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This paper proposes a low budget solution to detect and possibly track space debris and satellites in Low Earth Orbit. The concept consists of a space-borne radar installed on a cubeSat flying at low altitude and detecting the occultations of radio signals coming from existing satellites flying at higher altitudes. The paper investigates the feasibility and performance of such a passive bistatic radar system. Key performance metrics considered in this paper are: the minimum size of detectable objects, considering visibility and frequency constraints on existing radio sources, the receiver size, and the compatibility with current cubeSat's technology. Different illuminator types and receiver altitudes are considered under the assumption that all illuminators and receivers are on circular orbits. [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]

This paper presents results from experiments that utilize Global-Positioning-System-like terrestrial positioning signals in a passive radar system to track aircraft over a metropolitan area. Unlike communication signals, these positioning signals offer unique properties that are advantageous for a passive radar. Exploitation of these properties provides an opportunity to reduce hardware complexity and to enable passive radar systems with a lower cost. This paper provides a detailed description of the signal processing in the context of low-cost hardware. [ABSTRACT FROM AUTHOR]

In a recent paper, the problem of inverse synthetic aperture radar (ISAR) imaging moving ground targets was discussed and an effective solution based on a novel joint space-time adaptive processing (STAP)-ISAR formulation was proposed. In the case of bistatic geometry, ISAR imaging can be simply and effectively enabled via the bistatically equivalent monostatic theory, which allows for monostatic ISAR processing to be used to form bistatic ISAR images. However, the clutter nonstationary introduced by the bistatic geometry strongly degraded STAP performances if a monostatic STAP processing is used. In this paper, a new joint STAP-ISAR processing is introduced that works in the bistatic case. Theoretical figures and numerical simulations are used to prove the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

The multipeaked correlation functions of binary offset carrier (BOC) modulated signals introduce an ambiguity threat into the code tracking. Several techniques have been proposed to solve this problem, such as a class of two-dimensional (2-D) tracking techniques, in which the spreading code and the subcarrier are tracked separately. In this paper, the key properties of correlation functions and the theoretical tracking performance analysis considering noise and multipath environments of the general 2-D tracking loop are studied. Exact explicit expressions, taking into account the effects of local subcarrier waveform mismatching and the front-end bandlimiting, are provided for the correlation function employed in 2-D tracking. From these expressions, the key characteristics of 2-D correlation functions such as the shape, coupling relation between dimensions, and the effects of bandlimiting can be clearly seen. Moreover, general theoretical methods to predict the performance of the 2-D tracking techniques under the multipath and noise environments are provided. The results obtained in this paper can facilitate investigations in the area of 2-D tracking technique parameter optimization and performance assessment. [ABSTRACT FROM PUBLISHER]

To navigate in global navigation satellite systems (GNSS) signal-challenged environment, for example, foliage canopy, urban canyon, indoor, etc., high-sensitivity GNSS receivers are usually preferred for the improved acquisition and tracking capabilities. The core of high-sensitivity GNSS receiver design is to extend integration time coherently, which is optimal for improving post-signal-to-noise ratio, mitigating multipath and cross-correlation false locks, and avoiding squaring loss. In GNSS data channels, extending integration time coherently requires the navigation message data bit wipe-off. For stand-alone high-sensitivity GNSS receivers, bit wipe-off is usually achieved by using estimation algorithms (i.e., bit decoding) rather than accessing external networks (i.e., bit aiding). In this paper, the maximum-likelihood (ML) bit decoding is used to estimate the data bit values for bit wipe-off. Furthermore, the benefits of using advanced tracking algorithms?vector tracking and inertial navigation system (INS)-assisted tracking (i.e., ultratight coupling of GNSS/INS)?to improve ML bit decoding and navigation performance are analyzed. Two vehicular navigation tests are performed in dense foliage and an urban canyon environment. In the context of global positioning system L1 C/A signals, the field test results show that vector tracking and ultratight coupling can improve the successful decoding rate by up to 40% depending on signal strength. This paper also demonstrates how the signal power-based correlator selection method can address high bit error-rate problems when ML bit decoding is used for bit wipe-off in the signal-challenged environment. After implementing this algorithm, the position and velocity accuracy of the stand-alone high-sensitivity GNSS receiver has been improved about 50% after extending coherent integration time from 20 to 100 ms in the vehicular navigation tests. [ABSTRACT FROM PUBLISHER]

The automatic dependent surveillance—broadcast (ADS-B) system adopts an open communication mode, and the lack of designed-in security measures in the ADS-B system makes it vulnerable to various types of attacks (jamming, spoofing, etc.). In view of the low-bandwidth and less-data-bit features of the ADS-B, this paper studies the integrity and authenticity of information by signing messages and proposes an ADS-B message authentication method based on certificateless short signature. This method uses short signature and does not require certificate management and has efficient performance. Compared with the existing approach, the computation costs of the proposed method in the signature phase are reduced by 1/2, and the signature length is reduced by 3/4. Additionally, we used the extended NS2 simulation platform to simulate 1090ES data link in different scenarios of the network; the simulation results show that our solution is suitable for minimum operational performance standard of ADS-B. [ABSTRACT FROM AUTHOR]

This paper considers the localization of a moving target using a forward scatter radar consisting of a transmitter and an array antenna receiver. A direct positioning method based on maximum likelihood (ML) estimation is proposed and compared with the conventional two-step method in which the primary parameters, including Doppler shift and angle of arrival, should be determined in the first step. Moreover, closed-form expressions for Cramer–Rao lower bound of both methods are derived. The aforementioned methods are comprehensively compared in terms of positioning accuracy and computational complexity. Theoretical performance study, including determining the minimum required observation time for achieving a certain positioning error level and analyzing the best possible positioning accuracy in extreme scenarios, is carried out. The results of numerical simulations demonstrate that the proposed direct approach is more precise compared to the two-step method and can achieve higher resolution levels. [ABSTRACT FROM AUTHOR]

Random jitter or offset between the transmitter/receiver clocks is an important parameter that has to be accurately estimated for optimal detection of pulse position modulation (PPM) symbols for high-data-rate optical communications. This parameter, in general, is modeled as an unknown random quantity that depends on the clock drift between the transmitter/receiver clocks and the random motion between the transmitter and receiver stations. In this paper, we have modeled the time jitter for two scenarios—phase modulation jitter and frequency modulation jitter. The phase modulation jitter is modeled as a Gaussian random variable which is estimated with the help of a maximum a posteriori probability (MAP) estimator. The frequency modulation jitter is characterized as a random walk, and this leads to the modeling of the jitter as a state space variable in the context of a dynamical system. Since the observations are the photon counts in each slot of a PPM symbol (for both MAP estimation and tracking), the resulting dynamical model is highly nonlinear, and particle filters are employed for tracking the frequency modulation jitter. We evaluate the performance of both the maximum a posteriori estimators and the particle filters in terms of the relative mean-square error and probability of error. We conclude that with MAP estimation and particle filters that estimate/track the time offset, we achieve a significant performance gain in terms of probability of error as compared to systems that do not have a time synchronization system in place. [ABSTRACT FROM AUTHOR]

In helicopter satellite communication systems, the received signal is blocked by the helicopter rotor blades during propagation, incurring the unstableness of traditional methods of channel estimation due to unknown channel state. To this end, a joint occlusion detection and phase estimation algorithm is proposed by factor graph in this paper. For occlusion detection to acquire channel state, the occlusion channel model is established relying on a Markov chain with a priori blocking ratio, in which the probability distribution of the channel state is calculated by the proposed message-passing algorithm. To obtain the occlusion position and update the blocking ratio, a cross-correlation method is designed. Furthermore, a joint block detection and phase estimation algorithm is developed based on the proposed occlusion detection algorithm to realize phase estimation by iterative receiver, where occlusion detection, phase estimation, and decoding are performed in turn for one turbo iteration and the result of decoding is then sent back to the occlusion detection step for the next iteration. Simulation results show that the proposed algorithm is adaptive to the dynamical change in the blocking ratio and can achieve bit error rate performance that is close to the performance bounds based on perfect channel state. [ABSTRACT FROM AUTHOR]

In this paper, a generalized signal model is presented to accommodate both narrowband and wideband signals in a multi-input multi-output (MIMO) sensor system scenarios. The derived model is then used to define a MIMO ambiguity function based on the Kullback–Leibler divergence. Moreover, the proposed formulation is parametrized using the signal and channel correlation matrices to account for different waveform and sensor placement designs, thereby allowing a flexible modeling approach. A comparison between the proposed definition and the more conventional approach of summing the squared matched filter outputs is presented for different sensors and waveforms configurations. [ABSTRACT FROM AUTHOR]

In this paper, the detection of LoRa signals from a low-earth orbit (LEO) satellite is considered. The use of a LEO satellite to collect messages from the terminals, as an alternative to terrestrial base stations in rural areas, represents a major challenge due to the specific impairments of this scenario, e.g., Doppler effects and interfering signals. A redesign, whose validation is assessed through a system simulator operating in a realistic scenario, is thus required. [ABSTRACT FROM AUTHOR]

This paper presents the design, implementation, and the experimental verifications of a 2.4-GHz multibeam array receiver based on two-dimensional (2-D) spatially bandpass (SBP) digital plane wave (PW) filters. The digital array receiver consists of 2.4-GHz 16-element uniform linear array, and quadrature sampling is employed to process the broadband PWs at the baseband. Both 2-D finite impulse response (FIR) and infinite impulse response (IIR) digital filters having multiple trapezoidal-shaped passbands are employed as the 2-D SBP digital PW filters. Both 2-D FIR and IIR trapezoidal filters are implemented on a Xilinx Virtex 6 sx475t field programmable gate array chip that comes with the reconfigurable open architecture computing hardware version-2 (ROACH-2) platform. The 16 in-phase and quadrature-phase downconverted signals are sampled using the ROACH-2 platform and are processed using the digital architectures to form multiple beams. Example radio-frequency beam patterns corresponding to each filter architecture are measured and reported. The main lobes of the measured multibeam patterns are well aligned with the simulated multibeam patterns. [ABSTRACT FROM AUTHOR]

Spread spectrum (SS) signal acquisition in satellite communication is a very computation intensive technique, which hinders the development of real-time spread spectrum signal acquisition. In this paper, in order to achieve real-time acquisition, we propose a multi-GPU based SS signal acquisition algorithm. First, sliding correlation, the computation kernel, is formulated and efficiently parallelized by CUDA. Second, a CUDA-enabled SS signal acquisition algorithm is implemented by adopting the CUDA-enabled sliding correlation. Third, a multi-GPU based algorithm is implemented by using multi-GPU programming. The performance is evaluated in a real SS telemetry system. Real-time acquisition is achieved in all cases by using a single K40 GPU. Furthermore, an average of 374.7 $\times$ speedup (max 473 $\times$) in 6 datasets is achieved when using four K40 GPUs. Good scalability is observed when varying the parameters. [ABSTRACT FROM AUTHOR]

In this paper, we propose a framework, based on the combined use of single- and multiuser detection, to jointly optimize the achievable rates of two signals sharing the same frequency in the forward link of a multibeam satellite system. We then propose the application of the described framework to two different scenarios of interest. First, we consider a uniform coverage scenario, aimed at maximizing the average throughput per beam in a realistic coverage condition. We compare different solutions based on alternative frequency reuse (FR) schemes and different receiver strategies. We demonstrate that the use of multiuser detection can achieve significant gains over a reference strategy based on single-user detection. Next, we analyze a “hotspot” case, where resources are pulled from empty beams to serve a beam with a high service demand. Also in this case, we compare several strategies and FR schemes. We show that the best performance is achieved by a scheme adopting three colors and single-user detection. [ABSTRACT FROM AUTHOR]

In previous works, methods were explored for position estimation utilizing satellite-borne signals of opportunity, mainly the global positioning system (GPS). The GPS signal was exploited for use in a multistatic passive coherent location (PCL) system. The GPS signal is especially attractive for PCL applications because of the native capability to produce position and velocity estimation. This paper examines the signal specifications for PCL implementation and explores the potential limitations of the proposed solutions. GPS specific methods are developed for multistatic PCL velocity estimation in a three-dimensional plane. The method developed is combined with previously completed work of GPS PCL position estimation for a complete system design in range and Doppler. The PCL system is evaluated against conventional GPS position estimation and velocity estimation and proves to have comparable metrics of performance. Analysis and simulation are performed for verification and validation of the developed methods. [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]

When a target undergoes nonuniform rotational motion (RM), conventional motion compensation (MOCOM) approaches for monostatic inverse synthetic aperture radar (ISAR) systems fail to correct the nonuniform RM error in bistatic radar systems; this failure is caused by nonlinear phase relationships between scatterers in bistatic ISAR (Bi-ISAR) systems. To address this problem, in this paper, we propose a new MOCOM framework for Bi-ISAR imaging of nonuniformly rotating targets. In the proposed method, a newly devised correction is performed to ensure successful RM compensation, followed by translational motion compensation. Because the phase relationships of the scatterers become virtually linear after this correction, nonuniform RM can be successfully converted into uniform RM. Namely, using the proposed method, translational and rotational motion errors are correctly removed, and focused Bi-ISAR images are obtained even for a target involved in nonuniform RM. Furthermore, for more effective use of Bi-ISAR images, a method to restore sheared Bi-ISAR images is exploited in the proposed framework. In the simulations and experiments, we determined that the proposed method can provide high-quality Bi-ISAR images for nonuniformly rotating targets. [ABSTRACT FROM AUTHOR]

Wide area multilateration (WAM) estimates aircraft positions on the basis of time difference of arrival (TDOA) for air traffic control. To design the receiver positions, dilution of precision has been used. However, the conventional approach ignores failure in signal detection. Therefore, this paper proposes a novel WAM accuracy model for more realistic performance prediction. The proposed model employs a stochastic constellation, i.e., the constellation of available receivers virtually changes according to the signal detection performance. [ABSTRACT FROM AUTHOR]

This paper presents an innovative global navigation satellite system (GNSS) fault detection and exclusion approach for the adoption of satellite localization in the rail sector. Current global integrity monitoring systems cannot guarantee the safety level needed for such applications as train control where tolerable hazard rate in the order of 10−9/h is required. A new method, named two tiers, enabling to integrate local augmentation systems and global augmentation infrastructures, is presented. It is based on the comparison of single differences residuals among satellites for detecting signal in space (SIS) faults and double difference residuals among local augmentation stations and satellite-based augmentation systems ranging and integrity monitoring stations for detecting reference stations faults. GPS SIS faults described in literature and real GNSS raw data recorded on a train are taken into account. This study reports the performance analysis for the two-tiers approach carried out during relevant European projects. A test-bed architecture has been developed through the implementation of the algorithm in real time on a local augmentation operational center. Relevant performances have been tested on a rail track for validating the algorithm in real operative conditions. Significant results of the analysis are reported for SIS integrity assessment only. [ABSTRACT FROM AUTHOR]

A novel orthogonal frequency-division multiplexing (OFDM) multiple-input multiple-output (MIMO) radar is proposed in this paper in a scenario of coexisting with a communication system. For the purpose of avoiding interference to communication while maintaining MIMO radar's capabilities, such as measuring signal's direction of departure (DOD), a collocated antenna array is divided into several overlapped subarrays. Mutually OFDM waveforms exploiting an orthogonal space-time block code are transmitted through these subarrays to obtain waveform diversity, which can be separated at the radar receiver. A MUSIC-like DOD estimation algorithm and the Cramér–Rao bound of DOD are proposed for overlapped subarrays’ configuration in order to evaluate performance of the beamforming strategy. The beamformer is designed using an optimization technique at cost of maximizing the input SNR of radar while keeping communication rate constant. Due to nonconvexity of the optimization problem, a two-step optimization algorithm is proposed. Simulation experiments validate theoretical analysis of the proposed precoding radar. Experiments also show robustness of the precoding scheme and the ability of balancing the input SNR of radar and the communication rate. [ABSTRACT FROM AUTHOR]

This paper proposes a BeiDou-II navigation message authentication scheme to defend against the generating spoofing attacks. In this scheme, a digital signature is generated by the elliptic curve digital signature algorithm and is inserted into the reserved bits of the BeiDou-II navigation message. This signature will verify the integrity and authenticity of the navigation data to avoid entity masquerade and data tampering. Furthermore, the key exchange process is also designed through the short message service or digital certificate. The effectiveness and security of the scheme is evaluated in the simulation. The results indicate that the authentication of BeiDou-II navigation message can be realized in the harsh environment, and the resistance ability of civil BeiDou-II against a spoofing attack could be enhanced by this scheme. [ABSTRACT FROM AUTHOR]

This paper derives closed-form expressions for bistatic synthetic aperture radar (SAR) spatial resolution of a generalized system from the $k$ -space or the wavenumber domain. These spatial resolution equations have not previously appeared in the literature. From these equations, significant resolution is found in range and cross range, forecasting a forward-scatter bistatic SAR image when the elevation angles of each bistatic platform are significantly different. Simulation and laboratory tests demonstrated the forward-scatter resolution. [ABSTRACT FROM AUTHOR]

Chirp signals arise in many applications of signal processing. In this paper, we address the problem of detection of chirp signals that are encountered in a bistatic radar which we are developing for remote sensing of cosmic ray induced air showers. The received echoes from the air showers are wideband and characterized by very short sweep periods. This makes our astrophysical problem a challenging one, since a very short sweep period is equivalent to a very low energy chirp signal. Furthermore, the related parameters of the received echoes are nondeterministic since they are tied to the physical parameters of the air showers. The primary focus of this research is to address these challenges and to find an optimized detection approach in the additive white Gaussian noise channel for low signal-to-noise ratio (SNR) levels. Matched filters are commonly used in the radar systems when the chirp signal is known. In this paper, we revisit this context and use a “matched filter” as a basis of building a rake-like receiver which consists of a set of filters matched to quantized chirp rates, logarithmically distributed within the chirp-rate interval of interest. We examine the detection capability of the proposed structure through extensive theoretical and numerical analysis. Theoretical analysis and simulation results prove that the proposed detector has high detection capability for a range of chirp slopes in a low-SNR environment. [ABSTRACT FROM AUTHOR]

Satellite navigation is quickly becoming a multidimensional complex challenge, with four Global Navigation Satellite Systems (GNSS) fully or partially deployed, more than 90 GNSS satellites currently in the sky, and 12 open-service GNSS signal types available to the end-user. Research papers about GNSS performance typically focus on single or dual GNSS solutions, and there are still very few papers addressing jointly the four existing GNSS together. It is the goal of our paper to present a unified, fast, modular, and simple-to-program theoretical analysis of the four GNSS, focusing on performance limits in terms of spectral characteristics, tracking, multipath robustness, and positioning accuracy. A unified new compact approach for dealing with all the modulations employed in GNSS is also included.We show a unified generic framework for analyzing the multi-GNSS signals in a fast and simple-to-implement manner, without the need of simulating constellations or gathering measurement data. Our approach is also validated with Spectracom-based constellation data. Our analysis can be used as the starting point in further design process or system analysis. [ABSTRACT FROM AUTHOR]

Both economic incentives and policy trends motivate the study of spectrum sharing between radar and wireless communications as a means to mitigate spectrum scarcity. This paper proposes a novel, temporal signal processing algorithm for a radar to mitigate interference from cyclostationary, linearly modulated digital communications (LMDC) interference such as orthogonal frequency division multiplexing (OFDM) or code division multiple access (CDMA). The proposed algorithm has the form of a novel whitening filter followed by a matched filter, and as such it optimizes the statistical deflection among the class of all linear detection filters. The derived cyclostationary whitening filter has equivalent mathematical representations as: 1) the form of a multiuser detector followed by an interference canceler, and 2) a frequency-shift (FRESH) filter. Performance results indicate that the derived cyclostationary whitener can produce signal-to-interference-plus-noise ratio (SINR) improvements ranging from negligible to up to $\text{20 dB}$ for the parameters used in this study. The cyclostationary whitener leads to the greatest SINR improvements when the interference-plus-noise is dominated by only a few LMDC signals, such as might occur in a near-far scenario. [ABSTRACT FROM AUTHOR]

This paper investigates a radar-communications waveform sharing scenario. Particularly, it addresses the self-interference phenomenon induced by independent single-point scatterers throughout a low-complexity monostatic orthogonal frequency-division multiplexing (OFDM)-based radar receiver from a statistical viewpoint. Accordingly, an analytical expression of the postprocessing signal-to-interference-plus-noise-ratio is derived and detection performance is quantified in simulated scenarios for rectangular and nonrectangular pulses. Both metrics suggest that this phenomenon must be further handled. [ABSTRACT FROM AUTHOR]

Optical beam position on a detector array is an important parameter that is needed to optimally detect a pulse position modulation (PPM) symbol in free-space optical communications. Since this parameter is generally unknown, it is essential to estimate the beam position as accurately as possible. In this paper, we examine that an accurate estimate of the beam position is required in order to minimize the probability of PPM symbol detection error. Furthermore, we employ different estimators/trackers of the beam position (which could be stationary or time-varying), and compare the probability of error performance of the detectors using those estimators. The probability of error is calculated with the help of Monte Carlo simulations for the uncoded 8-PPM and 16-PPM systems, each of which employ photon counting maximum likelihood receivers. It is found out that the probability of error is minimum for the maximum likelihood estimate of the stationary beam position. Moreover, for the dynamically varying beam position, a particle filter with a sufficiently large number of particles provides a close-to-optimal probability of error performance. [ABSTRACT FROM AUTHOR]

The unambiguous estimation of high-order binary offset carrier signals in harsh propagation conditions is still an open problem in the literature. This paper proposes to overcome the limitations observed in state-of-the-art unambiguous estimation techniques based on the application of existing direct positioning techniques and the exploitation of the spatial diversity introduced by arrays of antennas. In particular, the ambiguity problem is solved as a multiple-input multiple-output estimation problem in the position domain. [ABSTRACT FROM AUTHOR]

In this paper, a time-frequency (TF) jammer mitigation algorithm based on a Kalman filter is proposed for global navigation satellite system (GNSS) receivers. A new state-space representation of the received signal is formed considering the rapidly changing characteristics of the jamming signal. Using the instantaneous frequency (IF) and chirp rate estimates obtained by the TF analysis, the proposed algorithm applies a Kalman filter for jamming signal estimation. The estimated jamming signal is then subtracted from the received data to remove the jammer signal. To cope with the time-varying accuracy in the jammer parameter estimates, the methods for adaptively adjusting the process and measurement noise variances using chirp rate estimates are described. Simulations are performed to verify the effectiveness of the proposed method for mitigating various types of jammers. The proposed approach improves the jammer signal removal performance by minimizing the effect of the Kalman filter model mismatch when compared to other techniques reported in the literature. [ABSTRACT FROM AUTHOR]

Spectrum congestion and competition over frequency bandwidth could be alleviated by deploying dual-function radar-communications systems, where the radar platform presents itself as a system of opportunity to secondary communication functions. In this paper, we propose a new technique for communication information embedding into the emission of multiple-input multiple-output (MIMO) radar using sparse antenna array configurations. The phases induced by antenna displacements in a sensor array are unique, which makes array configuration feasible for symbol embedding. We also exploit the fact that in a MIMO radar system, the association of independent waveforms with the transmit antennas can change over different pulse repetition periods without impacting the radar functionality. We show that by reconfiguring sparse transmit array through antenna selection and reordering waveform-antenna pairing, a data rate of megabits per second can be achieved for a moderate number of transmit antennas. To counteract practical implementation issues, we propose a regularized antenna-selection-based signaling scheme. The possible data rate is analyzed and the symbol/bit error rates are derived. Simulation examples are provided for performance evaluations and to demonstrate the effectiveness of proposed dual-function radar-communication techniques. [ABSTRACT FROM AUTHOR]

By using the multibeam smart antennas (MBSAs), the nodes can form a high-throughput wireless mesh network with concurrent packet delivery in multiple directions. In this paper, a novel medium access control (MAC) scheme is designed for multibeam airborne mesh networks. It has the following two features: First, IEEE 802.11-compatible, interference-minimized channel access via a special two-layer MAC architecture, where the upper-layer MAC supports the concurrent multibeam transmission/reception, and the lower-layer MAC takes advantage of MBSAs; second, adjustment of the parameters of the two MAC layers to support different mission priorities in airborne networks, including the allocation of time slots in the upper layer. The simulation results demonstrate the superior performance of our proposed multibeam MAC protocols. [ABSTRACT FROM AUTHOR]

In this paper, a Block Iterative Method with Adaptive Thresholding for Sparse Recovery (BIMATSR) is proposed to recover the received signal in an under-sampled distributed multiple-input multiple-output radar. The BIMATSR scheme induces block sparsity with the aid of a signal-dependent thresholding operator which increases the accuracy of the target parameter estimation task. We have proved that under some sufficient conditions, the suggested scheme converges to a stable solution. Moreover, different simulation scenarios confirm that the BIMATSR algorithm outperforms its counterparts in terms of the target parameter estimation. This superiority is achieved at the expense of slightly more computational complexity. It is also shown that in this application, random sampling scheme offers better performance than random Gaussian matrix, while it has simpler structure. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) inverse synthetic aperture radar (ISAR) imaging has been proven feasible by combining traditional ISAR imaging and interferometry. Such technique, namely Inteferometric ISAR (InISAR), allows for the main target scattering centers to be mapped into a 3-D spatial domain, therefore forming 3-D images under the form of 3-D point clouds. 3-D InISAR overcomes some main limitations of traditional 2-D ISAR imaging, such as the problem of cross-range scaling and unknown image projection plane. Despite the great advantage of 3-D imaging over traditional 2-D imaging, some issues remain, such as scatterer scintillation, shadowing effects, poor SNR, etc., which limit the effectiveness of 3-D imaging. A solution to these issues can be found in the use of multiple 3-D views, which can be obtained exploiting either multitemporal or multiperspective configurations or a combination of both. This paper proposes this concept and develops the image fusion algorithms that are necessary to produce multiview 3-D ISAR images. The effectiveness of the proposed technique is tested by using real data collected with a multistatic InISAR system. [ABSTRACT FROM AUTHOR]

In some well-known passive localization methods, when the number of equations equals the number of unknown object coordinates, there arise finite number of possible solutions. A localization disambiguation technique is required to identify the correct solution in these cases. As an example, the exact solution of time-difference-of-arrival (TDOA) based equations for four receiver sites, leads to a couple of solutions with only one of them being the true target position. This ambiguity conventionally is resolved by using angle-of-arrival measurements or using an additional receiver site, which increase the cost and complexity of the system. In this paper, a localization disambiguation technique for a moving target based on sequential measurements is presented. In the proposed method, the correct solution corresponding to each measurement is identified by determining the most sensible path of the target. The method analytically finds the position of the target and no extra measurement/site is required. Performance of the proposed method has been evaluated and verified in numerous simulations as well as real data based on TDOA measurements. [ABSTRACT FROM AUTHOR]

In this paper, first, two methods of positioning an indoor receiver with transferred global navigation satellite system signals from cellular base stations are proposed. Then, a technique of fusing the position results produced by the two methods through Kalman filter is introduced; thus, the positioning accuracy is improved. To mitigate the near–far problem generated by signals from base stations and satellites, a pulse shaping scheme and the strategy of assigning pulse patterns to base stations are investigated. [ABSTRACT FROM AUTHOR]

In this paper, we examine an FM-based passive coherent location (PCL) radar system and attempt to quantify its performance in the presence of various different jamming waveforms namely broadband noise and a single tone on the carrier. It is shown that effective jamming can be achieved with relatively low jamming power. The importance of an effective direct signal canceller is also shown as a means of suppressing the jamming signal. The effect of these jamming scenarios is shown through simulation and then compared to a representative measured result from a real FM-based PCL radar system. [ABSTRACT FROM AUTHOR]

This paper proposes a methodology for automatic, accurate, and early detection of amplitude ionospheric scintillation events, based on machine learning algorithms, applied on big sets of 50 Hz postcorrelation data provided by a global navigation satellite system receiver. Experimental results on real data show that this approach can considerably improve traditional methods, reaching a detection accuracy of 98%, very close to human-driven manual classification. Moreover, the detection responsiveness is enhanced, enabling early scintillation alerts. [ABSTRACT FROM AUTHOR]

Tracking with bistatic sonar or radar measurements is challenging due to the fact that the measurements are nonlinear functions of the Cartesian state. The performance of existing approaches, including the extended Kalman filter and sigma point Kalman filters, such as the unscented Kalman filter and cubature Kalman filter, may not be acceptable in terms of mean square error or tracker consistency (i.e., the tracker's state estimation error covariance is not statistically consistent with the actual estimation errors). This paper generalizes the sigma point Kalman filter (SPKF) as a converted measurement SPKF. The resulting converted measurement sigma point Kalman filter is demonstrated to have improved performance over the conventional SPKF when employed in a bistatic tracking scenario. [ABSTRACT FROM AUTHOR]

Future space missions will be driven by factors such as the need for reduced cost of spacecraft without diminished performance, new services, and capabilities including reconfigurability, autonomous operations, target observation with improved resolution and servicing (or proximity) operations. Small satellites, deployed as a sensor network in space, can through intersatellite communication (ISC) enable the realization of these future goals. Developing the communication subsystem that can facilitate ISC within this distributed network of small satellites require a complex range of design tradeoffs. For small satellites, the general design parameters that are to be optimized for ISC are size, mass, and power, as well as cost (SMaP-C). Novel and efficient design techniques for implementing the communication subsystem are crucial for building multiple small satellite networks with capability for achieving significant data-rates along the intersatellite links (ISLs). In this paper, we propose an alternative approach to radio frequency and laser ISLs for ISC among small satellites deployed as a sensor network in low earth orbit. For short to medium range ISLs, we present an LED-based visible light communication (VLC) system that addresses the SMaP constraints, including capability for achieving significant data rates. Our research is focused on the development of the physical layer for pico-/nano class of satellites with prime consideration for the impact of solar background illumination on link performance. We develop an analytical model of the intersatellite link (ISL) in MATLAB and evaluate its feasibility and performance for different intensity modulation and direct detection schemes. Using a transmitted optical power of 4 W and digital pulse interval modulation, a receiver bandwidth requirement of 3.5 MHz is needed to achieve a data rate of 2.0 Mbits/s over a moderate link distance of 0.5 km at a BER of 10−6. [ABSTRACT FROM AUTHOR]

Frequency diverse array (FDA) has received much attention, but little work about FDA receiver architectures and receiver processing algorithms have been reported. In this paper, we analyze the localization performance of FDA radar with a two-stage estimator. Three FDA receive architectures are discussed and compared with the phased-array (PA). In order to overcome disadvantages of the monochromatic FDA radar signal model adopted in the literature that ignores the coherent transmit gain in FDA radar, we formulate a bandwidth model for a general FDA radar received signals including both nonoverlapping and overlapping spectra. The localization performance are assessed by the Cramér–Rao bound (CRB) and receive signal noise ratio. The proposed model provides more accurate representation of FDA radar received signals and more reasonable comparisons between FDA and PA radars. Additionally, a fast two-stage estimator is proposed to reduce the two-dimensional multiple signal classification computation complexity. All proposed methods are verified by both theoretical analysis and numerical results. [ABSTRACT FROM AUTHOR]

Over the past decade, precise point positioning (PPP) has become a tool widely used in many Global Navigation Satellite Syste applications and the performance levels of the method are often close to those that can be achieved through a differenced approach. The aim of this paper is to analyze the dependency of the precision of a PPP solution output from the GIPSY-OASIS II software on the observing-session durations. In detail, starting from real data acquisitions spanning 24, 12, 6, 3 h, 1 h, and ½ an hour, all processed through the PPP, a continuous function has been defined and the related coefficients have been estimated. The original dataset consists of daily RINEX files provided from 44 sites of both International GNSS Service (IGS) and European Permanent Network (EPN) permanent networks acquired over one year. Each RINEX file was split into several shorter files according to the above listed time spans and processed by using GIPSY-OASIS II together with jet propulsion laboratory (JPL) precise post-processed products. The uncertainty of the proposed function was also estimated and a complete analysis of its compliance with the data sample has been provided. The estimated model is demonstrated to reach a millimeter accuracy level within a statistical confidence level of 99% using the experimental data. [ABSTRACT FROM AUTHOR]

This paper presents a novel filtering technique to estimate the position of a moving target based on discrete-time direction and velocity measurements. The velocity is assumed to be corrupted by an unknown constant bias, which is explicitly estimated in the process. A nonlinear system is first designed, describing the dynamics and observations associated to the target, followed then by a state augmentation that yields an equivalent linear time-varying system. An observability analysis for the latter is conducted based on necessary and sufficient conditions that are related to the target's motion. The final estimation solution resorts to a Kalman Filter with globally exponentially stable error dynamics. Its performance is assessed via realistic numerical simulations, including Monte Carlo runs and a comparison with both the standard extended Kalman filter and the Bayesian Cramér–Rao bound. A set of experimental results achieved within the scope of a realistic underwater mission scenario is also presented that allows to further assess the proposed technique. [ABSTRACT FROM AUTHOR]

In this paper, we propose a low-complexity adaptive reduced-rank interference suppression scheme based on alternating low-rank decomposition techniques for navigation receivers. The proposed scheme, which employs a generalized sidelobe canceler structure, makes use of a projection matrix for rank reduction followed by a reduced-dimension receive filter. An alternating optimization strategy based on recursive least squares is devised to compute the basis vectors of the projection matrix and the receive filter adaptively. Simulation results show that the proposed algorithm achieves a better performance than existing methods with a reduced computational complexity. [ABSTRACT FROM AUTHOR]

In this paper, an extended fast factorized back projection (FFBP) algorithm is proposed to focus the missile-borne bistatic forward-looking synthetic aperture radar (BFSAR) data. It is also based on the subaperture processing technique to reduce the computational burden, but represents the subimages on the elliptical polar coordinates referenced to the tracks of both transmitter and receiver. First, the imaging geometry and signal model of the missile-borne BFSAR are established. Then, considering the special configuration of the missile-borne BFSAR, a novel elliptical polar coordinate system whose origin is determined by the positions of both transmitter and receiver is established. The subaperture processing approach based on this coordinate system is presented. Successively, based on the range error analysis, the sampling requirements of the elliptical polar subimages are derived to provide a tradeoff between the imaging quality and efficiency. Finally, the computational burden of the proposed algorithm is discussed, and the speed-up factor of the proposed algorithm with respect to the direct back projection algorithm is derived. The experimental results with simulated data prove the validity and feasibility of the proposed algorithm. [ABSTRACT FROM AUTHOR]

This paper employs a special property of the Hadamard matrix to develop a novel compressive sensing (CS) based framework that eliminates the complex reconstruction step required by standard CS methods. In particular, time-difference-of-arrivals (TDOAs) of a signal at different receivers involved in positioning of unknown sources are directly estimated from CS measurements, without any attempt to reconstruct the full signals. Although this approach is designed for TDOA estimation, it is also capable of accommodating other CS applications. [ABSTRACT FROM AUTHOR]

In this paper, the distortion and defocus effects due to residual phase errors that are inherent in bistatic polar format algorithm are analyzed using time-domain Taylor series decomposition. A two-dimensional interpolation-based distortion correction method is developed, and accurate scene sizes limits are derived to mitigate the effects of defocus. The phase analysis is conducted with respect to arbitrary geometries, and examples are demonstrated for ideal linear and circular flight trajectories using a point target simulation. [ABSTRACT FROM AUTHOR]

In this paper, we investigate transmit beampattern optimization techniques for a multiple-input multiple-output radar in the presence of a legitimate communications receiver and an eavesdropping target. The primary objectives of the radar are to satisfy a certain target-detection criterion and to simultaneously communicate safely with a legitimate receiver by maximizing the secrecy rate against the eavesdropping target. Therefore, we consider three optimization problems, namely target return signal-to-interference-plus-noise ratio maximization, secrecy rate maximization, and transmit power minimization. However, these problems are nonconvex due to the nonconcavity of the secrecy rate function, which appears in all three optimizations either as the objective function or as a constraint. To solve this issue, we use Taylor series approximation of the nonconvex elements through an iterative algorithm, which recasts the problem as a convex problem. Two transmit covariance matrices are designed to detect the target and convey the information safely to the communication receiver. Simulation results are presented to validate the efficiency of the aforementioned optimizations. [ABSTRACT FROM AUTHOR]

A distributed multiple-input multiple-output (MIMO) radar system is capable of tracking multiple targets under the “defocused transmit-focused receive” (DTFR) operating mode, in which each transmitter forms a completely defocused beam to illuminate the whole surveillance region and each receiver adopts a focused beam to attain a higher resolution. However, when operating in this mode, there exists a resource optimization problem on the allocation of receive-beams. To address this problem, a receive-beam resource allocation (RBRA) strategy is proposed in this paper. The key mechanism is to implement the optimal allocation between receive-beams and targets based on the feedback information in the tracking recursion cycle, with the objective of improving the worst tracking accuracy with multiple targets. Since the posterior Cramér–Rao lower bound (PCRLB) provides a lower bound on the accuracy of target state estimates, it is derived and adopted as an optimization criterion. It is shown that the optimal RBRA is a multidimensional nonconvex assignment problem that is NP-hard. We propose an efficient convex relaxation optimization to solve it. Numerical results demonstrate the superior performance of the proposed strategy in terms of the worst case tracking root mean-square errors. [ABSTRACT FROM AUTHOR]

This paper handles the problem of optimizing the scanning policy of a surveillance system in the presence of Rayleigh targets in a mixture of K-distributed clutter and Gaussian noise. Adopting the detection rate (DR) and false alarm rate (FAR) as performance measures, we show that the said optimization amounts to solving a constrained DR maximization, wherein the free parameters are the transmit beamwidth, the dwell time, and the detection threshold, whereas the constraints concern the maximum FAR and time on target. A thorough performance assessment is offered, aimed at eliciting the influence of the signal-to-noise and clutter-to-noise ratios, the clutter shape parameter, and the rate of change of the target response. The results indicate that, although a uniformly optimum transmit policy does not exist, some trends can be devised, which appears of particular relevance especially for systems that can avail themselves of some degree of cognition. [ABSTRACT FROM AUTHOR]