Forward Scattering Radar for Ultra High Energy Cosmic Rays

Journal of Physics G: Nuclear and Particle Physics is committed to bringing its readers timely reviews in topics of interest to the nuclear and particle physics community, and an active programme of Topical Reviews is scheduled for future issues. Appropriate topics and authors are identified by the Editorial Board prior to invitation and commissioning of reviews, but any suggestions of suitable topics or offers of review contributions will be welcomed by the Board.

This letter presents a compact dual-polarized over-the-air testing antenna with low radar cross section (RCS) for the fifth-generation and emerging wireless communication systems. The proposed antenna design is achieved by slotting two orthogonal Vivaldi antennas in a cross-shaped form into a two-part three-dimensional printed radome and loading microwave-absorbing materials (MAMs) at the antenna's outer edges. The proposed antenna has a rocket-nest shape with a diameter of 63.4 and 113.68 mm length. The -10 dB impedance bandwidth (S11) is obtained from a range of 2.1 up to 6 GHz. The two-part radome is formed by polylactic acid and acrylonitrile butadiene styrene dielectric materials, respectively. Their respective dielectric constants are approximately 2.6 and 4.5 within the -10 dB impedance bandwidth. The MAMs geometry and distribution location are optimized to suppress induced current over the antenna. An antenna prototype has been fabricated and measured. The measured results agree reasonably well with the simulated results. The results show that the use of the two-part radome enables the achievement of low RCS with compact form factor. Furthermore, a low bistatic RCS of between 2 and 6 GHz is observed in simulation for angular ranges of θ ∈ [-30°, +30°] and Φ ∈ [-90°, +90°].

We present a review and analysis of the experimental data on cosmic rays with energies above 1014-1015 eV and up to 1020 eV. At the beginning a brief survey of the direct experimental data on the cosmic-ray energy spectrum and mass composition is given. We then consider the choice of the hadron interaction model at energies above 1015 eV that is essential for the interpretation of the experimental data on extensive air showers (EAS). Much attention is given to the well known knee in the primary energy spectrum at approximately 3*1015 eV and also to an analysis of this phenomenon in the framework of the recently developed Hall diffusion model. In doing so we use as yet scarce data on cosmic-ray anisotropy and mass composition at superhigh energies 1014-1017 eV. A considerable part of our review is concerned with cosmic rays of ultrahigh energies 1017-1020 eV. Using the galactic origin model for cosmic rays with energies 1014-1020 eV the possibility of explaining all the relevant data at energies above 3*1018-1019 eV is discussed: evidence of the 'ankle' in the primary energy spectrum, the upper bound of cosmic-ray anisotropy, and evidence for the increasing proton abundance. The prospects for future cosmic-ray investigations at superhigh energies are outlined and the great importance of development in the neutron and proton astronomy for energies above 1018-1019 eV is pointed out.

The authors consider Low Earth Orbited Satellite (LEOS) systems for personal communications. Their rather high power spectral density near the Earth surface could be used for aircraft detection. Bistatic radars that utilize communication signal transmitting from LEOS could operate in a passive mode. Moreover, the targets will be characterized by their bistatic radar cross-section (RCS) that could be 20 to 40 dB larger than the monostatic RCS. Another LEOS peculiarity is their high speed relative to the surface. This fact can be used for radar applications, namely: to design a bistatic synthetic aperture radar (SAR). Such a bistatic radar has a receiving subsystem near the acquisition scene (restricted usually by the horizon line) and the transmitter is the satellite transmitting communication signals. The direct communication line could be used for the heterodyne synchronization. It is important to mention, that a heterodyne signal should be formed that way in order to avoid or maximally reduce the Doppler frequency shift in the heterodyne circuit. In the present paper, using a particular example, the authors demonstrate the feasibility of this approach and evaluate parameters for system development. The example system demonstrates the feasibility of vessel detection at sea.

F-117 Nighthawk Stealth Fighter Jet is considered as the world's first true ‘Stealth’ aircraft and its considerably low Radar Cross Section (RCS) has been measured in this paper. RCS is an important parameter for radar that improvises on the detectability the target accurately. Varying two of the most important parameters of RCS such as frequency of the radar transceiver and aspect angle, the best outputs are recorded as the results. Using COMSOL Multiphysics software a virtual chamber has been created and the CAD model of F-117A is applied in it. With a planar electric field as the incident radar signal, the RCS has been recorded. In this paper, the bi-static RCS calculation method is considered for simulation. The results clearly depicted that better RCS can be obtained at the upper order frequency of S band wave range (above 3.5 GHz) and at some specific aspect angles the results exhibit higher detectability.

Ultra-high energy cosmic rays can be measured through the detection of radio-frequency radiation from air showers. The radio-frequency emission originates from deflections of the air-shower particles in the geomagnetic field and from a time-varying negative charge excess in the shower front. The distribution of the radio signal on the ground contains information on crucial cosmic-ray properties, such as energy and mass. A long standing challenge is to access this information experimentally with a sparse grid of antennas. We present a new analytic model of the radio signal distribution that depends only on the definition of the shower axis and on the parameters energy and distance to the emission region. The distance to the emission region has a direct relation to the cosmic rays mass. This new analytic model describes the different polarizations of the radiation and therefore allows the use of independently measured signals in different polarization, thereby doubling the amount of information that is available in current radio arrays, compared to what has been used thus far. We show with the use of CoREAS Monte Carlo simulation that fitting the measurements with our model does not result in significant contributions in both systematic bias and in resolution for the extracted parameters energy and distance to emission region, when compared to the expected experimental measurement uncertainties. This parametrization also enables fast simulation of radio signal patterns for cosmic rays, without the need to simulate the air shower.

Bistatic radars, specifically forward-scatter radars, are proposed as an alternative to standard monostatic radars against targets whose radar cross sections (RCS) have been reduced by passive means. Forward-scatter radars operate by detecting echoes from a targets forward-scatter RCS, which is insensitive to effects of passive RCS reduction techniques. However, the performance of the forward-scatter radar is compromised when the angular separation between the interference, which propagates directly from the transmitter to the receiver, and the target return is less than the Rayleigh resolution limit of the receiving antenna. This research presents the results of a parametric study of the ability of a forward-scatter radar to detect and measure the bearing of a large target, whose RCS is reduced via passive means. Super-resolution array processing techniques, particularly root-MUSIC (multiple signal classification), are used to overcome the traditional limitations resulting from the Rayleigh resolution limit of the antenna. The study compares the received power and the bearing measurement accuracy of the forward-scatter radar to that of an equivalent monostatic radar system. The results indicate that forward-scatter radars enjoy advantages in detection and bearing measurement when the backscatter RCS of the target has been reduced and when the target is close to the baseline. The results also indicate that, through the use of super-resolution array processing the capability of the forward-scatter radar to accurately measure the bearing of the target is dependent upon the amount of interference from the direct wave (i.e., the wave which propagates from the transmitter directly to the receiver) and the correlation between the direct wave and the target echo. Good bearing estimates can be achieved if the correlation coefficient is less than 0.95. Bearing measurements may be improved by suppressing the direct wave by either sidelobe control or null steering techniques. >

\nIncreased event statistics will be required to definitively answer the question of the origin(s) of Ultra-High Energy Cosmic Rays (UHECR). Using current technologies however, achieving the necessary statistics may be financially and practically impossible. We describe the status and plans of the TARA project, an effort to detect Ultra-High-Energy Cosmic Rays by their forward scattered or “bistatic” radar signature. Bistatic radar holds promise as a new remote sensing technique for UHECR, without the duty cycle limitations of nitrogen fluorescence detectors. Such a technique could prove key in advancing the study of UHECR beyond the constraints of the current generation of cosmic ray observatories. TARA consists of a low-VHF television transmitter illuminating the air above the Telescope Array (TA), and a set of radio receivers on the far side of TA approximately 50 km distant from the transmitter. We have collected radar data since April 2011 using a 2 kW transmitter at 54.1 MHz. Recently, we received permission to increase our broadcast power to 40 kW and our effective radiated power (ERP) to 6 MW. On the receiver end, we are employing software-defined radio receivers and developing real-time trigger algorithms based on the expected air shower radar echo. In addition to presenting an overview of the project status and future plans, we will present the most recent results of searches for coincidences between radar echoes and Telescope Array air shower events.\n

Field methods based on the finite-difference approximations of the time-domain Maxwell's equations and the potential-flow equation have been developed to solve the multidisciplinary problem of airfoil shaping for aerodynamic efficiency and low radar cross section (RCS). A parametric study and an optimization study employing the two analysis methods are presented to illustrate their combined capabilities. The parametric study shows that for frontal radar illumination, the RCS of an airfoil is independent of the chordwise location of maximum thickness but depends strongly on the maximum thickness, leading-edge radius, and leadingedge shape. In addition, this study shows that the RCS of an airfoil can be reduced without significant effects on its transonic aerodynamic efficiency by reducing the leading-edge radius and/or modifying the shape of the leading edge. The optimization study involves the minimization of wave drag for a non-lifting, symmetrical airfoil with constraints on the airfoil maximum thickness and monostatic RCS. This optimization study shows that the two analysis methods can be used effectively to design aerodynamically efficient airfoils with certain desired RCS characteristics.

High-energy astrophysics focuses on understanding the origins of the most energetic radiation and particles observed or theoretically predicted in the universe. This area of research probes the most mysterious and most difficult-to-observe astrophysical phenomena. Understanding these phenomena is worth the effort, however, because they may be key to understanding the origin of the universe and the fundamentals of particle physics. Recent observations of extreme events such as ultrahigh-energy cosmic rays and gamma ray bursts have led to significant progress in determining their sources and modeling the mechanisms that produced them. Relativistic jets accelerated from massive compact objects, such as neutron stars or black holes, have been implicated as the primary culprits for producing high- energy events. Here we present three Reviews of the more rapidly advancing areas of research, ultrahigh-energy cosmic rays, gamma ray bursts, and relativistic jets. The highest energy cosmic rays are thought to be extragalactic. However, their source remains a mystery, as discussed in a Review by Guenter Sigl (p. [73][1]). About 12 events have been detected by large-area, ground-based detectors that sample the secondary particles of the cosmic rays. These observations, combined with theoretical models, suggest that ultrahigh-energy cosmic rays come from the earliest processes in the formation of the universe, nonstandard particle physics, or acceleration of particles by energetic processes such as gamma ray bursts. Gamma ray bursts—which last for only a few seconds and generate about 1053 ergs per second of energy output, greater than any other astrophysical phenomenon—are now being detected at a rate of about one per day. Peter Meszaros (p. [79][2]) reviews our current understanding of these curious events. The Italian-Dutch satellite BeppoSAX in particular has helped to locate gamma ray bursts, and accurate locations make it much easier to observe the afterglows with ground-based telescopes and satellites. These observations have led to a model that favors the collapse of a massive object into a neutron star or black hole, generating jets that create gamma ray bursts. Relativistic jets, collimated flows of plasma that travel at almost the speed of light, may produce gamma ray bursts, ultrahigh-energy cosmic rays, and many other high-energy phenomena. David Meier, Shinji Koide, and Yutaka Uchida (p. [84][3]) review the latest observations of jet structures, mainly from radio and x-ray wavelength observations, and focus on magnetohydrodynamic simulations of relativistic jets that reproduce these structures. A neutron star or black hole, in a binary system or in the act of formation, will accrete mass and angular momentum. Differential rotation of this mass and momentum may twist the magnetic field lines of the system, producing bipolar jets, and simulations can provide the appropriate conditions to accelerate particles to produce ultra high-energy cosmic rays or create gamma ray bursts in the jet stream. Relativistic jets also make up the centerpiece of the first of three News stories. Mark Sincell (p. [66][4]) describes how the jets of recently discovered objects known as microquasars may shed light on the mechanisms behind quasars—galaxy-spanning structures powered by supermassive black holes. Next, Govert Schilling (p. [68][5]) tells of another attempt to link two similar-looking phenomena: faint high- energy blips known as soft gamma repeaters and anomalous x-ray pulsars. Finally, Richard Stone (p. [70][6]) travels to Tatarstan to report on a project intended to rival much larger Western ground- and space-based attempts to detect gravitational waves from binary pulsars Thus, the combination of numerical simulations, observations, and theory is improving our understanding of the origins of high-energy astrophysical phenomena. The future looks bright for resolving some of these great mysteries in the early 21st century. [1]: /lookup/doi/10.1126/science.291.5501.73 [2]: /lookup/doi/10.1126/science.291.5501.79 [3]: /lookup/doi/10.1126/science.291.5501.84 [4]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.66 [5]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.68 [6]: /lookup/doi/10.1126/science.10.1126/SCIENCE.291.5501.70

Instrumentation grade, coherent, bistatic radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise tests were performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay distance of 2.26 kilometers. The measurements were conducted using a standard Hewlett Packard HP 3048 A phase noise test set, and a variety of link configurations at a variety of frequencies from 100 MHz to 5 GHz. Further, system tests have been performed to incorporate a FOL into a coherent bistatic instrumentation radar system local oscillator path, and perform image processing on an emulated target. A Lintek elan system was configured to emulate bistatic operation by separating the transmit and receive local oscillator lines, allowing one of them to be excited through a fiber optic link. A target with one or two scatterers was emulated using a delay line between a sample of the transmit signal and a receiver injection port. This paper describes the theoretical effects of phase noise (close-in and higher frequency) on target scatterer imaging performance, describes the FOL tests and the architecture of the bistatic radar tests performed, and presents the results of the tests, including baseline system performance and performance using the FOL. Images representing closely spaced scatterers are presented, and the point spread function of a long dwell (40 seconds) data file, representing an ISAR process is presented. One of the limiting factors in the use of a FOL for long dwell times is that of the variable delay, caused by variation of the propagation velocity with laser frequency drift (dispersion), and the change in index of refraction with temperature. A method for mitigating the effects of this drift is presented.

LOPES STAR is a prototype detector for future experiments on the observation of radio emission of ultra high energy cosmic rays. Absolutely calibrated measurements of the electric field strength with the LOPES STAR detector were performed in coincidence with the well-established air shower detector KASCADE-Grande. The experimental configuration allowed a simultaneous observation of east-west and north-south polarised components of the electric field per antenna used. This thesis discusses in detail the influence of background sources on the detector configuration as well as the resulting self-trigger system. The implemented trigger suppresses strong background signals from the industrial environment on the site of the Forschungszentrum Karlsruhe (rejection > 99.9 %) and is the basis for a hardware self-trigger. Therefore, the system is adequate for any other detector site. Two different calibration methods are performed and cross-checked to convert the measured raw data into an electric field strength. Both methods result in the same frequency dependent calibration values within their uncertainties. Furthermore, the analysis of the radio emission of ten high quality shower events results in the most probable scale parameter of do = (137 ± 18) m. The comparison of selected events with Monte Carlo simulations on an event-by-event basis points out a good correspondence within the given uncertainties and confirms the geosynchrotron model. The overall angular resolution of the arrival direction results in only a few degrees. The presented methods and algorithms are developed for the trigger system and the analysis and are now standard tools for the data analysis in the LOPES collaboration.

This communication presents an approach to design a beam-steerable dipole array with low radar cross section (RCS). A 2-D periodical array of dipole antennas is colocated with absorptive frequency-selective reflection (AFSR) structures to effectively reduce the out-of-band RCS and maintain high gain at the in-band frequencies. The AFSR structures consist of 3-D bandstop frequency-selective structure (FSS) and pyramidal-shaped absorbers to provide a reflection band for the dipole array with enhanced front-to-back ratio. Compared with the reference array, the radiation performance of the proposed array is maintained with an aperture efficiency of 70% and a measured peak gain of 21.5 dBi. The beam-steering angle is up to ±45°. Meanwhile, more than 10 dB two-sided out-of-band RCS reduction is achieved with a bandwidth of 96.5% and 30% for copolarization and 142% for cross polarization, and the in-band RCS is unchanged.

In this article, a planar wideband circularly polarized (CP) slot antenna array based on metasurface (MS) with low radar cross section (RCS) is proposed. The array is comprised of CP MS‐based antennas fed with a sequentially rotated feeding network. For each CP antenna, the CP radiation is achieved due to polarization conversion of the MS. Simultaneously, the total MS patches of the array can be viewed as checkerboard composed of MS array and mirror array. A phase difference between the MS units and the mirror units ensures RCS reduction. Simulated results demonstrate that about 34.6% operating bandwidth in the frequency of 4.3–6.1 GHz (both S11 –10 dB and AR < 3 dB). In addition, over 5 dB RCS reduction is obtained in the entire operation band under normal incidence with respect to the array without MS for both x‐ and y‐polarizations. The simulated monostatic RCS less than −30.9 dBsm is observed at 5.0 GHz. A prototype of the array is fabricated and measured, which are in good agreement with simulated results.

Detecting and tracking small drones is significant for security requirements, but is challenging since small drones typically have a low radar cross section (RCS), small size, and hovering ability. Polarimetric analysis of returned radio wave is of great significance in detection and track of small drones. In this letter, the polarimetric analysis based on RCS and micro-Doppler signature of small drone rotor blade is first studied in VHF/UHF band. Both the simulated and measured RCS results confirm that the horizontal–horizontal polarization is preferred for drone detection in low-altitude angles at this frequency band. Moreover, the method of moments, a rigorous numerical method based on integral equation, is implemented to simulate the micro-Doppler signature generated by the rotating blade in VHF/UHF band. The results show that it is feasible to obtain the rotating speed from the micro-Doppler measurement, which is also verified by the experiment.