Inverse Synthetic Aperture Radar Simulator Implementation .

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T KOSTIS et al: INVERSE SYNTHETIC APERTURE RADAR SIMULATOR IMPLEMENTATIONInverse Synthetic Aperture Radar Simulator Implementation for an ExtendedNaval Target for Electronic Warfare ApplicationsTheodoros G. KostisSokratis K. KatsikasDept. of Information &Communication Systems EngineeringUniversity of the AegeanKarlovassi, Samos 83200,Greece.e-mail: tkostis@iee.orgDept. of Technology Education &Digital SystemsUniversity of Piraeus150 Androutsou Street, Piraeus 18534, Greece.e-mail: ska@unipi.grAbstract—Airborne Inverse Synthetic Aperture Radar systems provide excellent surveillance abilities for imaging naval targets.Considering this as a maritime surveillance threat we derive a simulator system in this paper that can be used to synthesize falsetargets that would appear as credible military fleets. The resulting electromagnetic signature synthesis allows the implementation ofreturns that appear to backscatter from areas of high reflectivity as arranged in military superstructures instead of commercialcounterparts. The conceptual model for this simulation is constructed by considering standard representation theory. Themodelling approach follows standard abstraction degrees for the three-dimensional synthetic environment, target fidelity andinverse scattering. The pace engine that acts as the computing moving force for the simulation is mathematically presented. Theinverse scattering signal follows standard ISAR theoretical guidelines. The data exchange standard between the various elements ispresented. Issues in implementation and project success are discussed by drawing upon the verification and validation analyses.Inverse Synthetic Aperture Radar; Simulator; Conceptual Model; Complex System; Extended Naval Target.Such simulator system implementations provide anadded value because they enable design engineers toenvision novel just-in-time countermeasure systemsdesigns and moreover allow the drafting of appropriateproofs of concept [14][15][16]. Therefore the problem thatthis paper is trying to address is the conceptual andimplementation steps towards the design of a High RangeResolution Radar (HRR) simulator for electronic warfarepurposes.The preferred method to deceive an ISAR system isthe use of a transponder which can analyze the threatsignal, inject a false reflectivity solution and send back theresult to the threat antenna [27, p. 180]. An importantfunction in a false target generator is the creation of thetarget’s reflectivity solution. We propose that this solutioncan be furnished by a simulator and its supportingdatabase of results. We argue that such a false targetgenerator can be achieved by using a virtual realityapproach supported by a corresponding database system.In this paper the target is a naval vessel of considerablesuperstructure dimensions (battleship class) which istracked by an airborne threat signal. Using an ISARsystem the threat sensor is able to resolve manyreflectivity centres on the target classifying the ship as anextended target. The common practice for the simulationof an extended target is the multiple scatterers model. Inthis manner the target’s radar cross section (RCS) isrepresented by many pointlike reflectors that togethersynthesize its electromagnetic identity back at the radarreceiver. For example high reflectivity centers like gunplacements in lower superstructure coordinates create highI. INTRODUCTIONHigh Range Resolution (HRR) radar systems mountedon airborne platforms provide enhanced surveillancefunctions of ship targets [8]. The superior resolution isperformed by diverse Inverse Synthetic Aperture Radar(ISAR) modes of operation [35]. Such ISAR modes havethe ability to resolve many features on a target thusproducing high resolution imaging details in the slant (x)and cross (y) range dimensions of the target. Thus theimaging output is two dimensional and even threedimensional with more processing in contrast toconventional radar which can only resolve a target as anintensity point on a scope. There is a lot of recent, activeand enthusiastic research motivation for ISAR systemscountermeasures because the results have big practicalapplications to the military remote sensing community [6][18]. We choose to employ mathematical modelling andsubsequent simulator implementations of HRR radarsystems as they are important tool in this countermeasureresearch process because at least they allow the formationof feasibility studies for designs that use a complete threedimensional synthetic environment.In this manner the simulator is placed in a virtualworldspace that can reproduce a real-life scenario, such asan attack of a sea-skimming missile to a naval friendlyasset. This reproduction can create a false naval target andperform deception tasks on the threat signal in real-time.IJSSST, Vol. 10, No. 344ISSN: 1473-804x online, 1473-8031 print

T KOSTIS et al: INVERSE SYNTHETIC APERTURE RADAR SIMULATOR IMPLEMENTATIONissues that can be utilised to transform the initial statesand resources of the input and produce a proper outputusing the FB-16 toolkit as the transfer function. We havepaid attention to specific constraints that govern the realmof ISAR Imaging in order for the output to be realistic.This is important because we aim to use the outcome ofthe project in order to recreate a realistic target as seen byhigh-resolution electromagnetic eyes [13].In order to determine the final draft of the ConceptualModel there are five steps that have to be completed:Related Work Review (RWR), Application DomainDefinition, Problem Space Decomposition, EntityAbstraction Degree and finally Entity RelationshipIdentification [22].valued outliers in the ISAR image that do not correspondto the real height coordinate [11]. The simulator should beable to cater for this case in order to be able to producevessels that could be classified as military candidates.Furthermore the target should be able to advance into thevirtual battlespace in a way that produces inconsiderableeffects on the ISAR imagery process. Because anytranslational motion of the target is not a major contributorto the ISAR imaging output due to its physical nature.Only pitch and roll motions must be allowed to synthesizethe high resolution image [32]. Otherwise the threat itselfwill drop the track as non-credible. Based on the above wecraft the outcome of the simulation to encompass all theabove issues in order to represent a credibleelectromagnetic return to the threat signal.The paper is organized as follows: In Section 2 theconceptual model of the application is introduced. InSection 3 the modelling approach that is deemed able tocover the aims of the project is described. Section 4explains the data exchange standard that was utilized forthis project. Section 5 deals with relevant verification andvalidation issues. In Section 6 the discussion aboutthequality focus of the model and project is presented.Finally in Section 7 concluding remarks are given.A. Related Work Review (RWR)[7] have clearly presented the mathematical basis ofthe Inverse Synthetic Aperture process. [29] havedescribed a technique used to simulate ISAR images of aship model while under angular motions such as yaw,pitch and roll. [25] have presented the theoretical analysisof SAR techniques as can be applied to ISAR imaging ofship targets. Emphasis is given in the exploitation ofinformation resulting from the point spread function. Alsofoundations are laid towards the study of interferenceeffects (glint). [9] have introduced the ISARLAB softwarepackage which is a comprehensive set of functions thatemulate the particular functions of an ISAR system. [5]have developed a simulation program which can generateISAR images of ships. The method is based on thelocalization of dominant scatterers and has applications inevaluating the performance of automatic ship classifiers.[19] have investigated the acquisition of top or side viewISAR images with the proper cross range scaling. Thetechnique is based on the measurement of slopes of thetwo main feature lines of the ship, which are the centerline and the stern line. This process has the advantage ofusing only the acquired image to complete its tasks. [20]have investigated methods to obtain three dimensionalradar cross section (RCS) images using the ISAR concept.Results are provided towards the degradations effect ofspecular multipath effects on the final image. [26] havedescribed a method of ISAR image classification based ona comparison of Range-Doppler imagery to existing threedimensional ship reference models. This technique uses asequence of ISAR images in order to estimate thedominant ship motion. In all above indicative work thereis no mention of the computing force that provides themotion of the radar and target platforms. Our work makesan attempt to fill in the details of an ISAR simulationanalysis in a virtual reality environment.II. CONCEPTUAL MODELThe Conceptual Model (CM) is the first step in thetask of producing an abstract representation for a systemand a draft for its corresponding software model usingalgorithms and data [2]. We start by applying theproperties of an intellectual problem to our case [4, p.23].The goal which can tell us how to judge the outcomewould be the output of the simulator. All findings shouldcorrespond to existing verified and validated ISARsimulators found in the related work review. Therefore theinitial state as well will be dictated by existing literature.Our main objective then would be to ensure that animprovement has been achieved towards the authoring ofan ISAR simulator especially for electronic warfarepurposes.Now we have set up the primary mechanism for thesuccessful communication among the SimulationEngineers (SEs), like software designers, systemengineers and system analysts and the Subject MatterExperts (SMEs), such as engineers who specialize in thefield of the simulation context. Analytically the SMEs aretasked with guiding the complete process of thesimulation since they can predict out of experience theright steps and avoid any representational pitfalls that canlead to a product unfit for its intended purpose.The Conceptual Model deals with the context of therepresentation and the methodology that will be laterfollowed in a concise and structured manner. We havecomprised a set of operations as a toolkit, called FB-16, inorder to capture the representation and methodologyIJSSST, Vol. 10, No. 345ISSN: 1473-804x online, 1473-8031 print

T KOSTIS et al: INVERSE SYNTHETIC APERTURE RADAR SIMULATOR IMPLEMENTATIONB. Application Domain Definition (ADD)Specifically the theatre of operations is the airborneassault on the Deutsche Kriegsmarine SchlachtschiffBISMARCK by the Royal Air Force Swordfish swarm inthe Battle of the Atlantic, back in 1941. The introductionof an historical perspective to the project providesorganization, stimulation and inspiration towards itscompletion and further maintenance. Therefore in thisparticular case the SMEs should first explain to the SEsthe operational behaviour of an ISAR system mounted ona Swordfish biplane when viewing the Bismarck, asshown in Figure 1.The task of Application Domain Definition is given tothe SMEs that have authoritative information about theactual situational context. Usually at this point in time theSMEs will hold several meetings with the SEs and discussthe theoretical and practical milestones that have to beobserved during the course of the project. Usually at thisstage the SEs will have only superficial knowledge aboutthe subject matter. On the other hand SEs that can performthe task of SMEs are valuable for any particular situation.An historical angle was introduced in order to makethe work more appealing to the involved parties.Figure 1. FB-16 Simulation geometry draftso the image can be processed by the focus imagingmethod. Drafting our simulator we should provide asolution to the threat signal that is motion compensated.It is now assumed that the threat radar’s signalbandwidth is sufficient for the successful range resolutionof the numerous superstructure elements. In this case thesignal signature of the target for wideband illumination inthe time domain is its Slant Range Profile (SRP). Theassociated frequency domain translation of the SRPsproduces the last step of the simulation, which is the ISARimage.The final touch to the ADD is the theoreticalfoundation of the ISAR image data format which is calledPolar Reformatting (PR). PR is an interpolation processwhere a set of samples uniformly spaced in cylindricalcoordinates is fitted into a rectangular grid. Therefore theoutput of the simulation is the interpolation of the ISARwaves to a rectangular grid. Again we draft our solution tocorrespond to a polar reformatting process.The theoretical representation of Inverse SyntheticAperture Radar systems is the dual of a SyntheticAperture Radar system in spotlight mode, as shown inFigure 2(a) [Wehner, 1998]. And to introduce the widerconcept of radar imaging we have included Figure 2(b).From Figure 2(b) we derive the final simulationtheoretical basis which is shown in Figure 2(c).The ADD for this project deals with the data collectionradar images of Slant Range vs Doppler Frequency for atarget under Spotlight Mode. These radar images areknown as Inverse Synthetic Aperture Radar (ISAR)images. There are two major techniques employed for thecreation of an ISAR image. First the concept of RangeDoppler (RD) imaging states that it is possible to achievea two-dimensional view of a ship target at sea by using therelevant RD algorithm. This algorithm is phase preservingand is best suited for applications using continuouslycollected data. The second concept is the motioncompensation of the scatterers. This method corrects anyunnecessary phase components of the reflected waveformsIJSSST, Vol. 10, No. 346ISSN: 1473-804x online, 1473-8031 print

T KOSTIS et al: INVERSE SYNTHETIC APERTURE RADAR SIMULATOR IMPLEMENTATION(a)3D EngineSystem DynamicsRadarPlatformMatchedFilterTransmitter Typeie Tx ParametersWaveform tformMotionProviderGround Clutterand other clutter agation of Radar Waves ModelingAntenna TypeTarget Typeie. Parabolic - Ray TracingHybrid - Custom - Custom Hybrid[Refraction - Diffraction]ie phased arrayparabolicie. rcsAll Other Noiseie. equipment noiseDynamic ErrorsReceiver Typeie. Platform MotionScanning MotionVibrationGlintDirectionalInterferenceie. JammerChaffie Rx ParametersGeneral Simulation ParametersInformationExtraction fromSignals Receivedie Code Specific ConsiderationsSampling RateResults Formation(b)(c)Figure 2. Theoretical Inverse Synthetic Aperture Radar process(a) Theoretical Explanation (b) Complete Model (c) ISAR Subsetsimulation should encompass must be drawn at thismoment.For this particular project a crude list of entities asshown in Table 1 would involve the radar transmit andreceive functions, the target properties, the environmentalimpact on the emitted signal and the digital signalprocessor simulator after the radar receive block.C. Problem Space Decomposition (PSD)The entities and processes that must be represented forthe successful accomplishment of the simulation aredefined. A basic memorandum of understanding betweenthe SMEs and the SEs regarding the level of detail that theIJSSST, Vol. 10, No. 347ISSN: 1473-804x online, 1473-8031 print

T KOSTIS et al: INVERSE SYNTHETIC APERTURE RADAR SIMULATOR IMPLEMENTATIONTABLE I. ENTITIES12345SimulationTarget cartesian coordinatesplus inherent amplitude &phaseRadar slant range and crossrange cartesian coordinateswith respect to the center ofthe targetSea-level distance fromradar to targetRadaroperationalparametersAspect angle from radar totarget6Pace Engine7ISAR processor details8ISAR system outputNow we can draw the necessary associations betweenthe entities and come up with the corresponding processes,as shown in Table 2. Again as above the agneticsignatureMost prominent appearsto be the middle of theshipFM height finder radar(altimeter) on airborneplatformISAR system particularsChange of aspect angleprovides the resolutionsabilityTarget movement due toforces of natureRange-DopplerprocessingSlant Range Profile andISAR Image of targetbetween the reality and the simulation is strongly takeninto account.TABLE II. PROCESSESABCDEFGHIJSSST, Vol. 10, No. 3SimulationProvide information to thePace Engine of targetCartesian coordinates to thepace engineProvide information to thePace Engine of radar twodimensional (slant rangerand cross range) coordinatesProvide information to thePace Engine of radar’s third(height range) dimensionalcoordinatesProvide information to theISAR Processor about theradar’soperationalparametersAspect angle variationFrom Pace Engine to rotatedpoints databaseFrom Pace Engine Databaseto ISAR ProcessorFrom points database toISAR processor48RealityPhysical presencemovement of targetPhysical presencemovement of radarandandMeasurement - capturesreality with a sensorInstrumentation–operational informationCaused by changes intarget/radar locationCaused by changes intimeRecording Process –processes history oftargetincomputermemoryComputer process –Range-DopplerProcessing - translatesrealitytocomputermemoryISSN: 1473-804x online, 1473-8031 print

T KOSTIS et al: INVERSE SYNTHETIC APERTURE RADAR SIMULATOR IMPLEMENTATIONFrequency of 1KHz. Other operational values can easilybe accepted by the simulator.D. Entity Abstraction Degree (EAD)The representational abstraction of the involvedentities is finalized in this step. The level of accuracy,precision, resolution and fidelity of the entities andprocesses is determined. For this project the level ofaggregation should involve a target that is made out ofone-hundred and forty-five points. The ISAR system willbe mainly operating at 5 GHz with a Pulse RepetitionE. Entity Relationship Identification (ERI)The relationships among the entities are identified inthis design phase. It is ensured that all constraints andboundary conditions are properly imposed by thesimulation context. All operational and functionalrequirements are taken into consideration, as shown inFigure3.Figure 3. Entity Relationship (E-R) IdentificationIII. MODELLING APPROACHsimulation process. The informatom can be defined as thesimplest result or group of results that can providesatisfactory validation in a simulation process. Then whenall points are processed and the complete databases areavailable, graphing results are produced [13].First the physical situation is analyzed. In this case aphased array antenna is transmitting and receiving acollection of numerous agile beams towards the targetrapidly scanning it in azimuth and elevation. Also thedigital signal processing block of the radar can produceresults either in sequential or parallel modes. Therefore asimulator design is needed for a parallel-to-parallel or aparallel-to-sequential physical process.In more detail the system is divided into its buildingblock, which may be call datatom (from data and atom).All data for one point is processed and the resultinginformation, which may be called informatom (frominformation and atom), is stored into one or manydatabases. The datatom can be defined as the simplestconfiguration that can provide satisfactory verification in aA. Modelling the Synthetic EnvironmentAll objects like radars and targets exist in a threedimensional matrix which is defined as the SyntheticEnvironment, as shown in Figure 4. Typical types ofobjects that exist in the Synthetic Environment includeRadar Platforms (airborne or terrain based), TargetPlatforms (airborne-ground based), Environment Volumes(clouds) and Terrain Platforms (sea, land). Moreover thoseplatforms can be stationary or in motion.Figure 4. Synthetic Environment ModellingIJSSST, Vol. 10, No. 349ISSN: 1473-804x online, 1473-8031 print

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The theoretical representation of Inverse Synthetic Aperture Radar systems is the dual of a Synthetic Aperture Radar system in spotlight mode, as shown in Figure 2(a) [Wehner, 1998]. And to introduce the wider concept of radar imaging we have included Figure 2(b). From Figure 2(b) we derive the final simulation

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