Each channel provides a signal from a group of Transmit/Receive modules comprising a section of the antenna. Channels typically consist of a full receive path, often with an independent local oscillator (LO) clock source. Such arrangement provides for beamforming flexibility on receive which can be applied in a digital domain. Consequently, the channel-to-channel phase and magnitude alignment is critical to maximizing the performance of the digital beamforming process and the accuracy of resulting detections and measurements. Herein, a novel method to improve such alignment using weather returns and achieve the improvement in the polarimetric variable estimates is described. [ABSTRACT FROM AUTHOR]

Multistatic radar is a promising option for the low-cost collection of multiple-Doppler weather observations. However, due to the use of low-directivity antennas at the receivers in these systems, they typically suffer from extremely high two-way sidelobe levels compared to monostatic radars. Doppler velocity estimation biases induced by sidelobe contamination have proved to be a significant obstacle to more widespread adoption of this technology. It has been noted in the existing literature that the technique of sidelobe whitening, first developed for use in monostatic systems, has the potential to mitigate this issue. However, the existing sidelobe whitening algorithm is not suitable for use in this application, as it is only capable of achieving whitening in the two-way antenna pattern, whereas an algorithm to be used in the multistatic case must be able to achieve this result in the transmit pattern alone. This article proposes an alternate sidelobe whitening technique based on the method of alternating projections which allows for effective whitening in the one-way antenna pattern. A multistatic weather radar time-series simulator is used in conjunction with numerical weather prediction data to demonstrate the effectiveness of this method in mitigating multiple-Doppler measurement biases. [ABSTRACT FROM AUTHOR]

Radar signal processing is a promising tool for vital sign monitoring. For contactless observation of breathing and heart rate a precise measurement of the distance between radar antenna and the patient's skin is required. This results in the need to detect small movements in the range of 0.5 mm and below. Such small changes in distance are hard to be measured with a limited radar bandwidth when relying on the frequency based range detection alone. In order to enhance the relative distance resolution a precise measurement of the observed signal's phase is required. Due to radar reflections from surfaces in close proximity to the main area of interest the desired signal of the radar reflection can get superposed. For superposing signals with little separation in frequency domain the main lobes of their discrete Fourier transform (DFT) merge into a single lobe, so that their peaks cannot be differentiated. This paper evaluates a method for reconstructing the phase and amplitude of such superimposed signals. [ABSTRACT FROM AUTHOR]

In order to improve the resolution of imaging radars, electrically large arrays and a high absolute modulation bandwidth are needed. For radar systems with simultaneously high range resolution and very large aperture, the difference in path length at the receiving antennas is a multiple of the range resolution of the radar, in particular for off-boresight angles of the incident wave. Therefore, the radar response of a target measured at the different receiving antennas is distributed over a large number of range cells. This behavior depends on the unknown incident angle of the wave and is, thus, denoted as range-angle coupling. Furthermore, the far-field (FF) condition is no longer fulfilled in short-range applications. Applying conventional signal processing and radar calibration techniques leads to a significant reduction of the resolution capabilities of the array. In this article, the key aspects of radar imaging are discussed when radars with both large aperture size and high absolute bandwidth are employed in short-range applications. Based on an initial mathematical formulation of the physical effects, a correction method and an efficient signal processing chain are proposed, which compensate for errors that occur with conventional beamforming techniques. It is shown by measurements that with an appropriate error correction an improvement of the angular resolution up to a factor of 2.5 is achieved, resulting in an angular resolution below 0.4° with an overall aperture size of nearly 200 $\lambda _{0}$. [ABSTRACT FROM AUTHOR]

Bats and birds are endangered by wind turbines, leading to a significant amount of fatalities in recent years, mainly due to a direct collision of the animals with the rotor blades or through barotrauma effects. The search for fatalities and the quantification of their number is an important tool to understand the adverse effects of wind turbines on bats and birds. This method has several problems: Cadavers do not always remain on site due to predators, only a part of the relevant area on the ground can be searched manually and not all fatalities can be found. This leads to the fact that searches, although extremely valuable in the analysis of animal mortality, are hardly used in practice due to the high effort and enormous costs. The present work closes this research gap by proposing a radar barrier approach that automatically detects animal fatality in real‐time. Therefore, three FMCW radar systems are attached to the tower of a wind turbine at different heights. Subsequent processing of time‐of‐flight information enables a binary classification of whether a fatality was observed or not. This study presents the results from numerical simulations as well as preliminary experimental results for a proof of concept. [ABSTRACT FROM AUTHOR]

In this article, we investigated the arrival times of plane waves incident on a circular dipole array in a borehole (CAB). Our theoretical and experimental findings suggest that the elevation angle of the received wave strongly affects plane wave arrival times in a borehole installation. If a plane wave is incident at the critical elevation angle (CEA), there is little arrival time difference among the signals at the CAB. On the other hand, when the plane wave is incident at an elevation angle steeper than the CEA, the wave arrival order at the CAB becomes opposite to the normal incidence case. In order to clarify the reason why these phenomena occur, we decompose a transmission matrix, which represents the effect of the borehole, into three components. We found that the azimuthal component of the electric field contributes to the observed phenomena significantly, and two of the three decomposed components cancel each other out at the CEA. For verification of the phenomena in a cross-hole borehole radar measurement, we performed a numerical simulation and carried out experimental verification at a field test site. Both data sets confirmed that the arrival time differences among the signals at the CAB were minimized at the elevation angle predicted to be the CEA. In addition, we found the reverse of the incident wave’s arrival order at the CAB. [ABSTRACT FROM AUTHOR]

A new architecture of self-injection-locked (SIL) quadrature radar sensor based on a metamaterial (MTM) leaky-wave antenna (LWA) to detect multi-target vital sign and location simultaneously is proposed for the first time. The angular information of targets is obtained by a 1-D MTM LWA with −50° to +30° beam-steering angle when the frequency varies from 1.85 to 2.85 GHz. Furthermore, the distance from the radar sensor to the targets is obtained by imposing the frequency-shift keying (FSK) modulation on the radar waveforms. Meanwhile, the vital sign signals of multiple targets can be extracted from the Doppler frequency spectrum. For the proof of concept, two frequencies, 2.24 and 2.45 GHz, are chosen to detect two targets at different locations, which correspond to the scanning angle at −10° and −40°, respectively. A first-order microwave differentiator followed by a quadrature coupler is employed to convert the radio frequency (RF) signals from the output of the MTM LWA to in-phase (I) and quadrature (Q) RF signals with frequency-dependent amplitudes, thereby eliminating the null point issues for radar detection. Experimental results show that the proposed SIL MTM radar sensor can detect the target location accurately, while the obtained vital sign results agree well with the ground truth. [ABSTRACT FROM AUTHOR]

The emergence of CubeSats has opened up the possibilities of advanced space missions with lower costs and faster development times. A major factor that limits the functionality of CubeSats is the absence of high-gain antennas (HGAs) that can sustain a high data-rate link for communications or provide the required spatial resolution for remote sensing. In this work, we discuss the development of one of the largest apertures at Ka-band for CubeSats—a 1 m mesh deployable offset reflector antenna, with a stowed volume of 3U ($10\times 10\times 30 \,\,\text {cm}^{3}$)—to enable precipitation radars that can achieve the required spatial resolution while meeting the stringent mechanical constraints posed by the small CubeSat volume. We detail the critical aspects of this antenna design, including RF characterization, antenna fabrication, and measurement. We also describe a novel deployment mechanism that facilitates the packaging of such a large aperture in a small volume. The antenna demonstrated a measured efficiency of 60%, with a half-power beamwidth of 0.60° at 35.75 GHz. The cost reduction afforded by CubeSats makes launching a constellation of such radar based CubeSats practical, allowing higher temporal sampling rates, which is essential for the observation of weather processes with a short-time evolution. [ABSTRACT FROM AUTHOR]

We develop and present a novel minimum estimation error variance waveform design method. The method optimizes the spectral shape of a unimodular radar waveform such that the performance of a joint radar-communications system that shares spectrum is maximized. We also develop the novel spectral water-filling successive interference cancelation data rate, which employs the continuous spectral water-filling algorithm to obtain the optimal communications power spectrum. We perform a numerical study to compare the performance of the new technique with the previously derived spectral mask shaping method. The global estimation rate and the data rate capture the radar and the communications performance, respectively. [ABSTRACT FROM AUTHOR]

A new approach to through‐the‐floor localization of a static person using an ultra‐wide band (UWB) radar is introduced in this article. The proposed solution is based on a multilateration method appropriately modified for target localization by a multistatic UWB radar employing 1 transmitting (Tx) and 4 receiving antennas (Rxi). Due to an appropriate radar antenna array layout, a geometrical interpretation of the considered problem of localization allows finding the target coordinates by a simple computation of 2 intersections of 2 pairs of ellipses. The mentioned ellipses are defined by the antenna layout and time‐of‐arrivals corresponding to the target that were estimated for the particular pairs of Tx–Rxi antennas. Experimental analysis of the proposed method of through‐the‐floor localization of a person confirmed its good accuracy, despite being such a complex scenario. [ABSTRACT FROM AUTHOR]

As a typical signal processing problem, direction-of-arrival (DOA) estimation has been adapted to a wide range of applications in radar-based systems. A high DOA resolution requires a large number of antenna elements which increases the overall cost. To minimise the cost, it is desirable to choose an optimum sub-array from a full array. To enable cognition, the subarrays are selected based on the present target scenario. By using deep learning (DL) based techniques, the authors show a cognitive sparse array selection technique. By using hardware simulations, they demonstrate the applicability of the deep learning (DL)-based sparse antenna selection network in direction-of-arrival (DOA) estimation problems. They show that the DL-based sub-arrays lead to a higher direction-of-arrival (DOA) estimation accuracy by 6 dB over random array selection. [ABSTRACT FROM AUTHOR]

For the synthesised aperture radar, the radar signal with a zero autocorrelation zone (ZACZ) is synthesised, which represents a sequence from two phase‐code shift keying (PCSK) pulses coded by adjacent pairs of the D‐code with additional linear frequency shift‐keying (LFSK) of the train pulse discrete value. A comparative analysis is performed of correlation characteristics of the synthesised signal with the signal without discrete modulation and the truncated M‐sequence. Total correlation characteristics of the ensemble from four signals are also analysed. It is shown that in ZACZ, the level of all side lobes of the autocorrelation function (ACF) of the synthesised signal is less than the level of the ACF side lobes of the M‐sequence. The total ACF of the ensemble of four signals both with modulation and without the discrete modulation has zero side lobes along with the whole time axis τ, and at mismatch in frequency in ZACZ, it has the side lobe level <−40 dB. [ABSTRACT FROM AUTHOR]

One option for the improvement of weather radar technology is the use of dual-polarized phased-array radar for weather observations. Several risk factors on this path have been identified and one of the most important ones is the existence of significant cross-polar patterns inherent to the phased-array antenna. These antenna patterns induce cross-coupling between returns from the two orthogonal radiation planes, which results in the biases of polarimetric variable estimates. Furthermore, the inductive and capacitive coupling in hardware behind the antenna may exacerbate the cross-coupling effects. This presents a formidable challenge because sufficient cross-polar isolation is difficult to achieve by the antenna hardware alone. Hence, additional approaches are required to reduce the biases due to cross-coupling. One proposed technique is a 180° pulse-to-pulse phase change of signals injected in either the horizontal or vertical ports of the transmission elements. This approach was analyzed for signals processed in the time domain but its effects in the frequency domain have not been investigated. Herein, these effects are considered in the presence of nondepolarizing scatterers. [ABSTRACT FROM AUTHOR]

This paper presents a whole new method for simulating a raw signal of synthetic aperture radar (SAR) with trajectory deviation. In this paper, a SAR raw signal is viewed as a spatial function in terms of radar location, at a fixed range frequency component of the radar signal. It is disclosed and verified that the SAR raw signals is band limited from the perspective of spatial spectrum analysis. Based on this finding, SAR raw signals under condition of trajectory deviation are obtained by interpolating the SAR raw signal along a set of ideally linear trajectories. The unprecedent performance achieved by the new method mainly includes the following. First, it provides a way to update existing simulators designed for ideally linear trajectory, giving them the capability to take trajectory deviation into consideration. Second, it is an efficient SAR raw signal simulator whose validity limit is greatly expanded compared with existing efficient simulators, as many constraints are eliminated without sacrificing computational efficiency or phase accuracy. Third, it is a uniform framework applicable for multiple SAR acquisition modes. Finally, the computational complexity of Monte Carlo experiment under various trajectory deviations is dramatically reduced, at the expense of storage memory requirement. Theoretical analyses are demonstrated by experimental results. [ABSTRACT FROM PUBLISHER]

This paper presents a novel low-cost technique for digital beamforming on receive in phased array radar. The presented technique can be used to synthesize multiple beam patterns in different angular directions and can be implemented after analog summation of the received signals using only one received pulse. Since the digital beamforming in our work is implemented after summation of the received signals, a reduction of the overall cost, size, and power consumption of the phased array front-end becomes possible in comparison to standard digital beamforming. The performance and effectiveness of the here-proposed beamforming technique are assessed by means of modeling and experimental verification. [ABSTRACT FROM PUBLISHER]

One of the main challenges to the use of phased array radar for weather observations is the implementation of dual polarization with acceptable levels of cross-polar fields induced by the antenna. For example, to achieve acceptable differential reflectivity bias (i.e., less than 0.1 dB) using simultaneous transmission and reception of horizontally and vertically polarized waves, the isolation between coaxial cross-polar and copolar beams needs to be in excess of 50 dB. This presents a formidable challenge, because sufficient isolation cannot be achieved at an affordable price by the antenna hardware alone. Hence, additional approaches are required to reduce the differential reflectivity bias due to cross-polar fields. One proposed technique is a 180° pulse-to-pulse phase change of signals injected in either the horizontal or vertical ports of the transmission elements. This approach mitigates the differential reflectivity bias but may increase the bias of the copolar correlation coefficient estimates and, in some cases, the bias of the differential phase estimates. Herein, a modified phase code is proposed, which maintains the differential reflectivity bias suppression but reduces the copolar correlation coefficient and differential phase bias with respect to the originally proposed phase code. [ABSTRACT FROM PUBLISHER]

We propose a polarization-sensitive directional borehole radar with a dipole array and loop antenna in a borehole (DALAB), along with an algorithm for radar signal processing. This radar enables us to discriminate between a vertical conducting cylinder and inhomogeneity in soil in 3-D space. The antenna configuration difference, antenna coupling, and borehole effects were calibrated using the method of moments (MoM) in the proposed algorithm. According to computer simulation, the magnitude of the polarization ratio (MPR) of the incident wave may be used for discrimination between signals from targets and background reflections. A prototype of the borehole radar with the DALAB was constructed and tested in laboratory and field experiments. In the laboratory, we confirmed that the DALAB can estimate the polarization state of an incident wave despite the presence of a water layer. In field testing, we confirmed that the MPR was nearly 0 dB, as theoretically estimated when the DALAB received electromagnetic waves radiated by a tilted dipole antenna. We also carried out single-hole measurements with the DALAB and found that the MPR of a reflected wave from a vertical conducting cylinder was more than 20 dB, whereas that from soil inhomogeneity was less than about 20 dB. This implies that we may discriminate between a conducting cylinder and soil inhomogeneity and determine target locations in three dimensions using the DALAB and the MPR. [ABSTRACT FROM PUBLISHER]

In this contribution, some possible effects of large band electromagnetic interferences on Ground Penetrating Radar (GPR) data are shown, and a possible way to counteract them is shown, too. The mitigation of the interferences is implemented thanks to a prototypal reconfigurable stepped frequency GPR system, that allows to program the integration time of the harmonic tones vs. the frequency. In particular, an algorithm for the measurement of the effects of the interferences in the field (linked to the signal to interference ratio) is proposed and tested vs. experimental data. The paper will show some advantages and some drawbacks of the proposed procedure. [ABSTRACT FROM AUTHOR]

Based on the impulse radar and the spread-spectrum (SS) radar, the through-wall imaging with three stationary targets or a moving target in a room are obtained, respectively, and compared in this paper. We then analyze the influences of different analog-to-digital converters (ADCs) and noise levels on imaging quality. Results show that the SS radar has a great advantage over the impulse radar in some ways, such as covert waveform, engineering realization, excellent anti-noise performance, and accurate tracking. Furthermore, the target localization accuracy is much easier to be affected by sampling frequencies of ADC. Besides, the stationary targets can still be recognized accurately in the image, even when one target is very close to the wall, and the receiving signals of the SS radar are added with 0-dB noise and sampled by a 4-bit 3-GHz ADC. [ABSTRACT FROM PUBLISHER]

One of the main challenges to the use of phased array radar for weather observations is the implementation of dual polarization with acceptable levels of cross-polar fields induced by the antenna. For example, to achieve acceptable differential reflectivity (Z_\mathrmDR) bias (i.e., less than 0.1 dB) using simultaneous transmission and reception of horizontally and vertically polarized waves, the isolation between coaxial cross-polar and copolar beams needs to be in excess of 50 dB. This presents a formidable challenge because sufficient isolation cannot be achieved at an affordable price by antenna hardware alone. Hence, additional approaches are required to reduce Z_\mathrmDR bias due to cross-polar fields. One such approach is pulse-to-pulse phase coding of the transmitted waves. Herein, this approach is evaluated using theoretical analysis and simulated and time series data from a weather radar. The main criteria for evaluation are the bias and standard deviation of Z_\mathrmDR estimates. The results indicate that phase coding has the capability to significantly decrease Z_\mathrmDR bias without a substantial increase in the standard deviation of estimates. [ABSTRACT FROM AUTHOR]

Conventional inverse synthetic aperture radar observes the target from one viewing direction and only obtains partial information. By using a distributed multiple-input multiple-output (MIMO) array, the target can be observed from multiple views and a better image can be obtained. The distributed MIMO radar imaging signal model is derived in this paper. Because the strong scatterers (scattering centers) of a target are usually sparsely distributed, a sparse signal recovery algorithm using homotopy between the ℓ1 norm and ℓ0 norm applicable to complex signals is proposed to recover the strong scatterers. This ℓ1 norm and ℓ0 norm homotopy method is then extended to the block sparse signal case. The equivalence between complex ℓ1 ℓ0 norms homotopy method and block size of two real ℓ1 ℓ0 norms homotopy method is demonstrated. This proves that a complex signal-based algorithm is better than a real signal-based algorithm, which only separates the complex signal into its real part and imagery part and does not use their block property. This method is compared with other methods, i.e., block orthogonal matching pursuit (BOMP), block CoSaMp, block smoothed ℓ0 norm based method (BSL0), spectral projected gradient (SPG L1), and block sparse Bayesian learning (BSBL). The signal recovery performance of ℓ1 ℓ0 norms homotopy method is better than the other methods except for BSBL. However, BSBL costs more computationally. Imaging using distributed MIMO antennas is simulated. The simulation results show that the image quality using distributed MIMO radar is better than that using monostatic (or bistatic) MIMO radar. [ABSTRACT FROM AUTHOR]

In a passive radar system, the bearing of the target is difficult to measure for the signal‐to‐noise ratio of the target's echo signal is generally very low in practise. A novel method to estimate the echo signal's angle is proposed. The cross‐ambiguity function of direct‐path signal and each antenna's receiving signal is calculated to form a virtual array signal, in which the desired echo signal can be enhanced at a certain time delay and Doppler shift, so it is able to estimate its angle. We make the theoretical analysis about how the desired echo signal is enhanced. Some experimental results of an amplitude modulation (AM) radio‐based passive radar are presented to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper addresses signal reconstruction for future multiple-input multiple-output synthetic aperture radars (SARs) equipped with a multichannel antenna to enable wide-area high-resolution imaging. To image large swaths without range ambiguities, these high-resolution wide-swath (HRWS) SAR systems use a low pulse repetition frequency (PRF). Such a PRF, however, causes the radar echoes received by each channel to be strongly aliased. By introducing new techniques, this paper extends the theory of multichannel signal processing for reconstructing the SAR signal from the aliased signals. The reconstruction performances of the proposed processing methods in terms of signal-to-noise ratio, resolution, point target ambiguity ratio, peak-to-sidelobe ratio, and signal-to-ambiguity-plus-noise ratio are investigated according to the PRF and compared with each other for an exemplary HRWS SAR system. [ABSTRACT FROM PUBLISHER]

Different from conventional phased-array, which provides only angle-dependent beampattern, frequency diverse array (FDA) employs a small frequency increment across the antenna elements and results in a range-angle-dependent beampattern. This beampattern offers a potential to localize the targets in two dimensions in terms of slant ranges and azimuth angles. However, it is difficult to obtain the target location information from a standard FDA radar due to the couplings in range and angle responses. In this paper, we propose a transmit subaperturing scheme on the FDA radar for range and angle estimation of targets. The essence is to divide the FDA elements into multiple subarrays and optimize the transmit beamspace matrix with the use of convex optimization. We also discuss several practical issues for designing the FDA radar system parameters. Since the subarrays offer decoupled range and angle responses, the targets can be located using the beamspace-based multiple signal classification algorithm. The range and angle estimation performance is evaluated by comparing with the Cramér-Rao lower bound. [ABSTRACT FROM AUTHOR]

The problem of fast code acquisition in dual-band Galileo receivers is tackled. The proposed techniques exploit the Galileo E1 and E5 Open Service signals structure, implementing a cross-band aiding approach that yields mutual assistance in code synchronization adopting a master/slave configuration. The schemes are analyzed in terms of mean acquisition time (MAT) introducing the procedure flow-graph and overall mean acquisition time (OMAT). Numerical evaluation shows enhanced OMAT performance w.r.t. autonomous dual-band acquisition, guaranteeing complexity reduction. [ABSTRACT FROM AUTHOR]

In the case of through-the-wall localization of moving targets by ultra wideband (UWB) radars, there are applications in which handheld sensors equipped only with one transmitting and two receiving antennas are applied. Sometimes, the radar using such a small antenna array is not able to localize the target with the required accuracy. With a view to improve through-the-wall target localization, cooperative positioning based on a fusion of data retrieved from two independent radar systems can be used. In this paper, the novel method of the cooperative localization referred to as joining intersections of the ellipses is introduced. This method is based on a geometrical interpretation of target localization where the target position is estimated using a properly created cluster of the ellipse intersections representing potential positions of the target. The performance of the proposed method is compared with the direct calculation method and two alternative methods of cooperative localization using data obtained by measurements with the M-sequence UWB radars. The direct calculation method is applied for the target localization by particular radar systems. As alternative methods of cooperative localization, the arithmetic average of the target coordinates estimated by two single independent UWB radars and the Taylor series method is considered. [ABSTRACT FROM AUTHOR]

A fast-scanning phased array weather radar (PAWR) with a digital beam forming receiver is under development. It is important in beam forming for weather radar observation with temporally high resolution to form a stable and robust main lobe and adaptively suppress sidelobes with a small number of pulses in order to accurately estimate precipitation profiles (reflectivity, mean Doppler velocity, and spectral width). A minimum mean square error (MMSE) formulation with a power constraint, proposed in this paper, gives us adaptively formed beams that satisfy these demands. The MMSE beam-forming method is compared in various precipitation radar signal simulations with traditional beam-forming methods, Fourier and Capon methods, which have been applied in atmospheric research to observe distributed targets such as precipitation, and it is shown that the MMSE method is appropriate to this fast-scanning PAWR concept. [ABSTRACT FROM PUBLISHER]

The middle and upper atmospheric (MU) radar at Shigaraki, Japan, is an excellent system to carry out observation using a spaced antenna (SA) technique. Over several years, numerous scientific studies were reported using SAs. In most of the studies, a SA is used to derive wind information, and the array is divided in to three segments and is grouped with three or six channels to obtain the baselines in the form of an equilateral triangle. In previous work, the wind had been estimated by the SA up to a maximum height of 10-14 km. In general, a SA-based wind estimation could not give good height coverage due to a smaller aperture being used for received signals. In this paper, the observation has been conducted for full-array transmission and reception of signal with 25 receiver channels of the profiler. The receiver channels are divided into three equal segments to form the baselines suitable for deriving horizontal wind vector components using full correlation analysis (FCA). FCA is considered as a conventional SA technique. The identification of receive channels for grouping and its phase centers was critical to obtain the good correlation value and to determine the velocity. The grouping is repeated with four different orientations of the array group. In each case, the wind velocity is estimated. The horizontal wind vector components obtained from all orientations are averaged to obtain mean horizontal wind vector components. The results are compared with the wind velocity estimated using a Doppler beam swinging (DBS) technique and that observed by the GPS sonde. It is observed that the new approach adopted using spatially distributed array grouping has yielded higher height coverage up to 20 km and in good agreement with the results obtained using DBS and GPS-sonde observations with a temporal resolution of 1.3 min in clear air condition. [ABSTRACT FROM PUBLISHER]

Probing signal waveforms play a central role in the signal processing performance of a multiple-input multiple-output (MIMO) radar. In practice, for a given desired beam pattern, we need to design a probing signal waveform whose beam pattern closely matches the desired one and whose autocorrelation and cross-correlation sidelobes are kept low. The latter properties are important to mitigate undesirable interference caused by multiple targets or scatterers. In this correspondence, we present an efficient optimization method to design a constant modulus probing signal which can synthesize a desired beam pattern while maximally suppressing both the autocorrelation and cross-correlation sidelobes at/between given spacial angles. We formulate this problem as an unconstrained minimization of a fourth order trigonometric polynomial and propose an efficient quasi-Newton iterative algorithm to solve it. Besides, we provide an analysis of the local minima of the fourth-order trigonometric polynomial and prove that any local minima is a 1/2-approximation of its global optimal solution. Numerical examples show that the proposed approach compares favorably with the existing approach. [ABSTRACT FROM PUBLISHER]

When the National Weather Radar Testbed (NWRT) was installed in 2004, a single-channel digital receiver was implemented so that the radar could mimic typical Weather Surveillance Radar (WSR) version 1988 Doppler (WSR-88D) capability. This, however, left unused eight other channels, built into the antenna. This paper describes the hardware instrumentation of a recently completed project that digitizes the radar signals produced by these channels. The NWRT is the nation's first phased array devoted to weather observations, and this testbed serves as an evaluation platform to test new hardware and signal processing concepts. The multichannel digital data will foster a new generation of adaptive/fast scanning techniques and space-antenna/interferometry measurements, which will then be used for improved weather forecasting via data assimilation. The multichannel receiver collects signals from the sum, azimuth-difference, elevation-difference, and five broad-beamed auxiliary channels. One of the major advantages of the NWRT is the capability to adaptively scan weather phenomena at a higher temporal resolution than is possible with the WSR-88D. Access to the auxiliary channels will enable clutter mitigation and advanced array processing for higher data quality with shorter dwell times. Potential benefits of higher quality and higher resolution data include: better understanding of storm dynamics and convective initiation; better detection of small-scale phenomena, including tornadoes and microbursts; and crossbeam wind, shear, and turbulence estimates. These items have the distinct possibility to ultimately render increased lead time for warnings and improved weather prediction. Finally, samples of recently collected data are presented in the results section of this paper. [ABSTRACT FROM PUBLISHER]

Conventional quadrature-polarimetric (quad-pol) synthetic aperture radar (SAR) systems operating from space are severely constrained by their limited range of useful incident angles and their reduced swath widths particularly at larger incidence. These limitations are due primarily to relatively severe range ambiguities in the cross-polarized measurement channels. The conventional approach for quad-pol SAR systems uses linear polarizations on both transmit and receive. Range ambiguities can be markedly reduced by adopting hybrid-polarimetric architecture. In this approach, the radar transmits circularly polarized waves but receives on orthogonal linear polarizations. The sense of the circular polarization-left or right-is reversed on alternate transmissions. Hybrid-polarimetric quad-pol architecture leads to hardware that is more readily calibrated because neither receive channel is cross polarized with respect to the transmitted polarization; hence, their mean signal levels are the same. The data provided by a hybrid-polarimetric quad-pol SAR may be transformed into the conventional linearly polarized scattering matrix, thus preserving compatibility with the rich heritage of analysis tools developed for such radars. [ABSTRACT FROM AUTHOR]

We consider the effect of imperfect separability in the received signals on the detection performance of multi-input multi-output (MIMO) radar with widely separated antennas. The mutual orthogonality among the received signals is often assumed but cannot be achieved in practice for all Doppler and delay pairs. We introduce a data model considering the correlation among the data from different transmitter–receiver pairs as unknown parameters. Based on the expectation maximization algorithm, we propose a method to estimate the target, correlation, and noise parameters. We then use the estimates of these parameters to develop a statistical decision test. Employing the asymptotic statistical characteristics and the numerical performance of the test, we analyze the sensitivity of the MIMO radar with respect to changes in the cross-correlation levels of the measurements. We demonstrate the effect of the increase in the correlation among the received signals from different transmitters on the detection performance. [ABSTRACT FROM AUTHOR]

Antenna effects on a monostatic multiple-input-multiple-output (MIMO) Radar for direction estimation are studied by analyzing the Cramèr-Rao lower bound (CRLB). The CRLB is derived for a multi-band MIMO Radar system, and is in a form that incorporates the characteristics of a practical antenna array. Two different uniform linear antenna arrays, one narrowband and the another wideband, are investigated by exploring the CRLB. The estimation performance for the real antenna array is compared to that for an ideal array composed of isotropic antenna elements. It is found out that the mutual coupling between antenna elements alters the radiation patterns a lot, and therefore tremendously affects the estimation performance. Furthermore, the study demonstrates that by distributing the transmitted power over several frequency bands, more accurate estimation can be achieved due to frequency diversity, and the negative effects of radiation pattern deviation can be alleviated. [ABSTRACT FROM PUBLISHER]

Snow avalanche victims have only a good chance to survive when they are located within a short time. This requires an active beacon for them to wear or a very rapid deployment of a search-and-rescue team with dogs. Customary ground-penetrating radar (GPR) instruments used on the snow surface are not able to reduce fatality numbers because they are slow to search a field. A potential alternative could be an airborne search using radar. An airborne radar search is technologically challenging because a very large data stream has to be processed and visualized in real time, and the interaction of the electromagnetic waves with snow, subsurface, and objects must be understood. We studied a two-step algorithm to locate avalanche victims in real time. The algorithm was validated using realistic test arrangements and conditions using an aerial tramway. The distance dependence the reflection energy with increased flight heights, the coherence between the use of more antennas and the detectable range, and the reflection images of different avalanche victims were measured. The algorithm detected an object for each investigated case, where the reflection energy of the scans was higher than for the scans of pure snow. Airborne GPR has a large potential become a rapid search method in dry snow avalanches. However, a fully operational version still requires substantial improvements in hardware and software. [ABSTRACT FROM AUTHOR]

Recently, rapid advances in radio detection and ranging (radar) technology applications have been implemented in various fields. In particular, micro-Doppler radar has been widely developed to perform certain tasks, such as detection of buried victims in natural disaster, drone system detection, and classification of humans and animals. Further, micro-Doppler radar can also be implemented in medical applications for remote monitoring and examination. This paper proposes a human respiration rate detection system using micro-Doppler radar with quadrature architecture in the industrial, scientific, and medical (ISM) frequency of 5.8 GHz. We use a mathematical model of human breathing to further explore any insights into signal processes in the radar. The experimental system is designed using the USRP B200 mini-module as the main component of the radar and the Vivaldi antennas working at 5.8 GHz. The radar system is integrated directly with the GNU Radio Companion software as the processing part. Using a frequency of 5.8 GHz and USRP output power of 0.33 mW, our proposed method was able to detect the respiration rate at a distance of 2 m or less with acceptable error. In addition, the radar system could differentiate different frequency rates for different targets, demonstrating that it is highly sensitive. We also emphasize that the designed radar system can be used as a portable device which offers flexibility to be used anytime and anywhere. [ABSTRACT FROM AUTHOR]

It has been recently shown that multiple-input multiple-output (MIMO) antenna systems have the potential to dramatically improve the performance of communication systems over single antenna systems. Unlike beamforming, which presumes a high correlation between signals either transmitted or received by an array, the MIMO concept exploits the independence between signals at the array elements. In conventional radar, the target's radar cross section (RCS) fluctuations are regarded as a nuisance parameter that degrades radar performance. The novelty of MIMO radar is that it provides measures to overcome those degradations or even utilizes the RCS fluctuations for new applications. This paper explores how transmit diversity can improve the direction finding performance of a radar utilizing an antenna array at the receiver. To harness diversity, the transmit antennas have to be widely separated, while for direction finding, the receive antennas have to be closely spaced. The analysis is carried out by evaluating several Cramer-Rao bounds for bearing estimation and the mean square error of the maximum likelihood estimate [ABSTRACT FROM PUBLISHER]

In this paper, we present a novel method for natural frequency extraction. Our algorithm is called late-time evolutionary programming (EP)-based CLEAN, and has many advantages compared to conventional methods. The accuracy of our algorithm is not affected by the false estimation of the number of natural resonance modes. Furthermore, our method is insensitive to random noise. Insensitivity is a very important characteristic in the resonance extraction algorithm since the late-time response usually has small energy. Using synthetic data, we show these characteristics by comparing them to Prony's method and the E-pulse technique. We also applied our method to the numerical data and B-52 measured data which is obtained at Michigan State University (MSU) arch range. [ABSTRACT FROM AUTHOR]

A novel passive wireless temperature sensor is presented, which aims at operating in harsh environments. For this purpose, the tag is designed quasi‐chipless and it uses a harmonic radar sensing principle: it combines the backscattering on the first harmonic of the transmitted frequency with frequency‐position encoding of the measured value. The harmonic radar approach brings significant advantages for the wireless readout, since it isolates the undesired radar clutter from the useful tag signal. The tag signal carries the desired temperature information encoded in the position of a resonance transmission peak. The tag is built up planar and consists of two patch antennas, a temperature‐dependent bandpass filter and a diode‐based harmonic generator. It works at 1.59 GHz and backscatters at 3.18 GHz. Wireless indoor measurements have proven the concept. The sensor has shown a temperature sensitivity of 1.2 MHz/K in the range of 22–109 °C. [ABSTRACT FROM AUTHOR]

Forward‐looking radars are essential sensors for many fast moving platforms such as fighter aircraft and missiles. Despite its multiple benefits, noise radar has not yet been used for this task because of the low Doppler tolerance of its waveform and the associated need for computational intense Doppler compensation. This Letter presents the very first experimental results of a continuous‐emission noise radar whose antennas are oriented along the direction of proper motion of a car operating as a radar platform at a significant speed. To the best of the authors' knowledge, no experiments of this kind have been published to date. Very likely, the results presented here are the first ones showing moving target detection by a noise radar on a fast moving platform. [ABSTRACT FROM AUTHOR]

We present a new contribution to the single-mode extraction pulse (S-pulse) technique. The S-pulse technique uses the natural frequencies of the target as discrimination features in such a way that the field scattered by a target after it passes through a S-pulse designed for this specific target has the form of a damped sinusoid. The S-pulses are usually constructed as a linear combination of cosine or polynomial basis functions, expressing a S-pulse function as a linear combination of these functions and fixing the linear parameters with conditions on the Laplace transform. The S-pulses are synthesized using complex exponential basis functions within which are extra parameters that can be used to tune their discrimination capabilities. The proposed S-pulses are compared with the ones in the literature using thin conductive wires as simulation targets and in this way we show that there is a distinct improvement in their discriminatory capacity. [ABSTRACT FROM PUBLISHER]

• Laboratory tests and computational models of rigid pavement with an asphalt layer. • Orientation of the antenna determines the amplitude. • 2 mm cracks detected in laboratory with a 1.6 GHz antenna but with no clear images. • 5 mm cracks are clearly detected regardless of the antenna orientation. • Antenna oriented perpendicularly to the crack produces higher amplitude hyperbolas. Cracks in road pavements are usual damages because of heavy and dense traffic. Cracks usually are hidden to visual inspections in the case of rigid pavements. In these cases, cracked cement concrete layers are located below the asphalt layer. Therefore, detection becomes a difficult task, being only possible in the case of major damage or even failure. As consequence, rehabilitation is complex due to the technical difficulty in finding those cracks. Non-destructive surveys are in many cases applied to detect those damages. However, the difficulty in data interpretation limits their use forcing the combination of different technologies and core samples. One of the most useful method, commonly applied to define the thickness of layers, is Ground Penetrating Radar (GPR). In order to understand better the GPR images obtained as consequence of damage in the case of rigid pavement, several tests were designed to determine the ability of GPR to non-destructively detect and quantify cracks in concrete samples covered by an asphalt layer. The laboratory experiment consisted of three testing configurations: (1) one concrete sample without cracks; (2) a model of a rigid pavement with the asphalt layer without cracks; (3) the same model with different crack widths. The test was performed with a 1.6 GHz centre frequency GPR antenna in two relative positions with respect the cracks, being always the radar line perpendicular to the crack axis. These measurements allowed also evaluate the effect of orientation of the crack with respect the antenna dipoles in the GPR images. In addition, numerical models were compared to the laboratory results. The results showed that the crack width was detected in most cases, even the thinnest ones (2 mm), being the higher amplitude anomaly corresponding to the case of the data acquired with the antenna parallel to the radar line. [ABSTRACT FROM AUTHOR]

In this paper, we set up a lightweight, frequency-domain radar system using a handheld vector network analyser combined with air-coupled and dielectric-coupled horn antennas. A new positioning system with a binary barcode ruler, reading webcam and an algorithm for its recognition was developed. The radar system was tested on several media using both the air-coupled and dielectric-coupled antennas, namely, on a laboratory model containing sand and on a concrete block with a metallic reinforcement. The resulting GPR images demonstrated the good functioning of the radar system. In our experiments, both antennas and the whole lightweight radar system provided radar images which permitted to clearly detect the buried objects. Furthermore, the dielectric-coupled horn antenna showed better coupling with the investigated materials, resulting in higher-quality radar images. • Dielectric-coupled TEM horn antenna • Lightweight radar system • Numerical, laboratory and real radar data • Remote positioning system [ABSTRACT FROM AUTHOR]