Annotation of the results obtained in 2022
Accurate localization of objects in space had always been the subject of extensive development with a clear outlook towards practical applications, such as navigation, transmitter’s pelengation, target’s tracking and more. Triangulation, being the most widely used approach to localization, is based on accurate, often sub-nanosecond, time-of-flight measurements, performed by several synchronized, high-frequency, and wideband receivers. During the last year of the project, we demonstrated a new localization approach, which does not rely on measuring the time of flight of broadband signals. The angular location of the target is estimated directly from micro-Doppler signatures that are processed from narrow-bandwidth measurements of baseband signals. This approach does not rely on fast or expensive hardware, nor require precise synchronization between the receivers. The system is based on a rotating wire or any other asymmetrical object, illuminated with a continuous wave and a small antenna receive array. The rotating beacon generates a micro-Doppler frequency comb, which originates from the periodic amplitude modulation of the scattered signal. Time delays between receivers are then assessed. The angular measurements permit accurately ranging the target with cm range and degree azimuth precision, as we demonstrated experimentally. The theoretical bound of accuracy is solely defined by signal to noise and time on target. On pathways to accomplishing those tasks, we have developed several superscattering devices and investigated their electromagnetic properties. Radar scattering cross section of complexed-shaped electromagnetic bodies in the vast majority of cases depends on an angle of incident illumination. As a result, a rotating body will produce a time-dependent back-scattered signal, detected by the receiver. Suppose the target undergoes periodic rotational motion with a constant angular velocity. In that case, the amplitude-modulated signal will inherit micro-Doppler signatures, which manifest themselves as a frequency comb at a baseband. Furthermore, the geometry of the rotating body as well as close proximity to other moving objects plays a role and, in several scenarios, allows extracting additional information about the rotating body. Following this logical sequence, we put an emphasis on (i) developing a scatterer, (ii) investigating its angular-dependent scattering characteristics, (iii) investigating different types of obstacles, limiting a line of sight to the rotating beacon, and (iv) demonstrating triangulation capabilities of the entire concept in case of a cluttered environment. (I) Scatterers - experimental demonstration of superdirectivity and superscattering phenomena is among the long-standing challenges in electromagnetic theory. Efficient computational algorithms can contribute to this endeavor by suggesting new designs bypassing commonly accepted limitations. We demonstrate several super scatterers designs using a stochastic optimization algorithm. Structure encompassing wires of different lengths and split ring resonators with different mutual positions and orientations demonstrated superior scattering capabilities, bypassing the single channel dipole limit by at least an order of magnitude. The subwavelength structures support several resonant higher-order multipoles which constructively contribute to the scattering, as we demonstrate experimentally. A new generation of genetically designed superscatterers may be used in a range of wireless applications, including navigation, point-to-point communications, smart beacons, and radar targets. (II) Angular properties Angular distribution of a scattered electromagnetic field can be described with the aid of multipole series. In lay terms, higher-order multipoles produce more complex far-field patterns and, as a result, can possess higher micro-Doppler signatures with a nontrivial hierarchy of peaks. However, the excitation of higher-order multipoles with an incident plane wave can be inefficient if subwavelength scatterers are considered. This is the result of symmetry restrictions and a lack of electromagnetic retardation effects, both constrained by small geometries. To bypath these issues, we performed an extensive electromagnetic optimization, using advanced genetic algorithms. As a result, we succeeded in spectrally overlapping several multipolar resonances, elevating both the scattering cross-section and imposing nontrivial angular dependence. The outcome of this investigation is a compact highly scattering device that produces high signal-to-noise object-unique micro-Doppler signatures, which become a subject to subsequent triangulation. (III) Investigating different types of obstacles, limiting a line of sight to the rotating beacon Detection of objects in a case of the limited line of sight to an interrogating system is a long-standing and still open challenge. In the case of optical detection, e.g., with the aid of a camera, the difference between opaque and transparent obstacles is clear and intuitive. However, wave-matter interaction scenarios with centimeter radiation are quite different. For example, optically opaque walls can become transparent to GHz radiation. However, material aspects here play a key role. For example, the is a severe difference between gypsum plasterboard and concrete constructions. To underline the differences we performed a set of experiments to retrieve the electromagnetic properties of several typical materials. We found that 2-way attenuation can vary between 10 and 40 dB, which still makes it possible to perform a reliable detection throughout the wall. (IV) demonstrating triangulation capabilities of the entire concept in case of a cluttered environment. Accurate localization of objects in space has been always a subject of extensive developments with a clear outlook on practical applications, including navigation, transmitter’s pelengation and many others. Triangulation, being the most widely used approach to localization, is based on time-of-flight measurements performed with several synchronized receivers. The accuracy of this layout heavily depends on transmitted signal bandwidth, which serves as a fundamental limitation given the rest of the system operates under ideal conditions. We demonstrated a new precise localization approach, which is free of bandwidth limitations. The system is based on a rotating wire, illuminated with a continuous wave and a small antenna receive array. The rotating wire, subject to the triangulation, generates a micro-Doppler frequency comb at the baseband signal, which originates from the periodic amplitude modulation of the scattered signal. Using this concept, we have demonstrated capabilities to localize a rotating scatterer with sub0cm accuracy, approaching millimeter-scale precision. After performing a large set of experiments in heavy clutter conditions, we identified the need to filter out multipath contributions. For this purpose, we have developed a new system, capable to detect the angle of arrival of an electromagnetic system. Direction of arrival (DoA) estimation is of primary importance in a broad range of wireless applications, where electromagnetic waves play a role. While a vast majority of existing techniques is based on phase lag comparison in antenna arrays, intensity-based approaches are valuable in a range of low-budget applications. Here we demonstrate a direct visible to a naked eye DoA device, based on a Fresnel zone plate lens, aperture, and a light-emitting diode indicator. Being a low-budget device, it still allows achieving up to 90° angle of view, 19° of angular resolution, and 11° of angular accuracy at 10 GHz operational frequency. The demonstrated approach provides fast DoA visualization and can be used to adjust point-to-point communication links, identify radio wave pollution sources at home conditions and several others. Having DoA information in hand, it will become possible to identify multipath contributions and improve the accuracy of our method, elevating it to new frontiers.

 

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Annotation of the results obtained in 2020
The research program contains three main work packages, which are designated to serve the main objective - accurate radiolocation of a rotating scatterer. During the first year we made significant progress in development of basic components, which include strong scattering from a single object, design and demonstration of new scatterers with angular-dependent differential cross section, and new approaches for radio-based triangulation. Hereinafter the main achievements will be briefly surveyed. 1) Strong scattering and it’s enhancement with the aid of metamaterials Increasing scattering efficiencies of subwavelength structures is a long-standing objective of applied electromagnetic theory. Quite a few fundamental bounds have been proposed and subsequently challenged with advanced designs. For example, the celebrated Chu-Harrington limit defines minimally achievable quality factors (Q factors) of subwavelength lossless resonators. On another hand, it is relatively well known that the maximal scattering efficiency of a resonant subwavelength object does not depend on its size if internal material losses are neglected. In a single resonance case, a fundamental limit of scattering is (2L + 1)λ2/(2π), where λ is the free space wavelength and L is related to orbital angular momentum of a multipolar resonance (L = 1 is the electric or magnetic dipolar case). A vast majority of reported designs operates at dipolar resonances. In the case of lossless structures the upper dipolar bound is 3λ2/(2π). While subwavelength structures can be quite efficient scatterers, the penalty of size reduction is a dramatic growth in the Q factor, described in a single resonance case by the ChuHarrington bound. A strategy to bypass this limitation is to involve several spectrally overlapping multipolar resonances. Volumetric reconfigurable metamaterial was designed and experimentally demonstrated, and the concept of tunable RF magnon resonance that is controllable with light was developed. The architecture is based on arrays of split ring resonators (SRRs) that serve as microscopic magnetic dipoles, which hybridize to a collective magnetic mode. Each SRR was loaded with a tandem of a varactor and a driving photodiode, which allowed the replacement of branched conducting wire network with optical fibers that are transparent to RF waves. It was shown that artificial magnon resonance within the metamaterial-based sphere can be controlled by light and that scattering cross sections are affected by it in real time. This demonstration can link to new solutions in the field of wireless communications and radio frequency localization, where real time control over scattering cross-sections is highly demanded. (4D Optically Reconfigurable Volumetric Metamaterials, D. Dobrykh, A. Mikhailovskaya, P. Ginzburg, and D. Filonov; Phys. Status Solidi RRL 2000159, 1-6 (2020), https://doi.org/10.1002/pssr.202000159). The second accomplishment concentrated on collocating several resonances within the same metamaterials-based structure. Design of spectrally overlapping resonances is typically challenging due to the inherent coupling, which affects both of the responses simultaneously. In other words, optimizing one resonance implies affecting another one in an uncontrollable fashion. Nevertheless, we have demonstrated a new design, where both electric and magnetic resonances are co-located in frequency. New type of metamaterial was obtained with parallel stacking of printed boards - one side of each board is occupied with electric resonators, while an opposite side with magnetic. Meandered dipoles and split ring resonators were used to achieve electric and magnetic response simultaneously. For the summary, we have demonstrated a metamaterial-based sphere, which has both permittivity and permeability negative. Those negative values are close to -2, which corresponds to an artificial plasmonic and magnonic resonances. This spectral overlap is shown to dramatically increase the overall scattering cross section of the device. 2) Investigation of scattering from rotating bodies and maximization of their time-dependent signatures Investigation of electromagnetic processes in moving coordinate systems requires applying a certain set of transformations to the laws of electrodynamics formulated for reference frames at rest. Relativistic effects could substantially change the regular form of Maxwell’s equations, material constitutive relations, and boundary conditions, especially in the case of accelerated motion, which in the most general case, requires applying the formalism of the general theory of relativity. Nevertheless, the majority of practical applications take place in regimes of slow motion and acceleration, where simplifying approximations can be applied. One of the practical examples dealing with nonrelativistic scenarios is radio detection and ranging (radar), where distant objects are probed with short pulses. Time delay between transmitted and received pulses enables extracting the range, while Doppler shifts hold information about velocities. Approximate electromagnetic analysis of the phenomenon relies on a set of static simulations, stitched together along a mechanical path of an object. Here, this approach will be referred to as adiabatic. Full characterization of a mechanical motion also requires knowledge of accelerations, which can be estimated using additional spectral analysis. For example, techniques for detection of helicopter propellers (axial rotation implies accelerated motion) exist. Accelerated bodies, in contrast to uniformly moving objects, produce much richer spectral signatures, called micro-Doppler shifts. In particular, objects illuminated by a field of frequency ω generate a frequency comb at a scattered field: the peaks are equidistant and situated at ω ± n, where is the angular frequency of rotation and n is an integer number. It is worth noting that the additional spectral features are generated by time-varying boundary conditions. Furthermore, it was shown that phase retardation effects along a rotating scatterer control the amplitudes of the peaks in the comb. Along with the phase accumulation effects, the symmetry properties of an object have crucial importance. For example, scatterers possessing reflection symmetry and rotating around their centers cannot generate even peaks in the comb. The degree of asymmetry is important for remote characterization of distant moving objects, and its impact will be investigated here by the example of an asymmetric scatterer. Our investigation is concentrated on studies of frequency combs produced by asymmetric rotating objects - here we perform theoretical, numerical, and experimental analysis with the help of a lock-in detection scheme. Our main emphasis is on electromagnetically small objects. 3) Triangulation of rotating object Identification of a rotating object in a clutter can be conceptually different from any typical triangulation scheme. The known approaches can be separated into two main domains - active pelengaiton and detection of a passive target. Active pelengation is based on identification of a transmitted signal. The latter may be unknown - in this case cross correlation at different reviving points (multi-static configuration) is performed with the purpose of identifying time delays. Those delays correspond to the time of flight, it takes a signal to propagate from the transmitter (the subject of pelengation) to the receivers. A knowledge of several time delays allows reconstructing the transmitter position. It is worth noting that the accuracy of this method is limited by the sampling rate at the receivers and cannot be more accurate than tens of centimeters cube if state of the art equipment is in use. It is worth emphasizing that the strong assumption of this method is that the target generates a signal. The second technique for target’s triangulation is based on an active radar-type search. Here a radar pulse is sent elsewhere and an antenna array listens for an echo. Antenna array performs co-called DOA measurement (detection of angle of arrival). Knowledge of the target’s angle from several positions allows performing the triangulation. This technique, being widely used for detecting airborne targets, obviously cannot operate in clutter. If multiple reflectors are present, DOA detection becomes impossible due to the dramatically low signal to noise ratios. During the first year of our research we have developed a new technique for triangulation. Our approach is conceptually different from the beforehand mentioned techniques and allows obtaining accuracies which are by an order of magnitude better than the current state of the art. In particular, we can locate an object with mm-scale accuracy. Our technique is based on physical aspects of micro-Doppler comb generation and, in particular, on polarization properties of the electromagnetic field. In this case we have demonstrated the ability to detect a passive(non transmitting) target in a clutter (a class room with many participants).

 

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Annotation of the results obtained in 2021
During the second year of the Research we approached the solution for a triangulation problem, where the rotating beacon has no direct line of sight to interrogating towers. Our approach is conceptually different from other existing solutions, which include Time of Arrival and Angle of Arrival. The first of the beforehand mentioned methods is based on measuring time difference in signals, received by spatially spread towers. The localization accuracy here is tightly related with the transmitted signals bandwidth and cannot be performed with quasi-continuous wave sources. Angle of Arrival method performs the localization by measuring angles of signals, scattered or transmitted from a beakone. Several observation towers, each one containing an antenna array is needed in this case. The accuracy here is linked with apertures of the receiving arrays. In a sharp contrast to the above mentioned methods, our approach does not rely neither on bandwidth nor on large antenna arrays. In this case, the advantages in case of non line of sight detection and detection in a clutter with a multipath interference are quite evident. For a practical realization of our concept we have developed several electromagnetic structures, having high scattering cross sections. This quantity also has a strong angular dependence, which allows filtering the field, scattered on a rotating object, from a clutter. Our next step was showing an efficient triangulation in a plane. Our experiments show the capability of object’s triangulation with centimeter accuracy. The bandwidth of the signals, involved in the process, was shown to approach 0. In a broader sense, we traded the bandwidth for a spatial extent of our system. Taking our methods for practical applications, we started to investigate dynamics in assemblies of stochastically moving miniature robots, where each one has to be monitored at a real time. Our experimental system allowed us to observe a range of new phenomena, including creation of spatial patterns, made by robot swarms. We demonstrated pathways to control the collective stochastic motion by tuning their interaction parameters. Our navigation system can be used to monitor multiple robots in cases where there is not a direct line of sight to each one of the participants.

 

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