Juha Jylhä

On efficient characterization of radar targets with scatterer sets for target recognition using commercial ray tracing software

The model-based radar target recognition requires the ability to simulate target signatures with adequate accuracy and computational speed. This paper proposes an entirely novel approach to utilizing readily available ray tracing software in the target characterization. The objective of the proposed approach is to give the target a compact description, from which arbitrary radar response can be simulated with efficiency. Certain essential computational aspects, such as ray density reduction and frequency-independency, are discussed. The simulated radar response is compared with a measurement, covering the case of a single frequency, high range resolution profiles, and inverse synthetic aperture radar images.

New aspects to knowledge-aided clutter analysis

Digital signal processing allows improvements in site-specific clutter prediction. With digital terrain maps and a flight obstacle register, land clutter origin can be solved. An efficient, knowledge-aided approach to extracting homogeneous clutter from radar signal is presented. Once homogeneous clutters statistic has been recognized, also mixture models can be constructed. The suggested aspects are illustrated through an air surveillance radar simulation. The enhancement attained in clutter analysis and thus in clutter models is the novelty of the presented aspects.

Incorporating a stochastic model of the target orientation into a momentary RCS distribution

The radar cross section (RCS) of a target describes its reflectivity towards the radar. It is a deterministic quantity-specific to the aspect angle of the target and the properties of the radar signal-and as such can be predicted with various methods when provided with an adequate physical model of the target. However, stochasticity is introduced when considering a target during flight and several factors increasing the uncertainty-typically about the location and orientation of the target in relation to the observing radar-have to be taken into account. It is possible to model these uncertainties when adequate information about physical properties of the target as well as about its flight path and motion mode is available. In this paper, we propose a method for incorporating such knowledge about the flight dynamics into the estimation of a momentary distribution for the RCS. We use a simulated trajectory to demonstrate the method and present comparisons to the Swerling I model.

Classification of items in a walk-through metal detector using time series of eigenvalues of the polarizability tensor

During the last decade, the safety regulations of the airports have been set to a new level. As the number of passengers is constantly increasing, yet effective but quick security control at checkpoints sets great requirements to the 21st century security systems. In this paper, we shall introduce a novel metal detector concept that enables not only to detect but also to classify hidden items, though their orientation and accurate location are unknown. Our new prototype walk-through metal detector generates mutually orthogonal homogeneous magnetic fields so that the measured dipole moments allow classification of even the smallest of the items with high degree of accuracy in real-time. Invariant to different rotations of an object, the classification is based on eigenvalues of the polarizability tensor that incorporate information about the item (size, shape, orientation etc.); as a further novelty, we treat the eigenvalues as time series. In our laboratory settings, no assumptions concerning the typical place, where an item is likely situated, are made. In that case, 90 % of the dangerous and harmless items, including knives, guns, gun parts, belts etc. according to a security organisation, are correctly classified. Made misclassifications are explained by too similar electromagnetic properties of the items in question. The theoretical treatment and simulations are verified via empirical tests conducted using a robotic arm and our prototype system. In the future, the state-of-the-art system is likely to speed-up the security controls significantly with improved safety.

Minimizing Fatigue Damage in Aircraft Structures

Aircraft structural health monitoring (SHM) refers to a process in which sensors assess the current (and predict the future) state of a structure in terms of its aging and deterioration to assure users or operators of its safety and performance. In addition to preventing failures, SHM extends aircraft life cycles. Consequently, adopting SHM is strongly motivated not only by flight safety but also by economic considerations. This article focuses on the optimization of aircraft usage as a new aspect of SHM and discusses a knowledge discovery approach based on dynamic time warping and genetic programming. In addition, it points out some of the challenges faced in applying artificial intelligence to aircraft SHM. This novel work reveals that AI provides a means to gain valuable knowledge for decision making on cost-efficient future usage of an aircraft fleet.

High-resolution acoustic imaging in air by synthetic aperture using pixel-wise matched kernels

This paper introduces a device and needed signal processing for high-resolution acoustic imaging in air. The device employs off the shelf audio hardware and linear frequency modulated (LFM) pulse waveform. The image formation is based on the principle of synthetic aperture. The proposed implementation uses inverse filtering method with a unique kernel function for each pixel and focuses a synthetic aperture with no approximations. The method is solid for both far-field and near-field and easily adaptable for different synthetic aperture formation geometries. The proposed imaging is demonstrated via an inverse synthetic aperture formation where the object rotation by a stepper motor provides the required change in aspect angle. Simulated and empirical results are presented. Measurements have been done using a conventional speaker and microphones in an ordinary room with near-field distance and strong static echoes present. The resulting high-resolution 2-D spatial distribution of the acoustic reflectivity provides valuable information for many applications such as object recognition.

Performance evaluation of radar NCTR using the target aspect and signature

In this paper, we present a framework for assessing and comparing the performance of non-cooperative target recognition methods that are based on radar signatures. The proposed framework aspires to quantify the performance considering the dynamic nature of the operating conditions. When evaluating the recognition performance, the training data is generated according to the operational scenario which specifies for instance the values for the true aspect angle, at which the target is observed, and the ability to estimate it, e.g. by a radar tracker. The performance of different classifiers can be assessed and compared considering the varying ability to observe the location and the orientation of the target. The proposed framework is demonstrated with the radar cross section as the feature for classification using values on a single frequency and high range resolution profiles.

Converting a 3D surface into a set of directional scatterers for high-resolution radar response simulation

It is practical and efficient to simplify targets to point scatterers in radar simulations. With low-resolution radars, the radar cross section (RCS) is a sufficient feature to characterize the scattering properties of a target. However, the RCS totals the target scattering properties to a scalar value for each aspect angle. Thus, a more detailed representation of the target is required with high-resolution radar techniques, such as Inverse Synthetic-Aperture Radar (ISAR). In straightforward simulation scenarios, high-resolution targets have been modeled placing identical point scatterers in the shape of the target, or with a few dominant point scatterers. As extremely simple arrangements, these do not take the self-shadowing into account and are not realistic enough for high demands. Our radar response simulation studies required a target characterization akin to RCS, which would also function in highresolution cases and take the self-shadowing and multiple reflections into account. Thus, we propose an approach to converting a 3-dimensional (3D) surface into a set of scatterers with locations, orientations, and directional scattering properties. The method is intended for far field operation, but could be adjusted for use in the near field. It is based on ray tracing which provides the self-shadowing and reflections naturally. In this paper, we present ISAR simulation results employing the proposed method. The constructed scatterer set is scalable for different wavelengths enabling the fast production of realistic simulations including authentic RCS scattering center formation. This paper contributes to enhancing the reality of the simulations, yet keeping them manageable and computationally reasonable.

Link between Flight Maneuvers and Fatigue

Structural integrity management is in a key role when operating with an ageing fleet. Preventive actions aid in keeping structures healthy and ensuring the designed lifetime for the aircraft. Our research focuses on exploitation of collected usage data by identifying actual in-flight events that cause the major fatigue life expenditure of the fatigue-critical structural details. In order to build a link between the events and damage, we have developed software for flight maneuver identification. As a latest advancement, we have created data models for several flight maneuvers and constructed a model library, referred to as template library. The library instructs the software about what the interesting events look like in the data. Our software together with the template library allows us to perform maneuver-specific fatigue assessment and achieve knowledge concerning the fatigue-criticality of various flight maneuver types. This lays a foundation for detailed analysis of the identified, nominally similar, maneuvers and identification of small crucial actions within the maneuvers that are behind the fatigue. In this paper, we consider the issues related to maneuver-specific fatigue assessment and present analysis results for four structural details of F-18 aircraft with a template library of seven flight maneuvers. We summarize the requirements and prospect of our fatigue analysis approach and prove its applicability.

On Sequential On-Line Outlier Detection and a Linescan Application

Industrial quality monitoring is increasing rapidly, and challenging signal environments with requirement of steady performance pose conflicting demands to on-line tests. The sequential probability ratio test (SPRT) and the Kalman filter (KF) are proposed as two tools for detection and recognition-oriented signal processing. A modified sequential test is suggested and applied to a linescan problem