Showing papers in "IEEE Aerospace and Electronic Systems Magazine in 2017"
TL;DR: In this paper, the authors describe the vulnerability of large-scale industrial control systems (ICSs) to cyber-physical attacks on both physical and cyber layers of the ICs, such as power grids, transportation systems, communication networks, oil and gas pipelines, water distribution and irrigation networks.
Abstract: Supervisory control and data acquisition (SCADA) systems are highly distributed systems used to control and monitor geographically dispersed assets—often scattered over thousands of square kilometers— in which centralized data acquisition is critical to system operation [1]. These large-scale industrial control systems (ICSs) have been playing an extremely important role in most safety-critical infrastructures [2], such as electric power grids, transportation systems, communication networks, oil and gas pipelines, water distribution and irrigation networks, and multiple facilities including heating, ventilation and air conditioning systems for buildings, and traffic control systems for airports—the list is long. These safetycritical assets, however, are becoming increasingly susceptible to cyber–physical attacks1 on both physical and cyber layers [3].
70 citations
TL;DR: In this article, a single pilot operates the flight deck with increased ground support from a dedicated ground human flight crew, providing a combination of strategic and tactical support to the single pilot in collaboration with the air traffic controllers (ATCo).
Abstract: Global air transport demand is increasing steadily, with the global revenue passenger kilometers (RPK) growing at an annual rate of 4% [1] and the number of passengers rising at an average annual rate of 10.6% [2]. By the end of 2016, it is estimated that 1,420 large commercial airliners will be produced, 40.5% more than was produced five years ago [2]. A consequence of this growth is an exacerbation of the existing global shortage of qualified pilots. Airlines have to hire more than 500,000 new commercial pilots until 2034 in order to meet this unprecedented air transport demand [3]. Additionally, the high costs associated with training and remuneration of pilots has been a substantial economic burden on air carriers, prompting active research into the concept of single-pilot operations (SPO) as an option for the future evolution of commercial airliners. SPO cockpits have already been developed for military fighters as well as general aviation (GA) aircraft, with small business jets like the Cessna Citation I obtaining approval for SPO as early as 1977 [4], however, the last decade has seen considerable interest in the implementation of SPO in commercial aviation. NASA has been conducting SPO-related studies since the mid-2000s [5], [6], while some recent research in Europe has focused on the technical [7] and operational [8] challenges of SPO. In the SPO concept of operations (Figure 1), a single pilot operates the flight deck with increased ground support from a dedicated ground human flight crew. The ground operators (GO) fulfil a role similar to that of a remotely piloted aircraft system (RPAS) operator, providing a combination of strategic and tactical support to the single pilot in collaboration with the air traffic controllers (ATCo).
57 citations
TL;DR: There is a need for further research in methods that allow for fault detection and recovery techniques to be easily realized and implemented with minimal risk of software errors.
Abstract: Ubiquitous sensing is pervasive in society for such applications as biometrics for health care, smart grids for power delivery, and avionics for transportation safety [1]. As society continues to rely ever more on sensors for various applications, there is a need to address the accuracy of sensor readings for health maintenance, signal identification, and control [2]. While there have been advances in information fusion [3] for avionics control [4] and user warnings [5], there is still a need for further research in methods that allow for fault detection and recovery techniques to be easily realized and implemented with minimal risk of software errors.
53 citations
TL;DR: Currently small unmanned aerial vehicles (UAV) pose a serious threat for the safety of flights, and a critical issue is to prevent UAVs being used for terrorist attacks, espionage, or other malicious activities against sites with critical infrastructure.
Abstract: Currently small unmanned aerial vehicles (UAV) pose a serious threat for the safety of flights. The Aviation Authorities are dealing with this issue worldwide. Recently (October 2015), the U.S. Federal Aviation Administration gave permission to test antidrone technology that would counter rogue drones flying within a fivemile radius of selected airports [1]. Airport safety is only one of the problems that the increasing number of UAVs can pose. A critical issue is to prevent UAVs being used for terrorist attacks, espionage, or other malicious activities against sites with critical infrastructure. Last but not least, UAVs flying in private area pose privacy concerns [2].
49 citations
TL;DR: The purpose of this article is to present and justify the reasoning behind the systematic use of KF-based tracking approaches instead of the well-established PLL-based architectures from both theoretical and practical points of view.
Abstract: Carrier synchronization is a fundamental stage in the receiver side of any communication or positioning system. Traditional carrier phase tracking techniques are based on well-known phase-locked loop (PLL) closed-loop architectures, which are still the methods of choice in modern receivers. Those techniques are well understood, easy to tune, and perform well under benign propagation conditions, but their applicability is seriously compromised in harsh propagation environments, where the signal may be affected by high dynamics, shadowing, strong fadings, multipath effects, or ionospheric scintillation. From an optimal filtering standpoint, the Kalman filter (KF) is clearly a powerful alternative, but the synchronization community seems still reluctant to exploit all the potential it has to offer. The purpose of this article is twofold: i) to review the basics and state of the art on both PLL and KF-based tracking techniques and ii) to present and justify the reasoning behind the systematic use of KF-based tracking approaches instead of the well-established PLL-based architectures from both theoretical and practical points of view. To support the discussion, two specific scenarios of interest to the aerospace community are numerically evaluated: robust carrier tracking of global navigation satellite systems' signals and synchronization in a deep space communications system.
43 citations
TL;DR: To deal with the tradeoff between large transmitting energy and high range resolution, the early unmodulated rectangular waveform has been replaced by more waveforms with a large time-bandwidth product (TBP), such as linear frequency modulation (LFM) and phase-coded waveforms, which have been widely used to realize the range compression via matched filter.
Abstract: During the World War II (WWII), radar was invented as an all-day, all-weather, long-range sensor. Over the past 75 years or so, radar has acted as the clairvoyance and clairaudience of humans and has had wide applications [1]–[19] in both defense and civilian fields, e.g., surveillance, reconnaissance, fire control, border monitoring, collision avoidance, and traffic control. Along with radar development, radar signal processing (RSP) always plays an important role. In 1943, North [4] pointed out that the matched filter should be the foundation of radar to detect targets from echoes generated by the known transmitting waveform. In 1950, Woodward and Davies [5] introduced the Bayesian statistics theory into the RSP field and proposed the constant false alarm ratio (CFAR) rule for target detection. Based on matched filtering and the CFAR rule, the embryonic framework of classic RSP has been formed and used thus far. In 1953, Woodward [6] further proposed an ambiguity function as a useful tool to evaluate the range and Doppler discrimination abilities of a transmitting waveform, which paves the foundation for radar waveform design. Furthermore, to deal with the tradeoff between large transmitting energy and high range resolution, the early unmodulated rectangular waveform has been replaced by more waveforms with a large time-bandwidth product (TBP), such as linear frequency modulation (LFM) and phase-coded waveforms. These waveforms with large TBP have been widely used to realize the range compression via matched filter.
40 citations
TL;DR: DTN is presently recognized as the only candidate protocol that approaches the level of maturity required to handle the inevitable long link delay, frequent and lengthy link disconnections, and heavy data loss inherent in space communications.
Abstract: Extensive work has been done in developing networking architectures and protocols for satellite/space communications and interplanetary networks. A variety of solutions have been proposed [1-9]. Numerous literature surveys [10-14] have also been conducted on these technologies. Delay/disruption tolerant networking (DTN) [1] architecture was proposed to enable automated network communications despite the long link delay and frequent link disruptions that generally characterize deepspace communications. DTN is presently recognized as the only candidate protocol that approaches the level of maturity required to handle the inevitable long link delay, frequent and lengthy link disconnections, and heavy data loss inherent in space communications [15].
39 citations
TL;DR: A complementary pulse pair that achieves the ultimate range sidelobe reduction zero sidelobe is an early and simple embodiment of radar waveform diversity (WD), presently a popular topic, however, the use of complementary pulse waveforms is not widely spread because of several drawbacks.
Abstract: Complementary pulse pair is a radar waveform that achieves the ultimate range sidelobe reduction zero sidelobe. It is an early and simple embodiment of radar waveform diversity (WD), presently a popular topic. However, the use of complementary pulse waveforms is not widely spread because of several drawbacks. The main problem is the sensitivity to Doppler shift. Usually the two complementary coded pulses are separated in time. Doppler shift causes a phase ramp as function of time. That ramp causes two problems: (a) the two pulses in a pair are centered on different average phases; (b) there is a phase ramp during each pulse. Problem (a) also known as slow-time mismatch, is handled by the pulse-topulse conventional Doppler processing, which provides slow-time phase compensation. Problem (b) requires fast-time compensation, not provided by a simple linear Doppler processor. It causes loss of the ideal delay-sidelobe cancellation resulting in near range-sidelobes. Those near sidelobes increase with longer codes and with higher Doppler shifts. At the same time a complementary pulse pair also causes a difficulty at low Doppler shifts.
33 citations
TL;DR: The Automatic Dependent Surveillance-Broadcast (ADS-B) system is one of the major surveillance systems for air traffic management, providing identity, position, and status information of all cooperating aircraft.
Abstract: The Automatic Dependent Surveillance-Broadcast (ADS-B) system is one of the major surveillance systems for air traffic management, providing identity, position, and status information of all cooperating aircraft. It is being used more and more frequently in modern air traffic management [1]-[4]. The ADS-B is a dependent and cooperative surveillance system used for air traffic control in which aircraft and surface vehicles periodically transmit their own information to any equipped listener for surveillance scope. The ADS-B system is one of the pillars of future air traffic systems and estimates suggest that about 80% of all commercial aircraft are now equipped with ADS-B transponders.
32 citations
TL;DR: A practical method that can reject virtually all spoofed ADS-B messages by monitoring the signal propagation time between senders and receivers is developed and called "ADS-B with Timestamp" (ADS-BT).
Abstract: Automatic Dependent Surveillance-Broadcast (ADS-B) is the foundation of NextGen Air Traffic Control (ATC) system, and all aircraft must be equipped with it by January 1, 2020. Despite the importance of ADS-B, it has been developed without security considerations and is subject to various types of attacks. A trustworthy ADS-B system will make the NextGen system more reliable and make air transportation safer. But current safety measures are inadequate and many research proposals are not yet practical or cost-effective. We developed a practical method that can reject virtually all spoofed ADS-B messages by monitoring the signal propagation time between senders and receivers. To measure the actual propagation time, the method uses a small timestamp value; hence, we call it "ADS-B with Timestamp" (ADS-BT). ADS-BT monitors the discrepancy between the time of flight based on the timestamp values and the time of flight based on the location data. In spoofed ADSB messages, the discrepancy between the two diverges over time, which allows us to identify spoofed ADS-B messages accurately.
26 citations
TL;DR: The focus of this article is on spacecraft on-board avionics software, that is, spacecraft- controlling code that flies into orbit with the spacecraft and discusses potential benefits and drawbacks of on- board Linux use.
Abstract: Spacecraft on-board computers are responsible for controlling the spacecraft platform, payloads, or other on-board devices. Their mission-specific software allows communication with ground or other on-board computers. Traditionally, on-board software has been written close to the hardware in assembly language, Ada, C, or C++, with or without a real-time operating system (RTOS) [1]. As the spacecraft computer hardware capabilities are increasing, spacecraft software is becoming larger and more complex, handling more tasks from payload data processing to landing a first stage of a launch vehicle on an ocean-going barge. Spacecraft will still continue to include very small embedded systems that can be developed without operating systems (OS), but some systems will also have large software bases, requiring efficient software development processes and reuse of existing software modules. The last decade has seen increasing use of Linux in spacecraft on-board software. This article presents common features of spacecraft on-board computers and software and discusses potential benefits and drawbacks of on-board Linux use. The focus of this article is on spacecraft on-board avionics software, that is, spacecraft- controlling code that flies into orbit with the spacecraft. Other types of computers are not included in this analysis; for example, many laptops on the International Space Station run Linux [2].
TL;DR: This paper considers the specific problem of anomalous change detection in hyperspectral images and discusses how images taken at two different times can be processed to detect changes caused by insertion, deletion, or displacement of small objects in the monitored scene.
Abstract: Exploitation of temporal series of hyperspectral images is a relatively new discipline that has gained a lot of attention from the image processing scientific community. In this paper, we consider the specific problem of anomalous change detection (ACD) in hyperspectral images and discuss how images taken at two different times can be processed to detect changes caused by insertion, deletion, or displacement of small objects in the monitored scene. We introduce the ACD problem using an approach based on the statistical decision theory and we derive a common framework including different ACD approaches. Far from being inclusive of all the methods proposed in the literature, this tutorial overview places emphasis on techniques based on the multivariate Gaussian model that allows a formal presentation of the ACD problem and the rigorous derivation of the possible solutions in a way that is both mathematically more tractable and easier to interpret. The unification of different approaches under a single rigorous statistical scheme provides both a tutorial overview of ACD techniques, and a useful instrument for researchers already familiar with the ACD problem. Dedicated preprocessing methods aimed at improving the robustness of the ACD process are also discussed. Real data are exploited to test and compare the presented methods, highlighting advantages and drawbacks of each approach. The tutorial aspect of the paper has suggested the use of a freely available data set. This should hopefully motivate the interested reader to experiment with the processing methods and performance evaluation chain presented herein.
TL;DR: Commensal1 or passive radar, which uses signals of opportunity to detect targets without affecting the functionality of the parent system, is proposed.
Abstract: Classical radar systems have been designed primarily for military operations. Of late many interesting ways of using radio-frequency spectrum for radar purpose have been under research. One such concept is commensal1 or passive radar, which uses signals of opportunity to detect targets without affecting the functionality of the parent system [1]–[7].
TL;DR: In a maritime domain awareness scenario, two main areas of interest can be recognized, specifically harbour protection and navigation safety, which are essentially connected to achieve an improved maritime navigational awareness.
Abstract: In a maritime domain awareness scenario, two main areas of interest can be recognized, specifically harbour protection and navigation safety. Harbour protection is currently a main concern and it is aimed at controlling the risk associated with possible attacks or illegal activities which can take place in the marine access areas. On the other hand, the objectives of the navigation safety are essentially connected to achieve an improved maritime navigational awareness by providing information on collision avoidance and general guidance to the navigators [1].
TL;DR: The Tracking Component Framework described here has been created as a first step to simplifying the rapid design and evaluation of target-tracking algorithms.
Abstract: The area of target tracking is a mature field with a great many algorithms having been developed over the years. Although this means that algorithms exist to suit a large number of specializations, such as radar, sonar, and image tracking, this also means that it can be difficult to determine which algorithm is the best. Additionally, the literature does not, for the most part, present complete algorithms for target tracking, but rather components of tracking algorithms. For example, measurements might be processed in various filters, which might themselves be incorporated into a framework for handling multiple dynamic models, which might be incorporated into a procedure for handling target measurement assignments, which could be part of a general track initiation and termination procedure, and all of which would tie into scheduling algorithms that select waveforms and dwell times for a radar. On top of that, various physical models are needed for handling atmospheric refraction and other effects. Consequently, the Tracking Component Framework described here has been created as a first step to simplifying the rapid design and evaluation of target-tracking algorithms.
TL;DR: Comparing between different techniques for the design and implementation of closed-loop control systems for buck converters is carried out and fuzzy logic controller and tuned fuzzy logic controllers are presented.
Abstract: Comparing between different techniques for the design and implementation of closed-loop control systems for buck converters is carried out. Controllers used for this comparison are integral, proportional plus integral (PI) controllers and artificial intelligence are represented in fuzzy logic controller (FLC) and tuned fuzzy logic controller (TFLC). Design and implementation of a control system demand the role of effective techniques that offer simple and pragmatic solutions in society to meet the performance requirements despite system disturbances and uncertainties [1]. The occurrence of nonlinear phenomena in buck converters makes their analysis and control difficult [2]. Classical linear techniques have stability limitations around the operating points [1]–[3]. Hence digital and nonlinear stabilizing control methods must be applied to ensure large-signal stability [3]. Fuzzy control has also been employed to control buck converters because of its simplicity, simplicity of design and simplicity of implementation [4], [5]–[10]. Fuzzy controllers are easily suited to nonlinear time-variant systems and do not require an accurate mathematical model for the system being controlled. They are usually designed based on expert knowledge of the converters [5]. In this survey, the design procedures of integral, proportional plus integral, fuzzy logic, and tuned fuzzy logic controllers are presented.
TL;DR: This paper describes two series of experiments undertaken by the author and coworkers in learning how to apply MIMO radar techniques to challenges in skywave Over-the-Horizon Radar (OTHR).
Abstract: Multiple-Input Multiple-Output (MIMO) radar has been an active topic in the radar research community for more than a decade [1]–[3]. MIMO based approaches have been investigated for improving target detectability, improving target localization, achieving higher spatial resolution, and clutter reduction using transmit or joint transmit-receive beamforming. This paper describes two series of experiments undertaken by the author and coworkers in learning how we might apply MIMO radar techniques to challenges in skywave Over-the-Horizon Radar (OTHR).
TL;DR: This work has shown that different bands of multispectral video streams can enhance video analytics capabilities in different ways, and the infrared bands can provide better separation of shadows from objects, and improved spatial resolution in scenes that are impaired by fog or haze.
Abstract: Video analytics plays an important role in a wide variety of defense-, monitoring- and surveillance-related systems for air and ground environments. In this context, multispectral video processing is attracting increased interest in recent years, due in part to technological advances in video capture. Compared with monochromatic video, multispectral video offers better spectral resolution, and different bands of multispectral video streams can enhance video analytics capabilities in different ways. For example, the infrared bands can provide better separation of shadows from objects, and improved spatial resolution in scenes that are impaired by fog or haze [16].
TL;DR: This work focuses on the system perspective of the technology, i.e. PCL, while relying on the technical depths of the PBR technology.
Abstract: Passive coherent location (PCL), passive covert radar (PCR), passive bistatic radar (PBR), passive radar, parasitic radar, and hitchhiking radar are all common names for passive radar systems exploiting cooperative, noncooperative transmitters, or transmitters of opportunity, [1, pp. 3]. Recent review of the various names resulted in the compromise PBR, [2, pp. 248]. However, it should be noted that the PCL name is still being used for full systems like Lockheed Martin's - Silent Sentry [3], [4], Thales' - Homeland alerter 100 [5], [6] seen in Figure 1, and Airbus “Passive Radar” [7]–[15]. The PCL systems may be considered as systems of PBR systems. This work focuses on the system perspective of the technology, i.e. PCL, while relying on the technical depths of the PBR technology.
TL;DR: In this article, the authors introduced a pilot channel that does not contain any data to allow very long coherent integrations and the introduction of a secondary code to offer better cross-correlations, to facilitate the synchronization with the data, and to help interference mitigation.
Abstract: The modern global navigation satellite systems (GNSS) signals, such as the Global Positioning System (GPS) L5 and L1C, and Galileo E5 and E1, have brought several innovations: the introduction of a pilot channel that does not contain any data to allow very long coherent integrations; the introduction of a secondary code to offer better cross-correlations, to facilitate the synchronization with the data, and to help interference mitigation; the introduction of new modulations to reduce the impact of multipath; and the use of higher chipping rates to have better accuracy and interference mitigation.
TL;DR: The introduction of computerized air traffic management applications and new digital communication links between aircraft and ground systems are introduced to ensure the sustainable growth of the European air transportation system in the coming decades.
Abstract: Today's very high-frequency (VHF) voice-based air–ground communication system for tactical aircraft guidance is suffering from the VHF band's increasing saturation in high-density areas [1]. The air–ground1 communication infrastructure is therefore undergoing modernization to ensure the sustainable growth of the European air transportation system in the coming decades [2], [3]. One major goal of this effort is the introduction of computerized air traffic management applications and new digital communication links between aircraft and ground systems.
TL;DR: The main applications of ground based passive coherent location (PCL) systems are in the domain of air traffic surveillance–air target detection, tracking, and localization and interest in using PCL radars in these applications is expressed primarily by military users.
Abstract: Today the main applications of ground based passive coherent location (PCL) systems are in the domain of air traffic surveillance–air target detection, tracking, and localization. Interest in using PCL radars in these applications is expressed primarily by military users, mainly because of the great advantages of silent and covert operation that passive radars provide. In this regard, passive radars benefit from the fact that they operate by using emissions already present (e.g. FM radio, DAB, DVB-T, GSM, Wi-Fi, etc.) rather than their own. Other advantages of passive radars relate to the possibilities of stealth target detection and gap filling in areas not covered by active radars. Another huge advantage of the passive radar lies in the fact that there is no need for requirements for spectrum allocation or powerful transmitters. Consequently, as the aforementioned points make passive radars great representatives of “green” technology, one can install them in “sanctuaries”–specific or specialized places such as high-density urban areas, sites located in the vicinities of hospitals, and/or other places where electromagnetic emissions are limited by law because of the risk of interference or simply through the public's fear of electromagnetic fields. Also, it is worth mentioning that PCL systems can be cheaper than active ones as the passive radar does not have its own transmitter and typically consists of a simple radio receiver with commercial off-the-shelf (COTS) hardware, and uses a commercially available PC as a computation.
TL;DR: For example, evolutionary algorithms have been widely used for parameter optimization in different engineering tasks as mentioned in this paper, such as the optimization of orbital maneuvers for satellites and spacecraft, which requires heuristic search techniques in finding the optimal transfer strategy that minimizes some specific criteria like control effort or fuel mass.
Abstract: Over the several last decades, evolutionary algorithms have been widely used for parameter optimization in different engineering tasks [1]-[3]. Engineering optimization has been widely involved in aerospace sciences because of its practicality in obtaining optimal solutions to different challenging problems including dynamics and control of nonlinear systems [4]-[6]. Such fields contain different aspects of aerospace engineering including missile systems [7], unmanned aerial vehicles [8], and hypersonic aircrafts [9]. One of the problems specific to aerospace engineering is the optimization of orbital maneuvers for satellites and spacecraft [10], [11]. This problem requires heuristic search techniques in finding the optimal transfer strategy that minimizes some specific criteria like control effort [12] or fuel mass [13].
TL;DR: In this article, the authors discuss potential transmit beamforming techniques for active LPI radars to increase their survivability against undesired interceptors, which can be more or less equal for the radar and the interceptor.
Abstract: Active radar using phased array has the ability to steer a high-gain beam toward any desired direction, while nulling in undesired directions to cancel jammers or interferences. However, the highpower transmission signals are often highly visible to intercept receivers; consequently, the radar may be detected and destroyed [1]. It is, thus, necessary to develop low probability of intercept (LPI) techniques to increase the system surveillance [2]. Note that although LPI technology has been defined for nearly half a century and used operationally for more than 20 years [3], which provides the user a significant advantage over his adversary by making it more difficult for a foe to detect an opponent, current LPI technologies concentrate on low observability against radars by reducing the radar cross section (RCS). Different from these existing RCS reduction methods against radar detection, this article discusses potential transmit beamforming techniques for active LPI radars to increase their survivability against undesired interceptors. However, active radar is not LPI by its nature, because the radar cannot avoid illustrating the interceptor. The interceptor is passive, and its location is probably unknown. The interceptor can have a good antenna with significant receiving aperture and a low-noise amplifier. All of these factors can be more or less equal for the radar and the interceptor. What is not equal is the large reduction of the radar signal on the return path from the target [4].
TL;DR: The effect of interaction between authentic and spoofing peaks on the tracking process of a GNSS receiver is analyzed and a spoofing countermeasure method based on monitoring the receiver's automatic gain control (AGC) gain level is proposed.
Abstract: GNSS signal quality monitoring and authenticity verification is gaining importance as different types of interference signals including jamming and spoofing are becoming more likely. There have been several studies on jamming and spoofing detection at various levels of GNSS receiver operation layers. Spoofing signals are structural interference that take advantage of the known structure of legitimate signals and try to deceive their target receiver into a false position and/or timing solution. This becomes much more important if the receiver is used in safety-of-life applications [1]–[5]. The features of spoofing signals are similar to those of authentic GNSS signals; therefore, a stand-alone GNSS receiver may face challenges in detecting this type of interference. Spoofing signals can be designed to mislead the tracking procedure of GNSS receivers by generating synchronized pseudo random noise (PRN) codes, thereby leading to counterfeit correlation peaks. This means that the PRN index and signal parameters such as Doppler frequencies and code delays of spoofing signals match those of the authentic ones. These fake correlation peaks can overlay the authentic ones, distort the normal shape of authentic correlation peaks, and gradually misdirect the tracking process of the target receiver. Detection and mitigation of spoofing attacks on GNSS receivers in tracking mode have become one of the important antispoofing topics. In [4]–[6], the effect of interaction between authentic and spoofing peaks on the tracking process of a GNSS receiver is analyzed. Most spoofing detection metrics are designed to detect a spoofing attack assuming there are only two states, namely, clean data or a spoofing attack [7]–[10]. More specifically the spoofing detection threshold for a given probability of false alarm is set in the presence of a clean data set. However, in real operational conditions there might be several situations in which the spoofing detection test statistics exceed the predefined threshold due to other sources of interference signals and cause false spoofing detection. For instance, [3] has proposed a spoofing countermeasure method based on monitoring the receiver's automatic gain control (AGC) gain level. It is shown that the presence of spoofing signals increases the power content of the received signals, leading to changes in the AGC level. However, the AGC gain can be disrupted by various interfering signals.
TL;DR: In this paper, reaction thrusters are used as an alternative to momentum exchange devices when disturbance torques exceed the control authority of momentum exchange device for attitude control during the thrusting or coast phase.
Abstract: Reaction thrusters (RTs) are used as an alternative to momentum exchange devices when disturbance torques exceed the control authority of momentum exchange devices. The reaction control system (RCS) can employ some rocket thrusters to provide attitude control during the thrusting or coast phase. Within the control loop, the RCS's target could be either achieving and keeping a certain attitude or controlling the rate of an attitude change. In the coast phase, some tasks such as preacceleration, settling of liquid propellant, damping of structural vibrations, providing a velocity increment to separate stages and payloads, and carrying out orbital and nonorbital maneuvers may be included in its functions. The propulsion perturbation torques, whose size is relatively large, are primarily produced because of the center of mass offset, which itself is produced as a result of static unbalance, transient gas flow phenomenon, and nozzle cant angle misalignments. Nozzle cant angle misalignments are produced because of manufacturing tolerances and pressurizations, which should be compensated by the attitude control system. In addition, rocket thruster plume impingement against surrounding structure or components produces sizable disturbance torques and cross-coupling effects that degrade the dynamic stability and increase the duty cycle that should be corrected by additional thrusters to restore vehicle attitude. During each maneuver, there are always undesirable angular rotations in consequence of some errors and uncertainties in the various components of a spacecraft. The attitude control system has to be capable of compensating these imperfect effects of the mechanical system.
TL;DR: Whereas the primary use of passive radars is in the defense sector, these same features make them attractive in commercial applications, and they can operate in a multistatic mode by using multiple transmitters.
Abstract: Passive radar systems operate by exploiting signals of opportunity that are designed for other applications, such as broadcasting, communications, and satellite navigation. Passive radars have recently attracted attention in military applications, mainly because of their covertness, low-cost implementation, and no requirement of signal emissions [1], [2]. Whereas the primary use of passive radars is in the defense sector, these same features make them attractive in commercial applications. Unlike conventional active radar systems, which typically operate in a monostatic mode, passive radar can operate in a multistatic mode (thus called passive multistatic radar, or PMR) by using multiple transmitters. Multiple networked receivers can also be exploited for an expanded PMR network size.
TL;DR: In this paper, a Skywave Over-the-Horizon Radar (OTHR) is employed for wide-area surveillance of aircraft and maritime vessels, which uses propagation via the ionosphere to achieve radar energy propagation to and from the target region.
Abstract: Skywave over-the-horizon radar (OTHR) is long-range beyond horizon radar technology that is presently employed by several nations for wide-area surveillance of aircraft and maritime vessels [1]. This radar class uses propagation via the ionosphere [2] to achieve radar energy propagation to and from the target region. The ionosphere is driven by varying solar interaction with the Earth, and this variability significantly influences the operational performance of a skywave OTHR. Considerable care in selecting radar-operating parameters is required to achieve the best performance.
TL;DR: The 6DOF vehicle state can be estimated accurately by fusing inertial sensor technology and the global navigation satellite system (GNSS), which has been a crucial step toward realtime guidance and flight control.
Abstract: Unmanned aerial vehicles (UAVs) have attracted significant attention from both civilian and defense industries over the last few decades. With the advances in low-cost inertial sensor technology and the global navigation satellite system (GNSS), the six degreesof- freedom (6DOF) vehicle state can be estimated accurately by fusing this information, which has been a crucial step toward realtime guidance and flight control [1], [2].
TL;DR: The Earth's geomagnetic poles represent the axis of a dipole field of best fit to the Earth's true magnetic field, and the magnetic field lines of the Earth extend many Earth radii into space and interact with the solar wind, leading to particle precipitation and horizontal ionospheric plasma drifts.
Abstract: The Earth's geomagnetic poles represent the axis of a dipole field of best fit to the Earth's true magnetic field. Canada currently hosts the Geomagnetic North Pole on Ellesmere Island, near 80 N 75 W. In the vicinity of the geomagnetic pole, the magnetic field lines of the Earth extend many Earth radii into space and interact with the solar wind, leading to particle precipitation and horizontal ionospheric plasma drifts. Particle precipitation produces the visible Aurora Borealis in the area shown as green in Figure 1, which we denote the auroral oval. North of the auroral oval is the region of the polar cap, where the solar wind drives horizontal plasma drifts, shown by the red contours in Figure 1. Previous over-thehorizon radar (OTHR) experiments have looked at operation in both the auroral oval and the polar cap.