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The utilization of polarization information is a hot topic in the bistatic radar field. Bistatic full-polarization scattering dataset and polarization decomposition theories are urgently needed. We have established an indoor bistatic full-polarization scattering measurement system to investigate the target polarization scattering characteristics (PSCs), and a dataset is obtained. Due to its excellent performance, bistatic Huynen target parameters (BHTPs) are selected as the PSCs analysis tool. Then, a new BHTP extraction algorithm is presented, which directly extracts BHTPs from the measured Sinclair matrix. It is a state-of-the-art algorithm for BHTP extraction because of its easy implementation, robustness, and clear physical meanings. Experiment results demonstrate that the proposed algorithm can extract BHTPs accurately. [ABSTRACT FROM AUTHOR]

Asan important means in low-altitude airspace surveillance, passive radar has been widely studied. With the increasing complexity of the low-altitude environment, the urgent need for radar's capability of automatic target recognition (ATR) emerges. In the field of ATR for passive radar, the method based on radar cross section (RCS) is an important direction for the simplicity of feature extraction. In this article, we exploit the compensated quasi-echo-power (CQEP), i.e., quasi-RCS, to realize ATR in passive radar. We propose an ATR method on the basis of the angular diversity. First, we divide the angular space, which consists of four components of the incident angle and scattering angle of electromagnetic wave, into multiple subspaces. Then, we build a subrecognizer in each angular subspace according to the CQEP distribution characteristics. Each receiving station of the multistatic passive radar recognizes targets in an angular subspace corresponding to its observation and outputs a preliminary decision. Based on the preliminary decisions of all receiving stations, a voting strategy is adopted to make the final decision on the unknown target class in the fusion center. Experimental results based on real-life target data show that the proposed method has a significant improvement on the average correct recognition rate with fusion of three receiving stations, which validates the proposed method. [ABSTRACT FROM AUTHOR]

Passive coherent location (PCL) systems using commercial FM radio broadcast stations as illuminators of opportunity (FM-PCL) require an antenna system with a null in its radiation pattern for suppressing the direct path interference (DPI) from the selected FM broadcast station. To maximize the surveillance coverage of the FM-PCL, ideally, the antenna should have a cardioid radiation pattern, which can be obtained by the ideal couplet of isotropic radiators. We carry out a theoretical and experimental analysis to show that such a cardioid pattern cannot be realized by a couplet of cylindrical wire antennas. Therefore, we introduce an alternative array configuration and at the same time seek a simple and low-cost antenna solution for FM-PCL. The proposed antenna is a two-element array of UWB wire bowties, which has an ultrawide bandwidth, a stable main beam, a large beamwidth, and a fixed array setup within the FM-PCL frequency range (88–108 MHz), leading to a simple antenna implementation. At the same time, as it does not require expensive RF and mechanical parts, a low-cost antenna solution for FM-PCL is obtained. Hence, the proposed antenna satisfactorily meets the goal of this work, i.e., simple and low-cost antenna solution for the FM-PCL passive radar. [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 global navigation satellite system (GNSS)-based passive radar shows potential for permanent maritime surveillance. In this article, the GNSS signals are exploited for vessel target imaging. From the obtained radar image, meaningful information about the vessel, such as its shape, position, length, and orientation can be extracted. In addition, the vessel is observed from different angles by spatially diverse GNSS satellites, and the multistatic geometry enables to enhance the imagery quality. The main drawback of GNSS-based passive radar stays in its limited power budget. And the inaccessible motion makes the noncooperative vessel smeared using conventional radar imaging methods. To address the problems, at first, each bistatic echo over a long observation time is integrated with the range and Doppler (RD) domain after removing the 2-D migrations. The signal-to-noise ratio (SNR) can be increased after the step. Then, with respect to a particular target velocity, the local Cartesian plane is constructed, and the multiple RD maps are projected and combined in the plane to obtain the multistatic image. In view of the inaccessibility of target kinematic parameters, such imaging processing is modeled as an optimization problem, where vessel’s velocity is set as a decision variable and the aim is to minimize the image entropy. Finally, the particle swarm optimization (PSO) algorithm is applied to solve the optimization problem, after which a well-focused vessel image can be obtained. In May 2021, we have successfully carried out the world’s first BeiDou-based passive radar maritime experiment, and the effectiveness of the proposed method is verified against the experimental data. [ABSTRACT FROM AUTHOR]

Active reconfigurable intelligent surfaces (RISs) are a novel and promising technology that allows controlling the radio propagation environment while compensating for the product path loss along the RIS-assisted path. In this letter, we consider the classical radar detection problem and propose to use an active RIS to get a second independent look at a prospective target illuminated by the radar transmitter. At the design stage, we select the power emitted by the radar, the number of RIS elements, and their amplification factor in order to maximize the detection probability for a fixed probability of false alarm and a common (among radar and RIS) power budget. An illustrative example is provided to assess the achievable detection performance, also in comparison with that of a radar operating alone or with the help of a passive RIS. [ABSTRACT FROM AUTHOR]

With the development of the JUpiter ICy moons Explorer (JUICE)/Radar for Icy Moons Exploration (RIME) [European Space Agency (ESA)] and Europa Clipper/Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) [National Aeronautics and Space Administration (NASA)] instruments, designed to study the subsurface of the Galilean moons, interest has been growing to study the performances of sounding radar orbiting these bodies. In the presence of strong Jupiter’s radio emissions, probing in a passive mode using these emissions as the emitter is considered. However, radar performances in this bistatic mode are dependent on the entire geometry of observation: the position of the source of emission, the spacecraft trajectory, and on the region probed. The 3-D Simulations are necessary to estimate the performances of these measurements. We analyze the influence of the geometry of observation by approximating Jovian radio bursts as radio impulses, simulating the signal scattered by a point target in a realistic 3-D geometry and computing the resolution. This allows a preliminary identification of scenarios of observation best suited for this radar mode. The influence of the correct localization of Jupiter’s emissions on the synthetic aperture radar (SAR) image is investigated as well, and interest in recovering the true position of Jupiter’s source is highlighted. [ABSTRACT FROM AUTHOR]

Global navigation satellite system (GNSS)-based passive bistatic radar (PBR) has many advantages benefiting from the bistatic configuration and the excellent properties of the GNSS signals. In this article, the possibility of moving target indication (MTI) using a GNSS-based PBR is analyzed and illustrated by experiments. The power budget is first evaluated to investigate the capabilities and limitations of the GNSS-based PBR. Due to the low power density of GNSS signals, long-time integration is required to achieve long-range surveillance. In our previous work, a Radon Fourier transform (RFT)-based long-time integration algorithm has been proposed for GNSS-based PBR. However, the RFT-based methods are computationally inefficient. In this article, a novel MTI algorithm based on high frame rate image sequence (HFRIS) is introduced to the GNSS-based PBR. Benefiting from the avoiding of the iterative multidimensional parameter searching, it has a higher computational efficiency than the RFT-based methods. At the same time, the possibility of multistatic positioning for moving targets using the GNSS-based PBR is analyzed. It is shown that the absolute position of the target in three dimensions could be determined if at least three satellites are utilized to measure the range delay of the target return. To confirm our analysis, experiments are conducted which detects the airplanes using the GPS signals as illuminators of opportunity. The collected data are processed by both the RFT-based method and the HFRIS-based method. Both methods have successfully detected the airplane target at a distance of about 5 km and the HFRIS-based method shows a far better performance in terms of processing efficiency. The multistatic positioning experiment is also conducted and the estimated position of the target is well consistent with the values acquired by the flight record, which shows the potential of the GNSS-based PBR on multistatic operations. [ABSTRACT FROM AUTHOR]

In a passive radar, when the reference antenna is set to receive a direct-path signal, target signals will be inevitably mixed in the reference channel due to the antenna pattern. In this case, adaptive clutter cancellation in the temporal domain will affect the target steering vector due to the correlation between the target signal in the reference and surveillance channels, which results in a direction of arrival (DOA) estimation bias. The spatial-domain adaptive clutter rejection will also cause a similar influence. To solve this problem, in this article, we derive and analyze the effect of the adaptive clutter rejection on the target steering vector. Two kinds of DOA estimation methods are hence proposed to correct the estimation bias. The applicable boundaries of these two methods are also discussed. Simulations and real radar data show that the proposed methods can correct the DOA bias caused by adaptive clutter rejection, thereby improving the location accuracy in a passive radar. [ABSTRACT FROM AUTHOR]

This article presents the pioneering results of successful target detection using a fully operational and cooperative 5G cellular network as a source of illumination in a passive radar system. The electromagnetic wave generated by the real-life base station allowed for remote sensing of a moving car in bistatic geometry, which shows the feasibility of passive radar in which new generation radio networks serve as an illuminator of opportunity. This article describes passive radar principles, the theoretical fundamentals of the 5G signal structure, and the proposed processing for 5G network-based passive radar. Also, the 5G passive radar demonstrator is described, and the trial results show the possibilities and limitations of using new radio standards in the context of passive radars. [ABSTRACT FROM AUTHOR]

This article puts forward a BeiDou-based passive multistatic radar (PMR) maritime moving target (MMT) detection technique via space–time hybrid integration (STHI) processing. Compared with passive bistatic radar (PBR), the utilization of multiple satellites provides an improvement in MMT detection performance, together with the capabilities of localization and velocity estimation. However, the multiple satellite transmitters cause the differences principally in bistatic range and Doppler centroid (DC) of the MMT. To integrate the PMR echoes, the biggest challenge is the two differences that need to be handled. In the proposed technique, first, the centroid-compensated keystone transform (CCKT) is proposed and applied to each PBR echo. It not only corrects range cell migration (RCM) but also equalizes the DC to the same. Then, the long-time integration is performed on each PBR echo, after which it is integrated into the range-Doppler frequency rate (DFR) domain. Finally, in order to settle the difference in bistatic range, an MMT position and velocity domain (i.e., the $X$ – $Y$ – $V$ domain) is constituted. The obtained multiple range-DFR maps are projected to the $X$ – $Y$ – $V$ domain, and then, the effective integration of multistatic echoes can be implemented. The final STHI result allows detecting the MMT reliably. Meanwhile, according to the 3-D position where the MMT is located in the $X$ – $Y$ – $V$ domain, the MMT can be localized, and its velocity can be estimated simultaneously. In May 2021, we have successfully carried out the world’s first BeiDou-based PMR MMT detection experiment, and the experimental results are given to prove the effectiveness of this technique. [ABSTRACT FROM AUTHOR]

Owing to the continuous random and nonperiodic characteristics of the passive signal, the random range sidelobe coupling (RRSC) effects of the strong clutter (i.e., the direct path signal and the strong multipath signal) will exacerbate the performance of space-time adaptive processing (STAP) algorithm in airborne passive radar (APR). This article analyzes the influence of RRSC caused by strong clutter on detection range cell of interest and illustrates that strong clutter should be suppressed before applying the STAP algorithm. A cascaded least square (CLS) algorithm based on the directional dependence of clutter Doppler frequency is proposed for side-looking APR, where the first least square (LS) algorithm is performed to achieve the amplitude estimation of the Doppler-shift echo signal, and the channel amplitude and phase error correction factor can be obtained by the estimated amplitude information of the direct patch signal and the APR geometric configuration. The second LS is employed to further estimate the amplitude information of the strong clutter, and the resulting weight vector can be utilized for clutter cancelation. To make the CLS algorithm more practical, a batch version of the CLS (CLS-B) algorithm is proposed in this article. The advantage of the method is that it cannot only cancel clutter along the clutter ridge but also effectively prevent the suppression of targets falling in the clutter Doppler band. In addition, the proposed CLS algorithm performs well in the presence of the channel amplitude and phase error. A series of simulations are conducted to test the proposed algorithms. [ABSTRACT FROM AUTHOR]

Passive operation is an important operational concept for future airborne radar platforms. Successful target detection requires an understanding of the underlying clutter statistics in order to set the detection threshold accurately. In addition, clutter statistics are important for modeling of passive radar performance and simulation of passive bistatic radar to aid in the development of new target detection algorithms. In this article, the clutter statistics of data from an airborne platform are studied over a range of bistatic geometries and receive polarizations. The illuminator is a digital video broadcast-terrestrial (DVB-T) station and the collected data spans both ground and maritime regions. The key contributions include a study of amplitude statistics using a number of common distributions and understanding the impact of the reference signal quality in terms of the clutter statistics. [ABSTRACT FROM AUTHOR]

Bistatic and multistatic radar observations are mostly conceived to exploit the signal phase, e.g., for interferometry, tomography, and ocean current applications. In these situations, the distance between the transmitting and receiving antennas (bistatic baseline) is generally small in order to, among other reasons, keep high the coherence between the backscattered (monostatic) and bistatic radar echoes. Less evidence can be found in the literature on the exploitation of the signal amplitude (i.e., the scattering coefficient) of monostatic observations combined with bistatic ones, thus implementing a multistatic observation. In this case, to increase the information content in the inverse problem (i.e., the retrieval of biogeophysical parameters), the observations to be combined must be sufficiently independent in order to improve the accuracy of the retrieval. This article aims at identifying suitable geometrical configurations of a passive satellite radar flying in convoy with an active spaceborne SAR at L-band. Few bistatic datasets exist to verify this concept experimentally, so that electromagnetic models can help understanding its potential in different fields. The applications foreseen in this article are the retrieval of soil moisture and vegetation biomass. A model-based investigation shows that retrieval performances can be improved by combining monostatic and bistatic measurements in geometric configurations requiring very large along track and across track baselines. In this way, the scattering mechanisms involved in the monostatic and bistatic geometry are sufficiently different so that their combination increases the retrieval performances with respect to a monostatic acquisition. [ABSTRACT FROM AUTHOR]

This article deals with the problem of detection performance prediction for passive bistatic radars that use illuminators of opportunity, such as radio or television transmitters. First, a simplistic free-space propagation model is introduced. Next, the effects of the transmitter and receiver antenna patterns are analyzed. Then, the influence of the pattern-propagation factor is investigated, considering the elevation patterns, multipath, and diffraction. The performance prediction of the free-space model and a more realistic model with antenna patterns and propagation effects are compared. The variation of the echo power level caused by the propagation effects predicted in the simulations is verified with real data acquired with an FM-based passive radar. The presented approach allows for a more realistic prediction of the passive radar performance to be achieved in comparison with a simple free-space propagation assumption. Some examples of passive radar performance optimization using the introduced methodology are presented. [ABSTRACT FROM AUTHOR]

Intelligent reflecting surface (IRS) is a promising technology being considered for future wireless communications due to its ability to control signal propagation. This paper considers the joint active and passive beamforming problem for an IRS-assisted radar, where multiple IRSs are employed to assist the surveillance of multiple targets in cluttered environments. Specifically, we aim to maximize the minimum target illumination power at multiple target locations by jointly optimizing the active beamformer at the radar transmitter and the passive phase-shift matrices at the IRSs, subject to an upperbound on the clutter power at each clutter scatterer. The resulting optimization problem is nonconvex and solved with a sequential optimization procedure along with semidefinite relaxation (SDR). Simulation results show that additional line-of-sight (LOS) paths created by IRSs can substantially improve the radar robustness against target blockage. [ABSTRACT FROM AUTHOR]

Passive radar signal processing is hindered by the need to sample a reference signal from the chosen illuminator, which is subject to sensor noise and interference from clutter returns. The reference signal may be demodulated and reconstructed to remove artifacts. However, without accounting for the transmitter's clock offset, the remodulated reference signal will be a poor match for the surveillance signal. This article evaluates clock offset estimation methods in terms of the radar ambiguity surface, thereby appraising them specifically for passive radar applications. Existing offset estimation methods are shown to meet variance bounds; however, these bounds worsen with increased clock offset. Ambiguity surfaces constructed from signals reconstructed with poor clock offsets exhibit power leakage. A new clock offset estimator is proposed, which refines coarse estimates made by an existing method. The new estimator is demonstrated to achieve accurate estimation of clock offset. Reference signals reconstructed with these estimates produce ambiguity surfaces with no significant power leakage. [ABSTRACT FROM AUTHOR]

In this article, we review and compare different methods for the localization of an illuminator of opportunity for passive radar systems employing cooperative targets. For assessment and comparison, we use numerical simulations and real-world radar measurements. To achieve reliable operation under real-world conditions, a correct and robust association of the bistatic radar target observations with ground truth information is necessary. We introduce a novel solution to this issue with an efficient preprocessing algorithm based on a planar approximation. [ABSTRACT FROM AUTHOR]

A new algorithm for passive bistatic radar (PBR) localization is proposed via joint exploitation of multiple illuminators of opportunity and measurements gathered by a colocated active radar. Ad-hoc constraints within the localization process are bestowed accounting for both a priori information on the PBR receive antenna main-beam size and the uncertainty characterizing active radar data, i.e., the root mean square error (RMSE) of range/bearing measurements. Hence, the estimation task is cast as an elliptic positioning problem, according to the constrained least squares framework. The resulting non-convex optimization problem is globally solved providing a closed-form estimate in Cartesian coordinates. The performance of the proposed estimator, in terms of RMSE, exhibits sensible improvement with respect to counterparts. The proposed technique is also applied to the case of a dynamic scenario for a bi-sensor surveillance system, where the active rotating platform acquires measurements at each scanning period. The results show the localization performance improvement achieved when the system is complemented with a PBR staring in a specific search sector of interest. [ABSTRACT FROM AUTHOR]

This article addresses the problem of clutter cancelation for slowly moving target detection and localization in multichannel passive radar onboard mobile platforms. A post-Doppler space-time adaptive processing (STAP) approach is exploited in the case of an angle-dependent imbalance affecting the receiving channels. While the clutter suppression capability is ensured by the adaptivity of space-time filtering, different solutions are compared, aimed at recovering the detection performance losses associated with channel calibration errors. A space-time generalized-likelihood ratio test (GLRT) scheme is considered, where the steering vector is not specified in the spatial domain, resulting in a noncoherent integration of target echoes across the receiving channels. This is compared with a fully coherent GLRT scheme where echoes from the stationary scene are exploited for the proper calibration of spatial steering vector mismatch. The first scheme proves to be a simple solution for target detection in the passive radar case, offering comparable clutter cancelation capability. The second scheme, at the expense of an additional stage, offers slightly better detection performance and preserves target direction-of-arrival (DoA) estimation capability. Finally, the STAP scheme is employed for the maximum-likelihood estimation of target DoA, evaluating the role of the steering vector calibration against the negative impact of channel imbalance. The effectiveness of the proposed approaches is tested against both simulated and experimental data from a DVB-T-based mobile passive radar. [ABSTRACT FROM AUTHOR]

The detection and tracking performance of passive radar systems can be enhanced by exploiting multiple emitters and one receiver site by allowing for multiple bistatic geometries. This article investigates the measurement accuracy of passive radar systems that employ the advanced television system committee (ATSC 1.0) signal as a signal of opportunity. The monostatic and bistatic modified Cramér–Rao lower bound for the range and velocity measurements are derived as a quantitative way to evaluate the performance variation when exploiting multiple transmitters. The effect of 3D target position on the signal-to-noise ratio, with respect to stationary transmitters and receiver, is highlighted. Furthermore, signal-to-interference-and-noise ratio modeling is included by considering the direct signal interference suppression performance for a set of transmitters. An example in Columbus, Ohio, for a single receiver and multiple digital television transmitters that are spatially distributed is presented to find the optimal emitter that yields the best range and velocity measurement in the analyzed area. Target-tracking experiments show an agreement between the optimal emitter map and the tracking quality for the set of transmitters. [ABSTRACT FROM AUTHOR]

This article examines methods for and results of determining the instantaneous kinematic state of a target, i.e., its velocity and heading, using passive multistatic radar. This is achieved by combining bistatic target range and Doppler information obtained by multiple bistatic transmitter–receiver pairs, to estimate the appropriate target velocity vector. This estimation is provided as a set of closed-form equations, derived for a general case applicable to an arbitrary number of transmitters. The feasibility of the proposed approach and its accuracy predictions, as a function of the number of transmitters, are experimentally confirmed via a dedicated experimental campaign with Global Navigation Satellite Systems as transmitters of opportunity, where two different and relatively slow-moving maritime targets were in the field of view of up to 12 satellite transmitters belonging to different satellite constellations (GPS, GLONASS, and Galileo) simultaneously. [ABSTRACT FROM AUTHOR]

In this article, localization of a maritime target using a passive multistatic radar system, consisting of a single receiver and a number of satellites as its transmitters, is investigated. The set of communication satellites and also the global navigation satellite system (GNSS), including GPS, Glonass, and Galileo, are employed as illuminators of opportunity. In the receiver, the reflected signals from the target along with the direct signal from the satellites are employed to extract a set of bistatic range measurements. Leveraging these measurements and the satellite’s position information, a novel localization method considering zero height of maritime target and appropriate combination of the measurement signals is presented. The proposed method is efficient for localization of maritime targets in cases with arbitrary bistatic range accuracy. The results of numerical simulations confirm the efficiency of the proposed method and its superior performance in comparison with the state-of-the-art methods. [ABSTRACT FROM AUTHOR]

Recent studies have proposed that Jovian decametric radiation (DAM) can be effectively exploited for probing the subsurface of Jupiter’s icy moons by using passive radio sounding. However, these studies were based on the assumption that Jovian noise is white and stationary. Therefore, additional investigations into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding and improve the acquisitions planning. In this article, we investigate these properties of the Jovian DAM to understand their impact on radar sounding performance. This is done by analyzing the recently available radio spectra acquired by the JUNO Waves instrument. Results are also evaluated for the specific case of the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) that have been selected for European Space Agency (ESA) and NASA missions to Ganymede and Europa. Our results show that the Jovian DAM is not perfectly white, but no severe distortions in the range response should be expected. The results on spatiotemporal occurrence show that Jupiter’s DAM has a variable probability of occurrence, which is rather sporadic for some frequency ranges. The results on RIME and REASON flybys show that the Jovian DAM occurrence probability is relatively low for selected sub-Jovian flybys at Europa. For the RIME Ganymede orbital phase, a large number of high occurrence passive acquisition opportunities are expected. The experimental results also show that a large bandwidth receiver would enhance the chance of recording Jovian noise. [ABSTRACT FROM AUTHOR]

This article explores the possibility to exploit global navigation satellite systems (GNSS) signals to obtain radar imagery of ships. This is a new application area for the GNSS remote sensing, which adds to a rich line of research about the alternative utilization of navigation satellites for remote sensing purposes, which currently includes reflectometry, passive radar, and synthetic aperture radar (SAR) systems. In the field of short-range maritime surveillance, GNSS-based passive radar has already proven to detect and localize ship targets of interest. The possibility to obtain meaningful radar images of observed vessels would represent an additional benefit, opening the doors to noncooperative ship classification capability with this technology. To this purpose, a proper processing chain is here conceived and developed, able to achieve well-focused images of ships while maximizing their signal-to-background ratio. Moreover, the scaling factors needed to map the backscatter energy in the range and cross-range domain are also analytically derived, enabling the estimation of the length of the target. The effectiveness of the proposed approach at obtaining radar images of ship targets and extracting relevant features is confirmed via an experimental campaign, comprising multiple Galileo satellites and a commercial ferry undergoing different kinds of motion. [ABSTRACT FROM AUTHOR]

Human sensing using WiFi signal transmissions is attracting significant attention for future applications in e-healthcare, security, and the Internet of Things (IoT). The majority of WiFi sensing systems are based around processing of channel state information (CSI) data which originates from commodity WiFi access points (APs) that have been primed to transmit high data-rate signals with high repetition frequencies. However, in reality, WiFi APs do not transmit in such a continuous uninterrupted fashion, especially when there are no users on the communication network. To this end, we have developed a passive WiFi radar system for human sensing which exploits WiFi signals irrespective of whether the WiFi AP is transmitting continuous high data-rate Orthogonal Frequency-Division Multiplexing (OFDM) signals, or periodic WiFi beacon signals while in an idle status (no users on the WiFi network). In a data transmission phase, we employ the standard cross ambiguity function (CAF) processing to extract Doppler information relating to the target, while a modified version is used for lower data-rate signals. In addition, we investigate the utility of an external device that has been developed to stimulate idle WiFi APs to transmit usable signals without requiring any type of user authentication on the WiFi network. In this article, we present experimental data which verifies our proposed methods for using any type of signal transmission from a standalone WiFi device, and demonstrate the capability for human activity sensing. [ABSTRACT FROM AUTHOR]

Maximizing the coherent processing interval (CPI) is crucial when performing passive radar detection on weak signal reflections. In practice, however, the CPI is limited by the target movement. In this work, the extent of the range and Doppler migration effects occurring when using a long CPI to integrate the returns from an $L$ -band digital aeronautical communication system (LDACS) based passive radar is studied. In particular, our simulations underline the extensive Doppler migration effect that arises even for nonaccelerating targets. To this end, the Keystone transform and fractional Fourier transform techniques are combined with the standard passive radar processing to enable the compensation of both range and Doppler migration effects. This nonmodel-based approach is, however, shown to have limitations, in particular for low signal-to-noise ratios and/or multitarget scenarios. To address these shortcomings, a novel model-based framework that allows to perform joint target detection and parameter estimation is developed. For this, a super-resolution sparse Bayesian learning approach is employed. This technique uses a multitarget observation model, which accurately accounts for the underlying range and Doppler migration effects and provides super-resolution estimation capabilities. This is particularly advantageous in the LDACS case since the narrow bandwidth generally limits the separation of closely spaced targets. The simulation experiments demonstrate the effectiveness of the algorithm and the advantages it provides when compared to the standard migration compensation approach. [ABSTRACT FROM AUTHOR]

This work addresses the target-detection problem in a passive multistatic radar consisting of one transmitter and several spatially separated receivers (i.e., single-input multiple-output (SIMO) configuration) without exploiting the reference channel (RC). Several solutions for this problem are developed within the Rao, likelihood ratio test (LRT) and geometrical representation frameworks as well as under different assumptions on parameter space of this problem. Thus, a unified framework for multistatic passive radar target detection is created. To do this, we formulate three different target detection problems as composite hypothesis testing problems, where they differ based on different assumptions on the parameter space of the alternative hypothesis, resulting in four new detectors. We analytically prove that most of the proposed target detection methods posses constant false alarm rate (CFAR) behavior against the noise variance uncertainties across different receivers. Then, through Monte-Carlo simulations, we examine the detection performance of the proposed detectors to show the capabilities of the proposed detectors. [ABSTRACT FROM AUTHOR]

In some radar applications, neither the channel parameters nor the transmitted signal are known a-priori at the radar receiver. As such, both the transmitted signal as well as the channel parameters are random variables. Given the knowledge of distributions of the unknown parameters, we propose a maximum a-posteriori (MAP) ratio test (MAPRT) detector that incorporates this knowledge into its decision making process. This MAP approach is a generalization of past approaches such as the generalized likelihood ratio test (GLRT) detectors, where the estimates of the unknown parameters are obtained using maximum likelihood (ML) estimation. Assuming no knowledge of transmit waveforms, we derive the test statistics for bi-static and multi-static radar systems, when the noise power is both known and unknown. For the case of known noise variance in the bi-static radar system, we derive an exact closed-form expression for the probability of false alarm (PFA) and an approximate expression for the probability of detection (PD). For the multi-static case with known noise power, we derive upper bounds on PFA and PD. Numerical simulations verify the accuracy of the derived expressions and demonstrate that the proposed MAPRT detector performs significantly better than the GLRT-based detectors, especially when the signal-to-noise ratio (SNR) associated with the surveillance channel or sample size assumes small values. [ABSTRACT FROM AUTHOR]

Polarimetric diversity has been recently shown to significantly improve the target detection performance in the passive radar systems if properly exploited to mitigate the competing interference. An adaptive processing scheme is presented in this article, leveraging the information conveyed using multipolarized receiving antennas and modeling the disturbance as a multichannel autoregressive process. Despite the fact that this approach operates with a limited number of the adaptive degrees of freedom, the long integration time exploited by a passive radar typically requires a substantial computational cost and ad hoc expedients for its application. Therefore, a modified cost-effective implementation of the conceived solution is proposed in order to reduce the computational burden while controlling the resulting loss. The authors extensively demonstrate the effectiveness of the proposed solution against the experimental data collected by an FM radio-based passive coherent location system. The experimental results show that the proposed processing scheme yields a remarkable improvement with respect to both the conventional processing at the single polarimetric channel and the state-of-the-art strategies exploiting the polarization diversity in passive radars. [ABSTRACT FROM AUTHOR]

Various correlation-based receivers have been proposed in passive bistatic and active monostatic radar exploiting orthogonal frequency-division multiplexing communications signals, but too little has been dedicated to establishing their relations and advantages over each other. Accordingly, this article provides an analytical comparison of the common filters in such waveform sharing scenarios, along with a performance analysis regarding three criteria: computational complexity, signal-to-interference-plus-noise-ratio, and resilience to ground clutter. The last two especially assess the possible detrimental effects of the random sidelobes (or pedestal) induced by the data symbols in the range-Doppler map. Although simulations show that none of the filters performs unanimously better, the ones employing circular correlations globally evidence attractive results. [ABSTRACT FROM AUTHOR]

In the detection theory framework, it is customary to assign a bound to the false alarm probability and to attempt to maximize the detection probability subject to this constraint. In the problem of moving target detection in passive radar with a noisy reference channel, we formulate a detection problem as a composite hypothesis-testing problem and solve it with the likelihood ratio test (LRT) principle, which is known as generalized LRT in the electrical engineering works of literature. In such a problem, we show that any uncertainty in the value of the direct signal-to-noise ratio of the reference channel, abbreviated as $\text{DNR}_r$ , can result in excessive false alarm probability of the proposed noisy-reference-channel-based detector in the low- $\text{DNR}_r$ regime. To facilitate efficient operation under uncertainty in $\text{DNR}_r$ , we propose a new threshold-setting strategy to adjust the level of the proposed detector. Through extensive Monte–Carlo simulations, we examine the above problem and investigate the efficiency of the proposed threshold-setting strategy. Besides, we apply the framework of the kernel theory to the target detection problem of a noisy and ideal reference channel passive radar to propose two new detectors. As such, we replace the inner products of the proposed tests with appropriate polynomial kernel functions allowing for richer feature space to be deployed in the detection, achieving better detection performance. In this case, our detection performance results show that the kernelized detectors offer more that 1-dB signal-to-noise ratio gain as compared to their conventional counterparts. [ABSTRACT FROM AUTHOR]

This article addresses the problem of direct signal interference (DSI) and clutter cancellation for passive radar systems on moving platforms employing displaced phase centre antenna (DPCA) approach. Attention is focused on the development of signal processing strategies able to compensate for the limitations deriving from amplitude and phase imbalances that affect the two channels employed on receive. First, we show that using the signal received from the illuminator of opportunity as a source for channels calibration might be ineffective when DSI and clutter echoes have different directions of arrival, due to the effect of angle-dependent channel imbalance. Then, a two-stage strategy is proposed, consisting of a preliminary DSI removal stage at each receive channel, followed by a clutter-based calibration approach that basically enables an effective DPCA clutter suppression. Different strategies for channel calibration are proposed, aimed at compensating for potential angle and range-dependent channel errors, based on the maximization of the cancellation performance. Effectiveness of this scheme is shown against the experimental data from a DVB-T based moving passive radar, in the presence of both real and synthetic moving targets. [ABSTRACT FROM AUTHOR]

The ability to use external illuminators of opportunity will be an important feature for future maritime radar systems. This article considers passive bistatic radar operation using the Sirius-XM and Optus-D3 broadcast satellites. These operate in different frequency bands and with different bistatic geometries and bandwidths. In order to evaluate radar detection performance, generate realistic sea clutter, and develop suitable detection schemes, we require an understanding of the sea clutter statistics. For the data studied here, we consider both the delay-Doppler and time-Doppler domains and include an analysis of the amplitude distribution, temporal correlation, and Doppler spectrum. [ABSTRACT FROM AUTHOR]

This article deals with the problem of target detection in a passive multiple-input multiple-output (MIMO) radar network involving noncooperative illuminators of opportunity (IOs) and geographically dispersed multichannel receivers. The signal model considers the direct-path interference (DPI). We employ the Rao test to solve the problem and derive a closed-form detector. Then, by considering the properties of the ambiguity functions of most IO waveforms, an approximated version of the detector with a simpler form is derived. Two detectors in the case of an active MIMO network are also provided as the upper bounds of the passive situations. Moreover, considering target echo signal fluctuation, we have analyzed the performance of the detectors in terms of probability of miss-detection when being used in a multistatic configuration and have specified the conditions under which the full diversity gain is achieved. Numerical results verify the effectiveness of the proposed detectors and demonstrate that the proposed detectors have performance very close to the active situation. [ABSTRACT FROM AUTHOR]

We study the problem of jointly estimating the delays and Doppler shifts of multiple targets in a bistatic passive radar system when the unknown signal from the illuminator of opportunity (IO) is modeled as a correlated stochastic process. We treat all the delays and Doppler shifts as continuous-valued parameters to avoid straddle loss due to discretization. A computationally efficient method based on the expectation maximization (EM) algorithm is proposed, which breaks up the complex multidimensional maximum likelihood estimation problem into several separate optimization problems, one optimization per target. The algorithm provides estimates of all the targets’ delays and Doppler shifts along with the estimate of the IO signal and each target's component signal in the surveillance channel. The Cramer–Rao lower bound (CRLB) is also derived. Simulation results are presented, where we compare the proposed algorithm with the conventional cross-correlation (CC) estimator, modified CC (MCC) estimator, EM with successive interference cancellation (EM-SIC) estimator, and CRLB. It is shown that when the targets are well separated on the delay-Doppler shift plane, the proposed method outperforms all the estimators. Moreover, when the targets are close in this plane, the CC and EM-SIC estimators fails to distinguish the targets, whereas the proposed method successfully estimates the delays and Doppler shifts and outperforms the MCC in this case as well. Finally as the signal-to-noise ratio increases, the performance of the proposed method approaches that of the CRLB. [ABSTRACT FROM AUTHOR]

The possibility to apply passive radar sounding techniques to the Jovian icy moons, making use of Jupiter’s strong decametric emissions (DAMs), has recently garnered large research interest. In this article, we propose a radar sounding simulation approach being able to simulate and compare passive and active acquisitions for a given scenario. The proposed simulator is based on the Stratton–Chu integral used with the linear phase approximation on triangular facets. The external field is modeled with plane waves, the direction, polarization, and amplitude of which can be freely chosen. The time-domain dependence can be either synthetically generated (e.g., Gaussian white noise) or taken from experimental measurements (e.g., waveforms recorded by a radio telescope observing Jupiter). For passive sounding, both autocorrelation and cross-correlation processing are considered. Validation tests were conducted on a series of ideal digital elevation models (DEMs), such as flat surfaces and subsurfaces, or Gaussian rough surfaces, and a good agreement with the existing literature was obtained. To illustrate the capabilities of the proposed simulator, we conducted additional more realistic experiments of radar sounding simulations, where we use both white noise and Jovian waveforms recorded by the LWA1 radio telescope. [ABSTRACT FROM AUTHOR]

We consider a hybrid active-passive radar system that employs a wireless source as a passive illuminator of opportunity (IO) and a co-channel active radar transmitter operating in the same frequency band to seek spectral efficiency. The hybrid system can take advantage of the strengths of passive radar (e.g., energy efficiency, bi-/multi-static configuration, and spatial diversity) as well as those of active radar (dedicated transmitter, flexible transmit beam steering, waveform optimized for sensing, etc.). To mitigate the mutual interference and location-induced timing uncertainty between the radar and communication signals, we propose two designs for the joint optimization of the radar waveform and receive filters. The first is a max-min (MM) criterion that optimizes a worst-case performance metric over a timing uncertainty interval, and the other a weighted-sum (WS) criterion that forms a weighted sum of the performance metric at each delay within the delay uncertainty interval. Both design criteria result in nonconvex constrained optimization problems that are solved by sequential convex programming methods. When timing uncertainty vanishes, the two designs become identical and admit a simpler solution. Numerical results are presented to demonstrate the performance of the proposed hybrid schemes in comparison with conventional active-only and passive-only radar systems. [ABSTRACT FROM AUTHOR]

By exploiting the sparsity of the scene containing only a few moving targets, a high-resolution and real-time range-Doppler map generation algorithm for passive bistatic radar is proposed. The proposed algorithm divides the long integration time into multiple short batches, from which a few batches are randomly selected on the basis of compressive sensing theory. A one-dimensional cross correlation is performed for each selected batch to obtain the range-compressed profile. Mean-value subtraction is then performed to suppress the direct path interference and stationary target reflections. Finally, an extended orthogonal matching pursuit algorithm is proposed for the effective estimation of target Doppler frequency. Practical application of this novel algorithm is examined by the detection of airplanes and ships via two synchronized general-purpose software-defined radio receivers. The results show that the proposed algorithm can achieve an improved resolution and a reduced sidelobe level compared to the conventional algorithms. [ABSTRACT FROM AUTHOR]

In this paper, we examine the problem of target detection for the multistatic passive radar. Passive radar systems leverage the existing wireless sources, such as radio/TV stations and cellular signals that are referred to as illuminators of opportunity (IOs), to illuminate the environment and provide surveillance functions. Usually, these IO source signals are sparse or locally sparse in the frequency domain. We develop a passive multistatic detector by exploiting the sparsity or local sparsity of the IO signals. To improve the computational efficiency, two fast implementations of the proposed detector are also introduced. Simulation results show that the proposed approaches outperform the conventional passive detection methods that model the IO signals as unknown without any specific structures. [ABSTRACT FROM AUTHOR]

This paper discusses the situation with cochannel interferences in the digital television terrestrial multimedia broadcasting (DTMB)-based passive radars. The cochannel interference comes from the station that works in the same frequency with the adopted illuminator of opportunity (IO), but broadcasts different contents. Based on the frame structure of the DTMB signal, the features of the cochannel interference are elaborated. Analyses show that several interfering peaks present in the range-Doppler map when the cochannel interference has the same frame header with the IO. These interfering peaks can be used to identify the existence of cochannel interferences. In addition, to eliminate the influences of the cochannel interference on the target detection, we propose a method that can effectively remove the interfering peaks. The simulation and field experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper addresses the exploitation of global navigation satellite systems as opportunistic sources for the joint detection and localization of vessels at sea in a passive multistatic radar system. A single receiver mounted on a proper platform (e.g., a moored buoy) can collect the signals emitted by multiple navigation satellites and reflected from ship targets of interest. This paper puts forward a single-stage approach to jointly detect and localize the ship targets by making use of long integration times (tens of seconds) and properly exploiting the spatial diversity offered by such a configuration. A proper strategy is defined to form a long-time and multistatic range and Doppler (RD) map, where the total target power can be reinforced with respect to, in turn, the case in which the RD map is obtained over a short dwell and the case in which a single transmitter is employed. The exploitation of both the long integration time and the multiple transmitters can greatly enhance the performance of the system, allowing counteracting the low-power budget provided by the considered sources representing the main bottleneck of this technology. Moreover, the proposed single-stage approach can reach superior detection performance than a conventional two-stage process where peripheral decisions are taken at each bistatic link and subsequently the localization is achieved by multilateration methods. Theoretical and simulated performance analysis is proposed and also validated by means of experimental results considering Galileo transmitters and different types of targets of opportunity in different scenarios. Obtained results prove the effectiveness of the proposed method to provide detection and localization of ship targets of interest. [ABSTRACT FROM AUTHOR]

Radar signal parameter estimation, in the context of the reconstruction of the received signal in a passive radar utilizing other radars as a source of illumination, is one of the fundamental steps in the signal processing chain in such a device. The task is also a crucial one in electronic reconnaissance systems, e.g., electronic intelligence systems. In order to obtain accurate results it is important to measure, estimate and precisely reconstruct the considered signal. Then it is necessary to identify it. The following paper presents a new approach to the estimation of the phase parameters of radar pulses. The novel method of chirp-rate estimation is described and used to analyze such signals. First, the method was examined by simulated pulses, and then true measured signals collected from the air traffic control radar were utilized. Using the phase accelerogram, an instantaneous chirp-rate was computed thanks to three novel estimators and the results were compared to results acquired using a different, already known, and estimation method of the phase coefficients based on matching filtration– extended generalized chirp transform. [ABSTRACT FROM AUTHOR]

Passive radar's ability to observe micro-Doppler phenomena from helicopters, fixed-wing propeller aircraft, and jet engine modulation is examined. Experimental results from a digital television based passive radar demonstrate its potential to observe characteristic signatures of multiple types of air targets and estimate helicopters’ primary rotor parameters. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of clutter cancellation and slowly moving target detection in digital video broadcast-terrestrial-based passive radar systems mounted on moving platforms. First, we show that conventional processing approaches based on the availability of multiple receiving channels might be ineffective in the considered scenarios due to the impossibility to control the employed waveform of opportunity. Therefore, an appropriate space-time adaptive processing scheme is proposed to cope with the Doppler spread clutter returns aiming at ground moving target indication applications. It exploits the benefits of the reciprocal filtering strategy applied at a range compression stage together with a flexible displaced phase center antenna approach. The effectiveness of the proposed scheme is demonstrated via application to a simulated dataset and then tested against experimental data collected by multichannel passive radar on a maritime moving platform. [ABSTRACT FROM AUTHOR]

Passive Radar signifying the localisation of a target by radar measurements without using own controlled emissions has been discussed, tried, reinvented, and matured within the last 80 years. Its advantages, like covert operation and saving the costs of a transmitter, are obvious. Military as well as civilian interests combined with the advances in technological developments have recently boosted research on passive radar and passive radar systems are currently approaching the market. This tutorial shall give an overview of the history, development, and processing in passive radar and enable the interested reader to further investigate the subject exploiting the presented material together with the cited references. [ABSTRACT FROM AUTHOR]

This paper addresses the exploitation of Global Navigation Satellite Systems (GNSS) as transmitters of opportunity in passive bistatic radar systems for maritime surveillance. The main limitation of this technology is the restricted power budget provided by navigation satellites, which makes it necessary to define innovative moving target detection techniques specifically tailored for the system under consideration. To this aim, this paper puts forward long integration time techniques able to collect the signal energy over long time intervals (tens of seconds), allowing the retrieval of suitable levels of signal-to-disturbance ratios for detection purposes. A local plane based technique is first considered, providing target detection in a plane that represents the section of maritime area covered by the radar antenna. As a suboptimum solution in terms of achievable integration gain, but more efficient from a computational point of view, a second technique is considered working in the conventional bistatic range and Doppler plane (basic plane based). Results against synthetic and experimental datasets show the effectiveness of the proposed techniques. [ABSTRACT FROM AUTHOR]

In this paper, we address the problem of target's range migration in passive bistatic radar exploiting long coherent integration times with fairly wideband signals of opportunity. We resort to the well-known keystone transform (KT) to compensate for the range walk effect and to take advantage of a higher coherent integration gain against targets with nonnegligible radial velocity. Specifically, an efficient implementation of the KT is proposed, based on the Lagrange polynomial interpolation, in order to reduce the computational load of the method that mostly depends on the required slow-time interpolation stage. The analysis conducted against simulated data shows that the conceived approach allows us to achieve theoretical performance, while further reducing the KT complexity with respect to alternative solutions based on cardinal sine functions or chirp-Z transforms. Moreover, the application against experimental datasets collected by a DVB-T-based passive radar proves the practical effectiveness of the proposed algorithm and highlights its suitability for real-time air traffic surveillance applications. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of target delay and Doppler frequency estimation for passive bistatic radar employing noncooperative illuminators of opportunity (IOs), where the receivers are contaminated by nonnegligible noise, clutter, and direct-path interference. A parametric approach is proposed by modeling the unknown signal transmitted from the IO as an autoregressive process whose temporal correlation is jointly estimated and exploited for passive estimation. An iterative estimator based on the expectation-maximization (EM) principle is utilized to solve this highly nonlinear problem. We also discuss the initialization of the EM-based estimator and a fast implementation based on the fast Fourier transform and interpolation techniques. In addition, we derive the Cramér–Rao lower bound for the estimation problem to benchmark the performance of the proposed estimator. Simulation results show that the proposed estimator behaves similarly to a clairvoyant EM estimator, which assumes knowledge of the IO waveform covariance matrix, and significantly outperforms other methods that ignore the waveform correlation. [ABSTRACT FROM AUTHOR]