Showing papers in "IEEE Transactions on Aerospace and Electronic Systems in 2022"

Refracting RIS-Aided Hybrid Satellite-Terrestrial Relay Networks: Joint Beamforming Design and Optimization

Abstract: Reconfigurable intelligent surface (RIS) has been viewed as a promising solution in constructing reconfigurable radio environment of the propagation channel and boosting the received signal power by smartly coordinating the passive elements’ phase shifts at the RIS. Inspired by this emerging technique, this article focuses on joint beamforming design and optimization for RIS-aided hybrid satellite-terrestrial relay networks, where the links from the satellite and base station (BS) to multiple users are blocked. Specifically, a refracting RIS cooperates with a BS, where the latter operates as a half-duplex decode-and-forward relay, in order to strengthen the desired satellite signals at the blocked users. Considering the limited onboard power resource, the design objective is to minimize the total transmit power of both the satellite and BS while guaranteeing the rate requirements of users. Since the optimized beamforming weight vectors at the satellite and BS, and phase shifters at the RIS are coupled, leading to a mathematically intractable optimization problem, we propose an alternating optimization scheme by utilizing singular value decomposition and uplink–downlink duality to optimize beamforming weight vectors, and using Taylor expansion and penalty function methods to optimize phase shifters iteratively. Finally, simulation results are provided to verify the superiority of the proposed scheme compared to the benchmark schemes.

SLNR-based Secure Energy Efficient Beamforming in Multibeam Satellite Systems

Abstract: Motivated by the fact that both security and energy efficiency are the fundamental requirements and design targets of future satellite communications, this letter investigates secure energy efficient beamforming in multibeam satellite systems, where the satellite user in each beam is surrounded by an eavesdropper attempting to intercept the confidential information. To simultaneously improve the transmission security and reduce power consumption, our design objective is to maximize the system secrecy energy efficiency (SEE) under the constraint of total transmit power budget. Different from the existing schemes with high complexity, we propose an alternating optimization scheme to address the SEE problem by decomposing the original nonconvex problem into subproblems. Specifically, we first utilize the signal-to-leakage-plus-noise ratio (SLNR) metric to obtain closed-form normalized beamforming weight vectors, while the successive convex approximation (SCA) method is used to efficiently solve the power allocation subproblem. Then, an iterative algorithm is proposed to obtain the suboptimal solutions. Finally, simulation results are provided to verify the superiority of the proposed scheme compared to the benchmark schemes.

Nonfragile Quantitative Prescribed Performance Control of Waverider Vehicles With Actuator Saturation

Abstract: The existing prescribed performance control (PPC) strategies exhibit the fragility and nonguarantee of the prescribed performance when they are applied to dynamic systems with actuator saturation, and moreover, all of them are unable to quantitatively design prescribed performance. This article aims at remedying those deficiencies by proposing a new nonfragile PPC method for waverider vehicles (WVs) such that the quantitative prescribed performance can be guaranteed for tracking errors in the presence of actuator saturation. First, readjusting performance functions are developed to achieve quantitative prescribed performance and prevent the fragile problem. Then, low-complexity fuzzy neural control protocols are presented for velocity subsystem and altitude subsystem of WVs, while there is no need of recursive back-stepping design. Furthermore, auxiliary systems are designed to generate effective compensations on control constraints, which contributes to the guarantee of the desired prescribed performance, being proved via Lyapunov synthese. Finally, compared simulation results are given to validate the superiority.

Joint Transmit Resource Management and Waveform Selection Strategy for Target Tracking in Distributed Phased Array Radar Network

Abstract: In this article, a joint transmit resource management and waveform selection (JTRMWS) strategy is put forward for target tracking in distributed phased array radar network. We establish the problem of joint transmit resource and waveform optimization as a dual-objective optimization model. The key idea of the proposed JTRMWS scheme is to utilize the optimization technique to collaboratively coordinate the transmit power, dwell time, waveform bandwidth, and pulse length of each radar node in order to improve the target tracking accuracy and low probability of intercept (LPI) performance of distributed phased array radar network, subject to the illumination resource budgets and waveform library limitation. The analytical expressions for the predicted Bayesian Cramér–Rao lower bound and the probability of intercept are calculated and subsequently adopted as the metric functions to evaluate the target tracking accuracy and LPI performance, respectively. It is shown that the JTRMWS problem is a nonlinear and nonconvex optimization problem, where the above four adaptable parameters are all coupled in the objective functions and constraints. Combined with the particle swarm optimization algorithm, an efficient and fast three-stage-based solution technique is developed to deal with the resulting problem. Simulation results are provided to verify the effectiveness and superiority of the proposed JTRMWS algorithm compared with other state-of-the-art benchmarks.

Multispectrally Constrained MIMO Radar Beampattern Design via Sequential Convex Approximation

Abstract: This article deals with the multiple-input–multiple-output (MIMO) radar beampattern design in an effort to the coexistence with multiple communication systems. A waveform optimization model accounting for the minimization of the beampattern integrated sidelobe level (ISL) along with the mainlobe width, peak-to-average power ratio, and energy constraints, as well as multispectral requirements where the interference energy injected by the MIMO radar in each shared frequency band in a particular direction, is precisely controlled to ensure the desired quality of service at each communication system. Through an equivalent reformulation of the original nonconvex problem, a polynomial-time sequential convex approximation (SCA) procedure that involves the tackling of a series of constrained convex problems is proposed to monotonically decrease the ISL with the convergence guaranteed to a Karush–Kuhn–Tucker point. Herein, to speed up the convergence, a fast iterative algorithm based on the alternating-direction-method-of-multipliers framework is introduced to globally solve the convex problems during each iteration of the SCA procedure. Numerical results are provided to assess the proposed algorithm in terms of the computational complexity, the achieved beampattern, and spectral compatibility with some competitive counterparts available in the open literatures.

On Parameter Identifiability of Diversity-Smoothing-Based MIMO Radar

Abstract: Diversity smoothing has been widely used for angle estimation with multiple input multiple output (MIMO) radar in the presence of coherent or correlated targets, and the parameter identifiability is an interesting issue. Previous works have shown sufficient conditions for some special cases, such as the coherent targets with conventional MIMO radar, and the array size are derived as a function of the target number and target structure. In this article, we further introduce the interelement spacings to build a complete parameter identifiability scheme. For monostatic MIMO radars, the antenna numbers and interelement spacings of transmit and receive arrays are derived as functions of the target number and the target structure. The optimization models are built to calculate the maximum number of detectable targets for a given target structure. Additionally, the conditions for the bistatic MIMO radar are derived from two-dimension viewpoint. It is shown that the new results improve upon previous spatial smoothing or diversity smoothing methods and recover them in special cases. Simulation results are presented that corroborate our theoretical findings.

The First Carrier Phase Tracking and Positioning Results With Starlink LEO Satellite Signals

Abstract: This letter shows the first carrier phase tracking and positioning results with Starlink’s low earth orbit (LEO) satellite signals. An adaptive Kalman filter based algorithm for tracking the beat carrier phase from the unknown Starlink signals is proposed. Experimental results show carrier phase tracking of six Starlink satellites and a horizontal positioning error of 7.7 m with known receiver altitude.

Predefined-Time Predefined-Bounded Attitude Tracking Control for Rigid Spacecraft

Abstract: In this article, we consider the attitude tracking control problem for rigid spacecraft with bounded external disturbances. We propose a predefined-time predefined-bounded attitude tracking control scheme based on a nonsingular predefined-time sliding-mode manifold. The proposed controller is continuous and it can achieve predefined-time predefined-bounded stability. That is, the attitude tracking errors are driven to a predefined-bounded region around the origin within a predefined time, which can be set as a tuning parameter during the controller design, independently of initial conditions. Finally, numerical simulations are carried out to evaluate the performance of the proposed control law.

Active Disturbance Rejection Control for Delayed Electromagnetic Docking of Spacecraft in Elliptical Orbits

Abstract: In this article, an active disturbance rejection control (ADRC) scheme is proposed for the electromagnetic docking of spacecraft in the presence of time-varying delay, fault signals, external disturbances, and elliptical eccentricity. By introducing an auxiliary variable, an intermediate observer (IO) is presented to estimate the relative motion information and the total disturbance resulting from fault signals, unknown mass, external disturbances, and elliptical eccentricity. Then, an ADRC scheme is developed to guarantee that the relative position, relative velocity, and the estimation errors of relative motion information and the total disturbance can all converge into the neighborhood of the equilibrium. With the proposed control scheme, the time-delay factor is fully considered in the controller design to avoid adverse effect, and the uniform ultimate boundedness stability of the entire closed-loop system is analyzed with a rigorous theoretical proof. In comparison to conventional ADRC and other state-of-the-art schemes, the proposed ADRC scheme has several advantages, e.g., high accuracy, strong robustness, and requires no prior knowledge of the fault, time-varying delay and states information due to IO-based relative motion information and total disturbance estimations. Finally, numerical simulations are performed to show the effectiveness of the proposed control scheme.

Appointed Fixed Time Observer-Based Sliding Mode Control for a Quadrotor UAV Under External Disturbances

Abstract: In this article, a study of the appointed fixed time control problem is presented for the quadrotor unmanned aerial vehicle attitude control system subject to external disturbances. Based on the appointed fixed time sliding mode variable and the adaptive technique, a novel sliding mode observer is first established to estimate the external disturbances within the appointed settling time. Subsequently, an appointed fixed time controller is proposed to track the desired attitude regardless of the initial states. To circumvent the singularity problem of the above controller, a novel improved adaptive sliding mode control law is designed by employing a nonlinear function and a modified sliding mode observer. Moreover, the chattering phenomenon is attenuated by replacing the sign function with hyperbolic tangent function. The appointed practical fixed time stability of the closed-loop attitude control system is proved and analyzed rigorously utilizing the Lyapunov theory. Finally, the effectiveness and fine performance of the proposed schemes are illustrated by numerical simulation results.

Adaptive Finite-Time Backstepping Global Sliding Mode Tracker of Quad-Rotor UAVs Under Model Uncertainty, Wind Perturbation, and Input Saturation

Abstract: In this article, an adaptive backstepping global sliding mode control method is designed at the aim of finite-time tracking control for attitude and position of the quad-rotor unmanned aerial vehicles in the existence of input saturation, model uncertainty, and wind perturbation. Whereas the existence of the input saturation leads to the complexity in the controller design, a compensation system is considered and applied in order to remove the limitations in the controller design. Then, a global sliding surface based on the tracking error, fast reaching part, and integral of virtual control input is considered. Also, for the approximation of the unknown upper bounds of model uncertainty and wind perturbation, an adaptive control procedure is used. Besides, the finite-time tracking control of desired attitude and position of quad-rotor is proved based on the Lyapunov theory concept using backstepping control process. At last, simulation results are provided in two different scenarios to show the effectiveness of the proposed method.

NN-Based Fixed-Time Attitude Tracking Control for Multiple Unmanned Aerial Vehicles with Nonlinear Faults

Abstract: This paper focuses on the fixed-time fault-tolerant attitude tracking control problem for multiple unmanned aerial vehicles (MUAVs) with nonaffine nonlinear faults. First, the command filter and neural networks (NNs) are employed to characterize unknown nonlinearities in MUAVs, and the update law of NN is developed via convex optimization technique. Second, the algebraic loop problem caused by nonaffine nonlinear faults is adequately solved by introducing the Butterworth low-pass filter. Then, the curve-fitting method is utilized to construct a piecewise virtual control signal to avoid the singularity problem in fixed-time control. Furthermore, based on the Lyapunov stability theory, a general fixed-time stability criterion is adopted to prove that the designed fault-tolerant attitude controller can guarantee the stability of MUAVs in fixed time. Finally, the effectiveness of the proposed control design method is verified via illustrative examples.

INTERLINK: A Digital Twin-Assisted Storage Strategy for Satellite-Terrestrial Networks

Abstract: Recently, low-orbit satellite networks have gained lots of attention from the society due to their wide coverage, low transmission latency, and storage and computing capacity. Providing seamless connectivity to users in different areas is envisioned as a promising solution, especially in remote areas and for marine communication. However, when jointly used with terrestrial networks composing satellite-terrestrial networks, the satellite moving speed is much faster than the ground terminal, which can cause inconsistent service from a single satellite, and therefore lead to frequent satellite handover. Moreover, due to the dynamic and time slot visibility of satellites, the topology of an intersatellite changes frequently, which results in loops during satellite handover, thereby reducing the utilization of links. To address these problems, we propose a digital twin-assisted storage strategy for satellite-terrestrial networks (INTERLINK), which leverages the digital twins (DTs) to map the satellite networks to virtual space for better communication. Specifically, we first propose a satellite storage-oriented handover scheme to minimize the handover frequency by considering the limited access time and capacity constraints of satellites. Then, a multiobjective optimization problem is formulated to obtain the optimal satellite by genetic algorithm. Finally, considering the timing visibility of the satellite links, a digital twin-assisted intersatellite routing scheme is introduced to improve the quality of data delivery between satellites. Simulation results demonstrate that the proposed INTERLINK can reduce both handover times and average propagation delay compared with its counterparts. Meanwhile, benefitting from integrated DT, both the quality of data delivery and the delay of intersatellite links are considerably improved.

3-D Near-Field Source Localization Using a Spatially Spread Acoustic Vector Sensor

Abstract: This article presents a new method for 3-D localization of an acoustic source signal by estimating its azimuth-angle, elevation-angle, and radial range. The proposed method exploits a spatially spread acoustic vector sensor, which is composed of a tri-axial velocity vector sensor and an isotropic pressure sensor. Unlike the spatially collocated case, the self-normalization of spatially spread acoustic vector sensor’s response is no longer independent of the source location, resulting in the inapplicability of the widely used “self-normalization” estimators (A. Nehorai et al. , 1994), (V. N. Hari et al. , 2012), (Y. Song et al. , 2013). The present article considers the near-field propagation’s path attenuation and phase difference among the sensor components and derives a source location estimation method without resorting to the “self-normalization” operation. Also, the proposed method is applicable to any nonfree-space propagation models at arbitrarily unknown path-loss exponent. In comparison with the existing methods, where the velocity vector sensor is spatially collocated, the proposed method can offer improved performance estimation due to the inherent extension of the vector sensor’s spatial aperture in the spread structure.

Prescribed Time Attitude Tracking Control of Spacecraft With Arbitrary Disturbance

Abstract: The problem of developing a controller for spacecraft to accomplish attitude maneuvers within a prescribed time, independently of the initial states, is addressed. A fixed-time disturbance observer-based control is established and proved to drive the error state to zero, in a prescribed time, in the presence of modeling and external uncertainties. The key advantages of the proposed control are that the gains are explicitly determined by the prescribed maneuvering time only and that it is also robust to instantaneous disturbances. The control approach enables an efficient design process for attitude control applications with time constraints. Simulation results are presented for the attitude control of a rigid spacecraft to verify the benefits of this control scheme.

Hierarchical Domain-Based Multicontroller Deployment Strategy in SDN-Enabled Space–Air–Ground Integrated Network

Abstract: The space–air–ground integrated network (SAGIN) is considered to be a significant framework for realizing the vision of “6G intelligent connection of all things.” A typical SAGIN consists of three parts: a space-based network composed of various orbiting satellites, an air-based network composed of aircraft, and a traditional ground-based network. Considering the cost of satellite launch, the network needs to be flexible and controllable. In order to ensure that the ground can handle satellite anomalies in real time by program, it is necessary to introduce in-orbit programmable networks, such as the software-defined network (SDN). In the network management architecture, if the controller plane in the SDN adopts the flat management scheme, the expansion of the control plane is limited due to the low efficiency of data synchronization among controllers. Compared with controller deployments on terrestrial networks, multicontroller deployments in the SAGIN face the following problems: the dynamic change of the satellite network topology, the large-scale network nodes, the increase or decrease in the number of aerial vehicles, and the unbalanced distribution of ground users. Therefore, it is of great significance to study how to optimize the deployment of multiple controllers in the SDN-enabled SAGIN. This article introduces an SDN into the SAGIN and designs a hierarchical domain-based SDN-enabled SAGIN architecture. A multicontroller deployment strategy for the hierarchical domain-based SDN-enabled SAGIN is proposed. First, we divide the SDN control plane into two layers, i.e., the primary controller layer is deployed on the ground network and the secondary on the space-based network. The SDN data plane is composed of space-based, air-based, and ground-based networks. Second, considering the average network delay and the controller load, a multiobjective optimization model is constructed. To determine the number of controllers and the relative positions of switch nodes and controllers, the clustering algorithm based on k -means is adopted to initially divide the data plane. Finally, to improve the global search ability of the algorithm, a multiobjective optimization algorithm based on a genetic algorithm is adopted. The simulation results show that the proposed strategy is effective in reducing the average network delay and improving the controller load balance. Compared to other algorithms, the average network delay is reduced by 13.3% and the controller load is improved by 10.33%.

On the Achievability of Submeter-Accurate UAV Navigation With Cellular Signals Exploiting Loose Network Synchronization

Abstract: A framework that could achieve submeter-level unmanned aerial vehicle (UAV) horizontal navigation in multipath-free environments with cellular carrier phase measurements is developed. This framework exploits the “loose” synchronization between cellular base transceiver station (BTS) clocks. It is shown through extensive experimental data that the beat frequency stability of cellular BTSs approaches that of atomic standards and that the clock deviations can be realized as a stable autoregressive moving average model. This BTS clock model is referred to as loose network synchronization. A rule-of-thumb is established for clustering the clock deviations to minimize the position estimation error, while significantly reducing the computational complexity. The presented models allow the UAV to achieve sustained carrier phase-based meter- to submeter-accurate navigation. To demonstrate the efficacy of the developed framework, this article presents three UAV flight experiments in Southern California, USA, utilizing signals from different cellular providers transmitting at different frequencies. The three experiments took place in open, semiurban environments with nearly multipath-free, line-of-sight (LOS) conditions, in which the UAV traveled 1.72, 3.07, and 0.61 km, achieving a horizontal position root mean squared error of 36.61, 88.58, and 89.33 cm, respectively, with respect to the UAV’s on-board navigation system.

INTERLINK: A Digital Twin-Assisted Storage Strategy for Satellite-Terrestrial Networks

Abstract: Recently, low-orbit satellite networks have gained lots of attention from the society due to their wide coverage, low transmission latency, and storage and computing capacity. Providing seamless connectivity to users in different areas is envisioned as a promising solution, especially in remote areas and for marine communication. However, when jointly used with terrestrial networks composing satellite-terrestrial networks, the satellite moving speed is much faster than the ground terminal, which can cause inconsistent service from a single satellite, and therefore lead to frequent satellite handover. Moreover, due to the dynamic and time slot visibility of satellites, the topology of an intersatellite changes frequently, which results in loops during satellite handover, thereby reducing the utilization of links. To address these problems, we propose a digital twin-assisted storage strategy for satellite-terrestrial networks (INTERLINK), which leverages the digital twins (DTs) to map the satellite networks to virtual space for better communication. Specifically, we first propose a satellite storage-oriented handover scheme to minimize the handover frequency by considering the limited access time and capacity constraints of satellites. Then, a multiobjective optimization problem is formulated to obtain the optimal satellite by genetic algorithm. Finally, considering the timing visibility of the satellite links, a digital twin-assisted intersatellite routing scheme is introduced to improve the quality of data delivery between satellites. Simulation results demonstrate that the proposed INTERLINK can reduce both handover times and average propagation delay compared with its counterparts. Meanwhile, benefitting from integrated DT, both the quality of data delivery and the delay of intersatellite links are considerably improved.

Acquisition, Doppler Tracking, and Positioning With Starlink LEO Satellites: First Results

Abstract: This letter shows the first acquisition, Doppler tracking, and positioning results with Starlink's low Earth orbit satellite signals. A generalized-likelihood-ratio-based test is proposed to acquire Starlink's downlink signals. A Kalman-filter-based algorithm for tracking the Doppler frequency from the unknown Starlink signals is developed. Experimental results show Doppler tracking of six Starlink satellites, achieving a horizontal positioning error of 10 m.

Fusion Recognition of Space Targets With Micromotion

Abstract: During the observation of micromotion targets in space, inverse synthetic aperture radar usually obtains the narrowband and wideband echoes simultaneously. In order to exploit their rich information in target electromagnetic scattering, shape, structure, and motion, this article proposes a recognition method of space micromotion targets based on decision fusion. The proposed method extracts physical features from the radar cross section and the joint time–frequency distribution from narrowband echoes, while extracts the data features from high-resolution range profiles and range-instantaneous-Doppler image by convolution neural network. Finally, particle swarm optimization is adopted to decision-level fusion so as to realize high-precision recognition. The recognition results of electromagnetic simulated data under various conditions have demonstrated the effectiveness and robustness of the proposed method.

Cost-Effective and Highly Reliable Circuit-Components Design for Safety-Critical Applications

Abstract: With the reduction of technology nodes now reaching 2 nm, circuits become increasingly susceptible to external perturbations. Thereby, soft errors, such as single-node-upset (SNU), single-event-transient (SET), double-node-upset (DNU), and even triple-node-upset (TNU), must be considered for safety-critical applications. This article first presents four advanced circuit components (i.e., advanced voters), that have very small overhead compared with the traditional voters. The proposed Advanced Triple-Modular-Redundancy (ATMR) and Advanced Quadruple-Modular-Redundancy (AQMR) voters only consist of four and six inverters, respectively, to provide effective tolerance against SNUs and DNUs. To further filter SETs, a Schmitt-trigger (ST) instead of an inverter at the output-level is used to construct the ATMR-ST and AQMR-ST voters. These proposed voters can also be extended to tolerate TNUs. Next, these voters are used for latch hardening, so that this article also presents a series of voter-based latch designs, to ensure high reliability with cost-effectiveness. Simulation results demonstrate the node-upset tolerance and/or SET-filterability of the proposed voters and voter-based latches, respectively. Simulation results also demonstrate that the proposed ATMR voter can reduce delay, power, and area by 55.2, 32.8, and 32.2%, respectively, compared with the traditional TMR voter; the proposed so-called high-impedance state-insensitive, TNU-tolerant, and SET-filterable latch can reduce delay, power, and area by 78.9, 15.8, and 28.6%, respectively, compared with the state-of-the-art TNU hardened latch.

BeiDou Satellite Radiation Force Models for Precise Orbit Determination and Geodetic Applications

Abstract: China's BeiDou satellite navigation system (BDS) has completed its full constellation in orbit since June 2020. Services have been evolved from regional (BDS-2) to global (BDS-3). This contribution evaluates the impact of solar radiation pressure (SRP) modeling on satellite orbits and geodetic parameters. To that end, we process 2 years of BDS observations (2019-2021), collected by a network of 100 ground stations. A physical a priori box-wing model based on the estimated optical properties is introduced. Various physical effects, such as radiator emission and thermal radiation of solar panels are considered. The ECOM (Empirical CODE orbit Model), ECOM+along-track and ECOM2 models are employed on top in the experiment. We show that without the use of the a priori box-wing model, the ECOM+along-track model shows clear better orbit solutions during eclipse seasons for BDS-3 satellites. This is proven to be mainly due to the thermal radiation of the solar panels. However, the along-track acceleration is highly correlated with LOD (length of day) and ECOM parameters. LOD estimates in this case are contaminated. When using the physical a priori box-wing model satellite orbital errors are greatly reduced for all the ECOM models. For instance, orbit misclosures of BDS-3 CAST (China Academy of Space Technology) satellites improve by a factor of two for the ECOM model during eclipse seasons. Furthermore, the use of the a priori box-wing model mitigates a great majority of the spurious signals in the geodetic parameters.

Slow-Time FDA-MIMO Technique With Application to STAP Radar

Abstract: Unlike the conventional phased array, the frequency-diverse array (FDA) employs a tiny frequency increment across the array elements, which is capable of providing range–angle–time-dependent beampattern, and thereby offers potential benefits in target localization and interference mitigation. Reported literature on FDA mostly focuses on its intrapulse (fast-time) property and ignores the interpulse (slow-time) feature induced by frequency offset. In this article, incorporating the neglected inherent slow-time coding feature, a novel slow-time FDA multiple-input multiple-output (FDA-MIMO) technique is proposed for the space-time adaptive processing (STAP) radar. The new coding scheme can separate transmitting (Tx) signals via slow-time Doppler filtering, and the problem of signal aliasing is tackled by appropriately designing the slow-time codes. At the receiving (Rx) side, two Rx schemes are devised for recovering range-dependent Tx degrees-of-freedom. Moreover, the relevant clutter rank and the rule for code design are derived in detail. The outstanding merits of the proposed slow-time FDA-MIMO STAP radar consist of the following: There is no specific restriction on the probing waveform; the negative effect of time-variant pattern is perfectly eliminated; and both the target localization accuracy and clutter suppression performance are superior over the state-of-the-art FDA radar systems. Numerical results corroborate the superiorities of the proposed waveform strategy for STAP application.

Hand Gesture Recognition Using Radial and Transversal Dual Micromotion Features

Abstract: Most of the work in hand gesture recognition (HGR) focuses on developing diverse classification algorithms based on micro-Doppler (mD) spectrogram, that is 1-D motion along the radial direction. In this work, we exert effort on the radar system and preprocessing methods to extract 2-D motions for HGR. Specifically, we utilize an interferometric radar with two widely spaced receivers to obtain both radial and transversal micromotion features of hand gestures. In the preprocessing stage, as pre-interferometry is nonlinear multiplication in time domain, both the increased noise level and unwanted cross-terms may reduce its usefulness for HGR. To solve these problems, we propose a post-interferometric preprocessing method in frequency domain, which is capable of reducing noise level of the obtained spectrogram and suppressing the nuisance cross-terms. We measure four pairs of symmetric hand gestures from three persons and compare the HGR accuracy using different preprocessing methods. Experimental results show that the mD processing combined with post-interferometry give the highest HGR accuracy of over 99%.

Phase-Time Method: Accurate Doppler Measurement for Iridium NEXT Signals

Abstract: Opportunistic positioning utilizing low Earth orbit satellite (LEO) signals mostly adopts Doppler positioning, the performance of which largely depends on the accuracy of Doppler measurement at the receiver. Traditional Doppler measurement methods for Iridium NEXT signals are limited and mostly implemented in frequency domain, which cannot avoid the limitation of fast Fourier transform operation. Aiming at this problem, this article proposes a method achieving accurate Doppler measurement of Iridium NEXT signal in time domain, namely phase-time method. This method achieves accurate Doppler measurement by measuring the change rate of the signal phase over time. Experiments were implemented using real Iridium NEXT signals, and the results have demonstrated that compared with the existing method, the Doppler measurement value obtained by the phase-time method possesses higher accuracy. Taking the Doppler measurement values obtained by the phase-time method as observations, the stability and reliability of Doppler positioning can be significantly improved. The proposed phase-time method is of great significance to LEO-signal frequency estimation in time domain, and further contributes to opportunistic positioning using LEO constellations.

Finite-Time Prescribed Performance Control for Space Circumnavigation Mission With Input Constraints and Measurement Uncertainties

Abstract: This article presents a 6-DOF attitude-orbit synchronous control problem for the space circumnavigation (SCN) mission with parameter uncertainties, time-varying uncertainties, and input constraints. In particular, from the perspective of engineering application, time-varying measurement uncertainties are taken into account of the 6-DOF attitude–orbit coupling kinematics and dynamics, and the analytical solution of the desired attitude is derived based on the measured relative orbit information with measurement uncertainties. To drive the active spacecraft approach to the faulty target safely, a time-varying exponential prescribed convergence boundary is introduced into the sliding surface. A finite-time disturbance observer is involved in equivalent tracking errors for compensating the mismatched uncertainties. In addition, an auxiliary system is designed to overcome the instability danger caused by input constraints. The stability of the controlled system is discussed in the nonautonomous finite-time stable framework, which is proved via Lyapunov analysis that the attitude-orbit tracking errors converge to the equilibrium within finite time. The simulation experiment with mismatched uncertainties and prescribed constraints shows the superiority of the designed control scheme.

Performance of UAV-Assisted Multiuser Terrestrial-Satellite Communication System Over Mixed FSO/RF Channels

Abstract: In this letter, performance of a multiantenna multiuser unmanned aerial vehicle (UAV)-assisted terrestrial-satellite communication system over mixed free space optics (FSO)/radiofrequency (RF) channels is analyzed. Downlink transmission from the satellite to the UAV is completed through FSO link which follows gamma–gamma distribution with pointing error impairments. Both the heterodyne detection and intensity modulation direct detection techniques are considered at the FSO receiver. To avail the antenna diversity, multiple transmit antennas are considered at the UAV. Selective decode-and-forward scheme is assumed at the UAV and opportunistic user scheduling is performed while considering the practical constraints of outdated channel state information (CSI) during the user selection and transmission phase. The RF links are assumed to follow Nakagami-m distribution due to its versatile nature. In this context, for the performance analysis, analytical expressions of outage probability, asymptotic outage probability, ergodic capacity, effective capacity, and generalized average symbol-error-rate expressions of various quadrature amplitude modulation (QAM) schemes such as hexagonal-QAM, cross-QAM, and rectangular QAM are derived. A comparison of various modulation schemes is presented. Further, the impact of pointing error, number of antennas, delay constraint, fading severity, and imperfect CSI are highlighted on the system performance. Finally, all the analytical results are verified through the Monte–Carlo simulations.

Differentiator-Based Incremental Three-Dimensional Terminal Angle Guidance With Enhanced Robustness

Abstract: In this article, an incremental guidancelaw with terminal angle constraint is proposed against maneuvering targets in the 3-D space. First, a sliding surface is constructed such that its first-order dynamics excludes the relative range and line-of-sight angles in the perturbation. This manipulation avoids unboundedperturbations induced by target maneuvers near collision. Then, a benchmark guidance law is derived via the nonlinear dynamic inversion (NDI) based sliding mode control (NDI-SMC). To further enhance guidance system robustness, an incremental nonlinear dynamic inversion (INDI) based SMC (INDI-SMC) 3-D guidance law is developed. The INDI-SMC guidance law exploits the first-order derivative of the sliding variable and guidance command output at the latest step, which leads to reduced perturbation and thus requires smaller gains than the NDI-SMC guidance law. A multivariable continuous differentiator is employed to estimate the sliding variable's first-order derivative for guidance law implementation. Moreover, the stability of the differentiator is analyzed and the guidance robustness under uncertainties is compared. Extensive numerical simulations and a Monte Carlo test are conducted to verify effectiveness and robustness of the proposed method.

On the Use of Reciprocal Filter Against WiFi Packets for Passive Radar

Abstract: This article derives a novel unified signal processing scheme for WiFi-based passive radar in order to limit its complexity and enhance its suitability for short range civilian applications. To this purpose, the exploitation of a reciprocal filtering (RpF) strategy is investigated as an alternative to conventional matched filtering at the range compression stage. Along with the well-known advantage of a remarkable sidelobes control capability for the resulting range-Doppler response, the use of a RpF is shown to provide additional benefits for the specific sensor subject of this article. Specifically, it allows to streamline the disturbance cancellation stage and to implement a unified signal processing architecture which is capable to handle the different modulation schemes typically adopted in WiFi transmissions. Appropriate adjustments are also proposed to the theoretical RpF in order to cope with the inherent loss in terms of signal-to-noise power ratio. The effectiveness of the proposed signal processing scheme encompassing the RpF strategy is proved against both simulated and experimental datasets.

Cybersecurity Attacks on Software Logic and Error Handling Within ADS-B Implementations: Systematic Testing of Resilience and Countermeasures

Abstract: Automatic-dependent surveillance-broadcast (ADS-B) is a cornerstone of the next-generation digital sky and is now mandated in several countries. However, there have been many reports of serious security vulnerabilities in the ADS-B architecture. In this article, we demonstrate and evaluate the impact of multiple cyberattacks on ADS-B via remote radio frequency links that affected various network, processing, and display subsystems used within the ADS-B ecosystem. Overall we implemented and tested 12 cyberattacks on ADS-B in a controlled environment, out of which 5 attacks were presented or implemented for the first time. For all these attacks, we developed a unique testbed that consists of 36 tested configurations. Each of the attacks was successful on various subsets of the tested configurations. In some attacks, we discovered wide qualitative variations and discrepancies in how particular configurations react to and treat ADS-B inputs that contain errors or contradicting flight information, with the main culprit almost always being the software implementation. In some other attacks, we managed to cause denial of service by remotely crashing/impacting more than 50% of the test set that corresponded to those attacks. We also implemented, and report some practical countermeasures to these attacks. We demonstrated that the strong relationship between the received signal strength and the distance-to-emitter might help verify the aircraft.s advertised ADS-B position and distance. For example, our best machine learning models achieved 90% accuracy in detecting attackers' spoofed ADS-B signals.