Advanced signal processing tools for ballistic missile defence and space situational awareness

The research presented in this Thesis deals with signal processing algorithms for the classification of sensitive targets for defence applications and with novel solutions for the detection of space objects. These novel tools include classification algorithms for Ballistic Targets (BTs) from both micro-Doppler (mD) and High Resolution Range Profiles (HRRPs) of a target, and a space-borne Passive Bistatic Radar (PBR) designed for exploiting the advantages guaranteed by the Forward Scattering (FS) configuration for the detection and identification of targets orbiting around the Earth.;Nowadays the challenge of the identification of Ballistic Missile (BM) warheads in a cloud of decoys and debris is essential in order to optimize the use of ammunition resources. In this Thesis, two different and efficient robust frameworks are presented. Both the frameworks exploit in different fashions the effect in the radar return of micro-motions exhibited by the target during its flight.;The first algorithm analyses the radar echo from the target in the time-frequency domain, with the aim to extract the mD information. Specifically, the Cadence Velocity Diagram (CVD) from the received signal is evaluated as mD profile of the target, where the mD components composing the radar echo and their repetition rates are shown.;Different feature extraction approaches are proposed based on the estimation of statistical indices from the 1-Dimensional (1D) Averaged CVD (ACVD), on the evaluation of pseudo-Zerike (pZ) and Krawtchouk (Kr) image moments and on the use of 2-Dimensional (2D) Gabor filter, considering the CVD as 2D image. The reliability of the proposed feature extraction approaches is tested on both simulated and real data, demonstrating the adaptivity of the framework to different radar scenarios and to different amount of available resources.;The real data are realized in laboratory, conducting an experiment for simulating the mD signature of a BT by using scaled replicas of the targets, a robotic manipulator for the micro-motions simulation and a Continuous Waveform (CW) radar for the radar measurements.;The second algorithm is based on the computation of the Inverse Radon Transform (IRT) of the target signature, represented by a HRRP frame acquired within an entire period of the main rotating motion of the target, which are precession for warheads and tumbling for decoys. Following, pZ moments of the resulting transformation are evaluated as final feature vector for the classifier. The features guarantee robustness against the target dimensions and the initial phase and the angular velocity of its motion.;The classification results on simulated data are shown for different polarization of the ElectroMagnetic (EM) radar waveform and for various operational conditions, confirming the the validity of the algorithm.The knowledge of space debris population is of fundamental importance for the safety of both the existing and new space missions. In this Thesis, a low budget solution to detect and possibly track space debris and satellites in Low Earth Orbit (LEO) is proposed.;The concept consists in a space-borne PBR installed on a CubeSaT flying at low altitude and detecting the occultations of radio signals coming from existing satellites flying at higher altitudes. The feasibility of such a PBR system is conducted, with key performance such as metrics the minimumsize of detectable objects, taking into account visibility and frequency constraints on existing radio sources, the receiver size and the compatibility with current CubeSaT's technology.;Different illuminator types and receiver altitudes are considered under the assumption that all illuminators and receivers are on circular orbits. Finally, the designed system can represent a possible solution to the the demand for Ballistic Missile Defence (BMD) systems able to provide early warning and classification and its potential has been assessed also for this purpose.The research presented in this Thesis deals with signal processing algorithms for the classification of sensitive targets for defence applications and with novel solutions for the detection of space objects. These novel tools include classification algorithms for Ballistic Targets (BTs) from both micro-Doppler (mD) and High Resolution Range Profiles (HRRPs) of a target, and a space-borne Passive Bistatic Radar (PBR) designed for exploiting the advantages guaranteed by the Forward Scattering (FS) configuration for the detection and identification of targets orbiting around the Earth.;Nowadays the challenge of the identification of Ballistic Missile (BM) warheads in a cloud of decoys and debris is essential in order to optimize the use of ammunition resources. In this Thesis, two different and efficient robust frameworks are presented. Both the frameworks exploit in different fashions the effect in the radar return of micro-motions exhibited by the target during its flight.;The first algorithm analyses the radar echo from the target in the time-frequency domain, with the aim to extract the mD information. Specifically, the Cadence Velocity Diagram (CVD) from the received signal is evaluated as mD profile of the target, where the mD components composing the radar echo and their repetition rates are shown.;Different feature extraction approaches are proposed based on the estimation of statistical indices from the 1-Dimensional (1D) Averaged CVD (ACVD), on the evaluation of pseudo-Zerike (pZ) and Krawtchouk (Kr) image moments and on the use of 2-Dimensional (2D) Gabor filter, considering the CVD as 2D image. The reliability of the proposed feature extraction approaches is tested on both simulated and real data, demonstrating the adaptivity of the framework to different radar scenarios and to different amount of available resources.;The real data are realized in laboratory, conducting an experiment for simulating the mD signature of a BT by using scaled replicas of the targets, a robotic manipulator for the micro-motions simulation and a Continuous Waveform (CW) radar for the radar measurements.;The second algorithm is based on the computation of the Inverse Radon Transform (IRT) of the target signature, represented by a HRRP frame acquired within an entire period of the main rotating motion of the target, which are precession for warheads and tumbling for decoys. Following, pZ moments of the resulting transformation are evaluated as final feature vector for the classifier. The features guarantee robustness against the target dimensions and the initial phase and the angular velocity of its motion.;The classification results on simulated data are shown for different polarization of the ElectroMagnetic (EM) radar waveform and for various operational conditions, confirming the the validity of the algorithm.The knowledge of space debris population is of fundamental importance for the safety of both the existing and new space missions. In this Thesis, a low budget solution to detect and possibly track space debris and satellites in Low Earth Orbit (LEO) is proposed.;The concept consists in a space-borne PBR installed on a CubeSaT flying at low altitude and detecting the occultations of radio signals coming from existing satellites flying at higher altitudes. The feasibility of such a PBR system is conducted, with key performance such as metrics the minimumsize of detectable objects, taking into account visibility and frequency constraints on existing radio sources, the receiver size and the compatibility with current CubeSaT's technology.;Different illuminator types and receiver altitudes are considered under the assumption that all illuminators and receivers are on circular orbits. Finally, the designed system can represent a possible solution to the the demand for Ballistic Missile Defence (BMD) systems able to provide early warning and classification and its potential has been assessed also for this purpose

Understanding BL Lac objects Structural & kinematic mode changes in the BL Lac object PKS 0735+178

Context. We present evidence that parsec-scale jets in BL Lac objects may be significantly distinct in kinematics from their counterparts in quasars. We argued this previously for the BL lac sources 1803+784 and 0716+714, report here a similar pattern for another well-known BL Lac object, PKS 0735+178, whose nuclear jet is found to exhibit kinematics atypical of quasars. Aims. A detailed study of the jet components' motion reveals that the standard AGN paradigm of apparent superluminal motion does not always describe the kinematics in BL Lac objects. We study 0735+178 here to augment and improve the understanding of the peculiar motions in the jets of BL Lac objects as a class. Methods. We analyzed 15 GHz VLBA (Very Long Baseline Array) observations (2cm/MOJAVE survey) performed at 23 epochs between 1995.27 and 2008.91. Results. We found a drastic structural mode change in the VLBI jet of 0735+178, between 2000.4 and 2001.8 when its twice sharply bent trajectory turned into a linear shape.We further found that this jet had undergone a similar transition sometime between December 1981 and June 1983. A mode change, occurring in the reverse direction (between mid-1992 and mid-1995) has already been reported in the literature. These structural mode changes are found to be reflected in changed kinematical behavior of the nuclear jet, manifested as an apparent superluminal motion and stationarity of the radio knots. In addition, we found the individual mode changes to correlate in time with the maxima in the optical light curve. The last two transitions occurred before a (modest) radio flare. The behavior of this pc-scale jet appears to favor a scenario involving non-ballistic motions of the radio knots, produced by the precession of a continuous jet within the ambient medium.Comment: Accepted for publication in A&A (Abstract reduced for astro-ph

Novel classification algorithm for ballistic target based on HRRP frame

Nowadays the identification of ballistic missile warheads in a cloud of decoys and debris is essential for defence systems in order to optimize the use of ammunition resources, avoiding to run out of all the available interceptors in vain. This paper introduces a novel solution for the classification of ballistic targets based on the computation of the inverse Radon transform of the target signatures, represented by a high resolution range profile frame acquired within an entire period of the main rotation of the target. Namely, the precession for warheads and the tumbling for decoys are taken into account. Following, the pseudo-Zernike moments of the resulting transformation are evaluated as the final feature vector for the classifier. The extracted features guarantee robustness against target's dimensions and rotation velocity, and the initial phase of the target's motion. The classification results on simulated data are shown for different polarizations of the electromagnetic radar waveform and for various operational conditions, confirming the validity of the algorithm

Observational Signatures of Supermassive Black Hole Binaries

Despite solid theoretical and observational grounds for the pairing of supermassive black holes (SMBHs) after galaxy mergers, definitive evidence for the existence of close (sub-parsec) separation SMBH binaries (SMBHBs) approaching merger is yet to be found. This chapter reviews techniques aimed at discovering such SMBHBs in galactic nuclei. We motivate the search with a brief overview of SMBHB formation and evolution, and the gaps in our present-day theoretical understanding. We then present existing observational evidence for SMBHBs and discuss ongoing efforts to provide definitive evidence for a population at sub-parsec orbital separations, where many of the aforementioned theoretical gaps lie. We conclude with future prospects for discovery with electromagnetic (primarily time-domain) surveys, high-resolution imaging experiments, and low-frequency gravitational-wave detectors.Comment: To appear in Chapter 5 in the book Black Holes in the Era of Gravitational Wave Astronomy, ed. Arca Sedda, Bortolas, Spera, pub. Elsevier. All authors equally contributed to the Chapter writin

Gamma-ray flaring activity from the gravitationally lensed blazar PKS 1830-211 observed by Fermi LAT

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope routinely detects the highly dust-absorbed, reddened, and MeV-peaked flat spectrum radio quasar PKS 1830-211 (z=2.507). Its apparent isotropic gamma-ray luminosity (E>100 MeV) averaged over $\sim$ 3 years of observations and peaking on 2010 October 14/15 at 2.9 X 10^{50} erg s^{-1}, makes it among the brightest high-redshift Fermi blazars. No published model with a single lens can account for all of the observed characteristics of this complex system. Based on radio observations, one expects time delayed variability to follow about 25 days after a primary flare, with flux about a factor 1.5 less. Two large gamma-ray flares of PKS 1830-211 have been detected by the LAT in the considered period and no substantial evidence for such a delayed activity was found. This allows us to place a lower limit of about 6 on the gamma rays flux ratio between the two lensed images. Swift XRT observations from a dedicated Target of Opportunity program indicate a hard spectrum and with no significant correlation of X-ray flux with the gamma-ray variability. The spectral energy distribution can be modeled with inverse Compton scattering of thermal photons from the dusty torus. The implications of the LAT data in terms of variability, the lack of evident delayed flare events, and different radio and gamma-ray flux ratios are discussed. Microlensing effects, absorption, size and location of the emitting regions, the complex mass distribution of the system, an energy-dependent inner structure of the source, and flux suppression by the lens galaxy for one image path may be considered as hypotheses for understanding our results.Comment: 14 pages, 6 figures, 2 tables. Accepted by the The Astrophysical Journal. Corresponding authors: S. Ciprini (ASI ASDC & INAF OAR, Rome, Italy), S. Buson (INAF Padova & Univ. of Padova, Padova, Italy), J. Finke (NRL, Washington, DC, USA), F. D'Ammando (INAF IRA, Bologna, Italy

Stellar Processes Near the Massive Black Hole in the Galactic Center

A massive black hole resides in the center of most, perhaps all galaxies. The one in the center of our home galaxy, the Milky Way, provides a uniquely accessible laboratory for studying in detail the connections and interactions between a massive black hole and the stellar system in which it grows; for investigating the effects of extreme density, velocity and tidal fields on stars; and for using stars to probe the central dark mass and probe post-Newtonian gravity in the weak- and strong-field limits. Recent results, open questions and future prospects are reviewed in the wider context of the theoretical framework and physical processes that underlie them. Contents: [1] Introduction (1.1) Astrophysical context (1.2) Science questions (1.3) Scope and connections to related topics [2] Observational overview: Stars in the Galactic center (2.1) The central 100 parsecs (2.2) The central parsec [3] Stellar dynamics at extreme densities (3.1) Physical processes and scales (3.2) The stellar cusp in the Galactic center (3.3) Mass segregation (3.4) Stellar Collisions [4] Probing the dark mass with stellar dynamics (4.1) Weighing and pinpointing the dark mass (4.2) Constraints on non-BH dark mass alternatives (4.3) Limits on MBH binarity (4.4) High-velocity runaway stars [5] Probing post-Newtonian gravity near the MBH (5.1) Relativistic orbital effects (5.2) Gravitational lensing [6] Strong star-MBH interactions (6.1) Tidal disruption (6.2) Dissipative interactions with the MBH [7] The riddle of the young stars (7.1) The difficulties of forming or importing stars near a MBH (7.2) Proposed solutions (7.3) Feeding the MBH with stellar winds [8] Outlook (8.1) Progress report (8.2) Future directionsComment: Invited review article, to appear in Physics Reports. 101 p

Flux Density and VLBI Measurements of the IDV Source 0917+624

Extragalactic compact flat spectrum radio sources are known to be highly variable and about 30% of them show intra-day variability (IDV). The quasar 0917+624 was known to show prominent IDV behaviour from the time it was first studied as a short-time variable source. Meanwhile, however, its variability ceased and yet the reason for that is not clear. The working hypothesis for the observations, on which the main part of this thesis is based, was that structural changes of the variable part of the source have caused the variability decline. These changes could be revealed using high resolution observing techniques, namely VLBI. Several epochs of single-dish flux density measurements performed with the Effelsberg 100m radio telescope between 2001 and 2004 as well as several multi-frequency epochs of VLBI observations of 0917+624 over a period of eight years (1999 to 2007) have been analysed concerning particularly kinematic and polarimetric properties and the results are interpreted in terms of Intraday Variability

High-resolution VLBI Studies of the Blazars TXS 2013+370, OJ 287, and 3C 454.3

Blazars are the most luminous sub-class of active galactic nuclei (AGN). Powered by an accreting supermassive black hole (SMBH), these systems are characterized by an axisymmetric pair of powerful relativistic jets that emanate from their central region and are closely aligned to the line of sight of the observer. Thanks to this geometrical coincidence, blazars constitute a unique case of astrophysical objects in which we can study the extreme physical conditions associated with the jet launching region and the strong magnetic fields in the vicinity of the central engine. To date, even though numerous observational and theoretical studies have been established for an improved understanding of the underlying jet physics, a number of questions are pending to be answered. The origin of the high-energy emission and the seed photon field, the true nature of the observed jet base, and the jet phenomenology are investigated in this thesis. For this purpose, we employ a state-of-the-art observational technique called very-long baseline interferometry (VLBI). Simultaneous radio observations at 86 GHz using the largest radio antennas in the world, as well as complementary observations of ground array elements with the space radio telescope RadioAstron, give us the unique opportunity to focus on three case studies: the blazars TXS 2013+370, OJ 287 and 3C 454.3. In the blazar TXS 2013+370, quasi-simultaneous VLBI observations at 15, 43 and 86 GHz, along with space-VLBI data at 22 GHz, allowed us to investigate the jet base with an angular resolution of ≥ 0.4 pc. In combination with broad-band variability observations and γ-ray data, the high-resolution VLBI imaging revealed the ejection of new jet features, accompanied by flaring activity in radio/mm- bands and γ rays. The analysis of the transverse jet width profile constrained the mm-VLBI core to be located within ≤ 2 pc downstream of the jet apex, and also showed the existence of a transition from parabolic to conical jet expansion at a deprojected distance of ~54 pc from the core. The estimation of the intrinsic jet parameters allowed us to determine the magnetic field strength in the 22 GHz VLBI core region to be B(SSA)= 0.36 ± 0.16 G. Cross-correlation analysis of the broad-band variability revealed a strong correlation between the radio-mm and γ-ray data, with the 1 mm emission lagging ~49 days behind the γ rays. Based on this, we infer that the high energy emission is produced at a distance of the order of ~1 pc from the jet apex, suggesting that the seed photon field for the external Compton mechanism originates either in the dusty torus or in the broad-line region. The study of the blazar OJ 287 was based on a unique data set, comprising 15, 43 and 86 GHz VLBI observations along with contemporaneous space-VLBI data at 22 GHz, which enabled us to reconstruct for the first time the fine structure of the innermost compact region with the record-resolution of 10 μas. Total intensity and polarization images of the source revealed a complex structure, with the brightest core feature being located a ~10 pc downstream from the innermost jet component. The source modeling enabled us to perform a spectral decomposition analysis of the VLBI knots, which led us to the determination of the synchrotron turnover parameters. Additionally, through spectral index mapping, we studied the spectral evolution along the radio jet. The results of the spectral fitting were used to obtain the magnetic field strength of the innermost jet features. We calculated the equipartition Doppler factor to be ≥ δ(eq)=2.9±0.26. A variation of the Doppler factor along the jet was also observed, and its origin is unclear. The most likely explanation is the presence of a strong jet bending towards the line of sight, a local change of the plasma speed, or a superposition of both. Polarization imaging and Faraday rotation analysis based on two independent methods helped us to probe the magnetic field topology on sub-parsec scales. A rotation measure analysis in the core region revealed a rotation between -440 to -1100 rad/m^2 (obtained from a pixel-based analysis and single-dish flux density variability measurements). By combining the imaging and the rotation measure (RM) analysis, we report indications of the existence of a helical magnetic field in the OJ 287 core region, which is in agreement with similar polarization studies. Furthermore, during our observational interval, a prominent flaring event took place, which allowed us to study the observed brightness temperature evolution in the VLBI core region. We report a rising trend for the innermost VLBI feature during the flare evolution, rising from T(b)=(33.6±0.8)x10^11 K up to 5.5±0.9x10^12 K. Using the computed δ(eq), we estimated that the intrinsic brightness temperature is T(int)≈10^12 K in the core region, which is significantly above from the equipartition limit of ~5x10^10 K. This implies that the VLBI core in OJ 287 is particle-dominated. Lastly, for the blazar 3C 454.3 we analyzed 24 epochs of VLBI data at 43 and 86 GHz. In this thesis, we present for the first time a five-year structural and kinematic study of the innermost jet region of this source, probed by the ultra-high resolution of 50 μas (≥0.4 pc). The results of the analysis revealed that the flux density distribution along the jet is described by components that move with apparent speeds between 6 c to 26 c, as well as by stationary features. We trace the appearance of 7 new VLBI components during the observing interval of 2013-2017. The detected knots show unusual behavior in their velocity pattern, with a transition from fast motion to apparent stationarity and merging when they reach a radial distance of 0.5 mas from the core. The nature of this region was investigated by studying the spectral index variability and the linear polarization of an indicative epoch; however, the physical interpretation remains ambiguous, with possible scenarios involving local jet bending, standing shock or plasma instabilities. Also, indications of trailing jet components have been found, which are related to rarefaction in the near wake of the most prominent propagating disturbances. Lastly, we report that the newly detected features are ejected at different position angles, pointing to more complicated ejection scenarios related to accretion disk precession, jet nozzle precession, or jet instabilities

Gamma-Ray Emission from Microquasars

Microquasars, X-ray binary systems that generate relativistic jets, were discovered in our Galaxy in the last decade of the XXth century. Their name indicates that they are manifestations of the same physics as quasars but on a completely different scale. Parallel to this discovery, the EGRET instrument on board of the Compton Gamma Ray Observatory detected 271 point like gamma-ray sources 170 of which were not clearly identified with known objects. This marked the beginning of gamma-ray source population studies in the Galaxy. We present in this thesis models for gamma-ray production in microquasars with the aim to propose them as possible parent populations for different groups of EGRET unidentified sources. These models are developed for a variety of scenarios taking into account several possible combinations, i.e. black holes or neutron stars as the compact object, low mass or high mass stellar companions, as well as leptonic or hadronic gamma-ray production processes. We also show that the presented models for gamma-rays emitting microquasars can be used to explain observations from well known sources that are detected in energy ranges other than EGRET's. Finally, we include an alternative gamma-ray producing situation that does not involve microquasars but a specific unidentified EGRET source possibly linked to a magnetized accreting pulsar.Comment: PhD Thesis supervised by G.E. Romero and presented at the University of Buenos Aires (UBA) on December 15th, 200