Network Models Within ACESNetwork Models Within ACES Erin M. McDonald, Rebecca J. Gassler, and Brian...

244 JOHNSHOPKINSAPLTECHNICALDIGEST,VOLUME23,NUMBERS2and3(2002)

E. M. McDONALD, R. J. GASSLER, and B. L. HOLUB

T

NetworkModelsWithinACES

Erin M. McDonald, Rebecca J. Gassler, and Brian L. Holub

heAPLCoordinatedEngagementSimulation(ACES)isbeingdevelopedtosup-port distributed weapons coordination (DWC) analysis. One key factor affecting theDWCdecisionprocessisthequalityoftheinformationonwhichthedecisionsarebased.InACES,networksfacilitatetheexchangeofsituationawarenessinformationtogener-ateasimulatedtacticalpictureandtosupporttheexchangeofengagementcoordinationinformation. The networks modeled within ACES can be configured to represent theupdaterates,protocols,andinformationexchangesupportedbythecurrentcapabilityofoperationalsystems.However,ratherthanattemptingtoemulatethespecificbehavioroftheoperationalnetworks,ACESfocusesonmodelingthosenetworkcharacteristicsthatcreatecertainbehaviorsbelievedtoimpactengagementcoordinationsuchasdualdesig-nationsandmessagelatency.ThisarticledescribesthefeaturesofthenetworksmodeledwithinACES.

INTRODUCTIONEngagement operations depend on a clear tactical

picturetosupporttheefficientuseofordnanceamongfiring units and to effectively defeat a raid. Unfortu-nately, thetacticalpicture seen inoperations today isoftenambiguous.Errors in thedetectionand trackingphase can ripple through the correlation process andaffecttheclarityofthetacticalpicture.Correlationisthe process whereby contacts or tracks from multiplesensorsareassociatedtoasingleobject;incorrectcor-relationcanresultinmultipletracksbeingreportedforthesameobject,ormultipleobjectsbeingreportedwiththesametracknumber.Pastengagementcoordinationanalyseshaveassumedthatasingleintegratedairpic-tureexists,withoneandonlyonetrackperobject.Thispresentsanunrealisticsimulatedenvironmentinwhichtheimpactofanimperfecttrackpictureonengagement

operations cannot be determined. The APL Coordi-natedEngagementSimulation(ACES),whichisbeingdevelopedtosupporttheanalysisofweaponscoordina-tion alternatives, provides a more realistic simulationenvironment that is capableof reflecting the ambigu-ities often present in real-world tactical pictures. AnoverviewofACESisprovidedinthearticlebyBurkeandHenlyelsewhereinthisissue.

TheunitsrepresentedinACESexchangebothsitu-ationawarenessinformationandengagementcoordina-tion information in accordance with the capabilitiesand constraints of modeled networks. Informationexchangedviathesenetworksfacilitatesthegenerationofatacticalpictureandenablessimulationofengage-ment coordination concepts that require data to beexchanged among potential firing units. Currently,

JOHNSHOPKINSAPLTECHNICALDIGEST,VOLUME23,NUMBERS2and3(2002) 245

NETWORK MODELS WITHIN ACES

ACES includes representations of two types of net-works:(1)theTimeDivisionMultipleAccess(TDMA)Data Link (TDL), which captures many of the keyfeatures,characteristics,andprotocolsofLink16,and(2)theSensorBasedNetwork(SBN),whichsimulateshighupdateratecompositetrackformation,similartotheCooperativeEngagementCapability(CEC).

ACEShasbeendesignednottoprovidehigh-fidelitynetworkmodeling,buttodemonstratetheimpactthatnetworkperformancecanhaveonthesituationaware-ness at each platform and on engagement operationseffectiveness.ACESfocusesonmodelingthecommuni-cationscharacteristicsthatcreatecertainbehaviorsthatimpact engagement coordination. Therefore, the datasent across the networks in ACES do not correspondexactlytothemessagedefinitionsgiveninthemessagestandarddocumentationforexistingdatalinks.Insomecases,notalldata identifiedwithinadefinedmessagearesentbecausecertaindatafieldswithinthatmessageare not used by the model. However, the sizes of thenetworkmessages as defined in themessage standardsarealways taken intoaccountwhendetermininghowmuch information can be transmitted across the net-works,giventhespecifiedbandwidthconstraints.

At this stage in ACES development, the TDL ismorematurethantheSBN.Thisarticlethereforepro-videsmoredetailaboutthestructureandoperationoftheTDL,whiletheSBNisdiscussedatahigherlevel.DevelopmentoftheSBNisanobjectiveforFY2002.

TDMA DATA LINK

Architecture The TDL is a generic representation of Link 16;

itscharacteristicsandprotocolsarebasedon informa-tionfoundinLink16documentationsuchastheLink16 operational specification1 and the Tactical DigitalInformationLink(TADIL)Jmessagestandard.2Unitlocationandstatusinformation,aswellasengagementcoordination information, is exchangedover thisnet-work.Processed track reportsarealsoexchangedovertheTDL,producingaremotetrackfile(RTF)ateachunit.Thetimingofthisinformationexchangeisdeter-minedbytheTDLnetworkdesign.

Link16architecturefeaturesareusedintheACESrepresentation of the TDL network. Link 16 networkdesignisbasedona12-sframeconsistingof1536timeslots, each being 7.8125 ms in length. During devel-opmentofaLink16network,timeslotsarefirstallo-catedtovirtualcommunicationcircuitscallednetpar-ticipation groups (NPGs). NPGs are differentiated bythe functions they support. For example, the Surveil-lanceNPGisusedforreportingsituationawarenessandtheEngagementCoordinationNPGisusedtosupporttheexchangeofengagementinformation.Dividingthe

networkintofunctionalgroupsinthiswayallowsunitstoparticipateononlytheNPGsthatsupportthefunc-tionstheyperform.

The amount of network capacity given to an NPGis determined by considering priorities, including thenumber of participants on the NPG, expected volumeofdatatobereported,updaterateoftheinformationtobereported,andrelayrequirements.Relayisrequiredifmessageshave tobe transmittedover thehorizon.Theadditionofaone-hoprelaydoublesthenumberoftimeslotsrequiredtosupportanNPG.Oncenetworkcapac-ityisassignedtotheNPGs,NPGcapacityisassignedtoparticipating platforms. The number of time slots allo-catedtoeachNPGandeachunitdetermineshowoftenatransmitopportunityoccursforthatNPGandhowmuchinformationgetstransferredduringthatopportunity.

As inLink16, theTDLnetworkarchitecturecon-tainsNPGs,andthetimingoftheinformationexchangeisdeterminedbytimeslotallocations.FortheTDLmod-eledinACES,thetimingoftheinformationexchangeoneachNPGisdivided intoTDLtimesteps, that is,the interval in which each unit participating on theNPGgetstotransmitatleastonce.Thefollowingpara-graphsdescribehowtheTDLtimestepforeachNPGisdetermined.

Figure 1 illustrates the allocation of time slots toNPGs and units as well as the determination of eachNPG’s time step. For simplicity, when modeling theACES TDL network, these slots are assumed to bespacedevenlythroughoutthe12-sframe;inanactualLink 16 network, however, this may not be the case.UsingthisassumptionmakesitsimplertocalculatehowoftenatimeslotassignedtoaspecificNPGoccurs.

TodetermineanNPGtimeslotinterval,

(1)

Therefore,everynthtimeslotwillbeallocatedtotheNPG(theresultofthisdivisionisroundedoffforesti-mation purposes). Since each time slot is a time unitof7.8125ms,atransmitopportunityforthisNPGwilloccurevery(n)(7.8125ms).

NotethatinEq.1thetotalnumberoftimeslotsinthenetworkisdividedbythenumberoftimeslotsallo-catedtotheNPG,withoutrelay.Thisisbecausenrepre-sentstheintervalbetweentimeslotsthatareallocatedtounitsforthetransmissionoforiginaldata.Therelaytimeslotsareallocatedtoadesignatedrelayplatform,whichrebroadcaststhosedatatounitslocatedbeyondthelineofsight.

The length of the time step may be calculated bymultiplyingthenumberofsecondsbetweenNPGtrans-mission opportunities by the number of transmission

Total # time s lots in the network

Total # time s lots allocated to the NPG (without relay)= n .



opportunitiesneededtoensurethateachunitparticipatingontheNPGhastransmittedatleastonce.Attheendofthisinterval,alloftheinformationforthatNPGissentacrossthenetwork;thismethodallowsACEStodeter-minethetimingofTDLinformationexchangewithoutgettingdowntothetimeslotlevel.

The information transmitted over the TDL network is separated intogroups according to NPGs. For example, track data are transmitted overtheSurveillanceNPG,whileengagementcoordinationdataaretransmit-tedover theEngagementCoordinationNPG.Rulesdictatingwhenmes-sagesshouldbesentandwhichunitshouldsendthemarebasedontherulesidentifiedinRef.2.

ATDLnetworkmayallocatetimeslotcapacitytoNPGsthatarenotofinteresttousersofthemodel.Currently,onlytheSurveillance,EngagementCoordination,andPreciseParticipantLocationInformation(PPLI)NPGsaremodeledinACES.EventhoughtheinformationexchangeovercertainNPGsmaynotbesimulated,thetimingcalculationsinthemodeltakeintoaccountthetimeslotsallocatedtothoseNPGs.ThetimeslotsnotcoloredinFig.1representthosethatareallocatedtoNPGsnotspecificallymodeledinACESandthosethatarerelayslots.

InaTDLnetwork,asinaLink16network,therecanbedifferentgroupsofunitsparticipatingoneachNPG(notallunitsparticipateonallNPGs),and the unit transmission sequence for each NPG can be different. TheexamplegiveninFig.1assumesthatthreeunits(denotedA,BandC)areparticipatingon eachNPG shown.At the endof the time step for eachNPG,theinformationfromallthreeunitsissentacrossthenetwork.OntheSurveillanceNPG,unitBisgiventwiceasmanytimeslotsaseitherunitAorunitC.Therefore,twiceasmuchinformationwillbesentfromunitBattheendoftheSurveillancetimestep.

ThetrackdataexchangedovertheTDLproducesanRTFateachunit;thisRTF is oneof the sources for the tactical picture.A local-to-remote

track correlation process is exe-cutedoneachunit and results ina series of logical decisions thatdetermine what track data eachunittransmits.

Determination of Reporting Responsibility

TDL tracks are correlated tothelocallyheldtracksinanefforttopreventdual tracks frombeingpropagatedonthedatalinks.Thecorrelationprocessitselfinvolvesastatisticalcomparisonofvaluescal-culatedfromthestatevectorsasso-ciatedwithatrackandtheirasso-ciated covariance matrices. Eachlocaltrackmaycorrelatewithonlyoneremotetrack.ThiscorrelationprocessisdiscussedingreaterdetailbyBates et al., this issue.Once alocal track iscorrelated toaTDLtrack, the track quality (TQ) isused todeterminewhichunithasreporting responsibility (R2) forthetrack.

Each track reported over theTDL is sent with TQ, which isa measure of the reliability of thetrackdata.Thisvalueisalsocalcu-latedonlocallyheldtrackstoper-formTQcomparisonstodetermineR2.TQisanintegerbetweenzeroand15.Zerorepresentsanon–realtimevalue(whichcouldbeavaluesent by a unit that is not a datalinkparticipant,relayedbyanon–realtimesystem,orassociatedwithtrackdatanotderivedfromaninte-gratedsensor).

TQ is computed by combiningtheestimatedpositionandvelocityerrorstoformthevalue

B Tp v

i x,y,z i i= +⎣ ⎦=∑ 2 2 2 , (2)

where p is the position error infeet,visthevelocityerrorinfeetpersecond,and∆Tisthetimestep(definedas6s inRef.2).TheseBvaluesarecomparedwithtabulatedvalues inRef.2 todetermineTQ.InACES,afunctionwascreatedtorepresentthetableofTQvalues,

Figure 1. Time step determination for a TDL network.

A 12-s frame contains 1536 time slots

0 1536

A B C A B

T-PPLI

C B A C B A C

T-EC

B C B A B C B A B

T-SURV

T-PPLI = time step forPPLI messages

T-EC = time stepfor EC messages

T-SURV = timestep for SURVmessages

Time slots allocated to Precise Participant Location Information (PPLI) NPG

Time slots allocated to Engagement Coordination (EC) NPG

Time slots allocated to Surveillance (SURV) NPG



TQ=Floor[22.19–1.954*Log10B], (3)

where “Floor” is a function that rounds the argumentcloser to zero. This function could potentially returnvaluesgreater than15.Because15 is thehighestpos-sibleTQvalue,thesimulationassignsavalueof15toanycomputedvaluegreaterthan15.

ThisTQvalueisusedtodeterminewhichunithasR2 for the track. Only the unit that holds R2 for agiventracksendsdataforthattrackoverthelink;intheory,thisensuresthatonlyoneunitisreportingonatrackandthebesttrackinformationissentacrossthelink.Ifaunitdeterminesitholdsalocaltrackthatisnotbeingreportedonthelink,itassumesR2forthattrackandbeginsreportingit.Ifaunitdeterminesithasalocaltrackthatcorrelateswithagivenremotetrack,itdetermineswhoshouldhaveR2basedonacompari-sonoftheTQs.Forairtracks,aunitmusthaveaTQatleast2greaterthanthevaluebeingreportedtoassumeR2.Forspacetracks(e.g.,ballisticmissiletracks),thedifferenceisreducedtoatleast1orgreater.

EachtrackreportedontheTDLhasauniqueremotetracknumber(RTN),giventoitbytheunitthatorigi-natedthetrackreport.Whenaunitcorrelatesitslocaltracktoaremotetrack,itassignsthattrack’sRTNtoitslocaltrack.WhenaunitassumesR2foratrack,itkeepsthesameRTNoriginallyreportedtoensurecontinuityinthetrackreporting.Figure2illustratesthedecisionchainthatresultsfromthetrackcorrelationprocess.

IftrackshavethesameTQ,R2goestotheunitwiththe higher unit number. In accordance with Link 16rules,aunitmayalsoassumeR2foratrackiftheunitholds local data on the track but has not received aremotereportonthattrackforaspecifiedtimeperiod.

The correlation process can also help detect andresolvetrackidentificationandcategoryconflictsthatmay cause confusion in the tactical picture. ACESdoesnotcurrentlyresolvetheseconflictsbecausecat-egorizationand identification functionshavenot yetbeen implemented; all tracks are assumed tobehos-tilespacetracks.However,futureversionsofthemodelwillhavethecapabilitytoresolvesuchconflicts.

SENSOR BASED NETWORKWhiletheTDLisdesignedtosimulateabroadcast

data link network, the SBN is designed to simulate apoint-to-point sensornettingnetwork.TheSBN sup-portstheexchangeofsensordatafrommultiplesourcestoformcompositetracks.SBNarchitecture,timing,andinformation exchange are based on the concepts andmethodsemployedbyCEC.

The SBN represents a network, such as CEC, inwhich high data rates are achieved by employing apairwisemultipleaccessscheme.Thistypeofnetworkreliesondirectionalantennas,whichallowoneunitinthenettotransmitdatatoasecondunit,whileathirdunitinthenetissimultaneouslytransmittingdatatoa fourth unit, as illustrated in Fig. 3. In the figure,unitA1 sends data to unitA2,while unitA4 sendsdatatounitA3.Thisprocesscontinuesuntilalloftheunitsinthenethavetransmitteddatatoeveryotherunitwithinthenetlocatedwithinthenetwork’seffec-tiverange.

In some instances, units may be located beyondline of sight; in these cases, a relay is required forthe data to reach their destination (as in the casewith TDL). In Fig. 3, units A1 and A3 are too far

Figure 2. Correlation-related decision chain for the TDL.

away from each other for directdataexchangetooccur.Therefore,units A2 and A4 act as relaysto exchange information betweenunitsA1andA3.

Each set of pairwise transmis-sionstakesplacewithinaperiodoftimecalleda“frame.”Thisisdiffer-ent from theTDL framediscussedpreviously. The time step ACESuses to transfer SBN informationisdefinedas the interval inwhichallunitsontheSBNnethavetheopportunity to transmit their datatoeveryotherunitonthenet(thesamerationaleasusedinTDL).Atthe end of this interval (denotedTSBNinFig.3),alloftheSBNinfor-mationissentacrossthenetwork.

Currently, only track states aretransmitted across the SBN, buttheimplementationofthisnetwork

Remotetrack file

Correlatetracks

Localtrack file

Identifytracks not

locally held

Check forconflicts Initiate new

track message

Assume R2for track

Conflict No conflict

Confict resolution

Environment/category conflicts,

ID conflicts

CompareTQsLocal track has

higher TQRemote track has

higher TQ

Does unithold R2?

Does unithold R2?

Yes No Yes No

No action Take over R2 Relinquish R2 Update

No local trackmatching remote track

Local track does not correlatewith any remote track

Tracks same



willallowothertypesof information,suchasengage-ment coordination data, to be sent. ACES does notsendmeasurementdatatoeachplatformviatheSBN.Instead,platformtrackswitherrorsaretransmitted,andthenoneachplatformalltheinformationprovidedinthosetrackstatesisfusedintoasingletrack.Thiscor-respondstotheintentofACEStomodelthebehaviorofthenetworkasitimpactsengagementcoordination,ratherthantoemulatespecificcharacteristicsofopera-tionalsystems(e.g.,CEC).Eachtrackhasitsowncom-positetracknumber(CTN),whichisusedbyallunitsonthenetworktodenotethattrack.EveryplatformontheSBNmaintainsacompositetrackfile(CTF)usingthisCTN.

SBNcompositetrackmanagementisillustratedinFig.4.Whenaunitreceivesdataonanexistingtrackfrom the network, it updates the data for that trackbeing held in the CTF. If the local sensor data donot correlate to any of the composite tracks, thoselocaldataareassignedanewCTNandaretransmittedacrossthenetworktotheotherunitsduringthenextSBNcycle.IfthelocalsensordatadocorrelatetoanexistingtargetintheCTF,theappropriateCTNwillbeassignedtothoselocaldata.IfthelocalsensordatahaveaCTNassigned,thoselocaldatawillbesenttootherSBNunitsandaddedtotheexistingcompositetrackdataforthattarget.

EachSBNunitwillperiodicallyattempt to correlate each track intheCTFtotheotherstoverifythatthere is only one composite trackfor each object, eliminating dualtracks. All platforms employ iden-tical algorithms to maintain thesameCTFforallSBNparticipants

A1

A2

A3

A4

Time = 0

Frame Frame Frame Frame

TSBN

A1A4

A2A3

A3A4

A2A1

A2A3

A1A4

A1A2

A4A3

Figure 3. Time step determination for an SBN.

(unlikeTDL,whereeachplatformwillemployitsownalgorithmstomeettheR2processingrules).

FLEXIBILITY OF NETWORK MODELSThenetworksinACEScanbemodeledatavariety

of different levels. Both the TDL and SBN can bemodeledsothatalldesiredinformationisexchangedwith minimal latency, representing the ideal case.These networks can also be modeled to reflect thecurrent operational characteristics of the systems onwhichtheyarebased.Operationallimitationsmaybeimposed, including restricting information exchangetocurrentmessagestandards,constrainingbandwidth,and constraining specified update rates. ACES alsoprovidestheflexibilitytomodelthenetworksatlevelsbetween ideal and operational. For example, a casemayberuninwhichthebandwidthisconstrainedbutupdateratesarefasterthanthosespecified.

To illustrate theflexibility ofnetworkmodeling inACES, consider the example shown in Fig. 5, whichfocuses on the information being exchanged over theTDLduringengagementcoordination.Theinformationexchange requirements shown are information itemsthatneedtobesenttosupportadistributedengagementcoordinationconcept (see thearticlebyShaferetal.,thisissue).

Intheidealcase,nolimitsareplacedonthetypeofinformationthatcanbeexchangedoverthenetworks,andthisinformationistransmittedwithnoconstraintsonbandwidthorupdaterate.Ifthenetworksweremod-eledtoreflectcurrentoperationalcapability,however,engageabilitydatacouldnotbesentovertheTDL,asthecurrentmessagestandardforLink16(thesystemonwhichtheTDLisbased)doesnotsupportthosedata.MissileinventorywouldbesentintheplatformstatusmessageoverthePPLINPG,asdefinedinthemessagestandard.Because theunitsparticipatingon thePPLINPG would likely be allocated fewer time slots thanthose participating on the Engagement CoordinationNPG, inventory information will be transmitted lessfrequently. Introductionofbandwidthconstraintsandadherencetospecifiedupdateratesmayresultinhigherlatenciesduringinformationexchange.

The networks in ACES currently run under theassumptionthatthereisperfectconnectivity.However,network connectivity may degrade because of factorssuch as atmospheric propagation effects or jamming.

SBN data

SBN dataSBN data

Local trackfile

If track has anassigned CTN Composite

trackproduction

Correlatetracks CTF

Create newCTN

Assign CTNto local track

Correlate tracks,eliminate duals

Local track does notcorrelate with anycomposite tracks

Local track doescorrelate with anycomposite tracks Every few seconds

compare all tracksfor dual tracks

Figure 4. SBN composite track management process.



Futureversionsofthemodelwillprovidethecapabilitytorunthenetworksunderconditionsofdegradedcon-nectivity,addinganotherdegreeofflexibilityandreal-itytothenetworks.

FUTURE ANALYSIS USING ACES ThemodelingofthenetworkswithinACESallows

fortheexplorationofhowcertainnetworkcharacteris-ticscanimpactengagementcoordination.Forspecifiedengagementcoordinationschemes,aspectsofnetworkperformance can be examined to determine the mostcriticalrequirementsforsuccess.Theseaspectsinclude

Figure 5. Networks within ACES can be modeled to reflect the ideal case, the operational case, or levels in between.

the type of information that mustbe exchanged, the timeliness withwhich that information must beexchanged,andhostsystemimple-mentation of network rules andprotocols.

The flexibility of the networkmodeling can also support futureanalyses that focuson informationexchange.Thismodelcanbeusedtoexplorewhetherupgradesormodi-ficationsareneededtothecurrentoperationaluseofexistingsystems.Forexample,ACEScanhelpiden-tifywhethernewmessagesorhigherupdateratesmaybeneededtosup-port certain engagement coordi-nation concepts. ACES can alsobe used to explore the differencesbetweenapplyingadatalinksystemviceasensornettingsystemtosup-porttheseconcepts.

Analysessuchasthesearecriti-caltodeterminingnetworkrequire-mentstosupportengagementoper-ations. By providing a realistic,

flexible simulation environment able to support thistypeofanalysis,ACESstandsoutasauniquecapabil-ity that will be valuable as engagement concepts areexploredinthefuture.

REFERENCES 1Navy Center for Tactical Systems Interoperability, Link 16 Opera-

tional Specification OS-516.2, Change 2(3Mar2000). 2Department of Defense Interface Standard, Tactical Digital Infor-

mation Link (TADIL) J Message Standard, MIL-STD-6016A (Apr1999).

ACKNOWLEDGMENTS:TheauthorswouldliketoacknowledgethemembersoftheACESteamfortheirdedicationandcommitmenttothecreationofthismodel.WewouldalsoliketogivespecialthankstoMichaelBurke,JoshuaHenly,andKenShaferfortheirvaluablecontributionsinthereviewofthisarticle.

THE AUTHORS

ERIN M. McDONALD is a member of the Senior Professional Staff in the AirDefenseSystemsEngineeringGroup.ShereceivedaB.S.inelectricalengineeringfromVillanovaUniversityin1989.ShehassupportedBMC4IstudiesatAPLsince1997,providingtechnicalleadershipineffortstorelateC4ISRperformancetowar-fightingeffectivenessandsupportingprojectssuchasJointInterfaceControlOfficer(JICO)requirementsdevelopment.Ms.McDonald’sbackgroundisincommunica-tionssystemsanalysis,particularlythedesignandanalysisofTADILsystems.Here-mailaddressiserin.mcdonald@jhuapl.edu.

Informationexchange

requirement

PPLI NPG

Platformstatus

message

SurveillanceNPG

Trackmessage

Engagement CoordinationNPG

ECmessage

Engagementstatus

message

Target positionand velocity

Missileinventory

Engageability

Trackdata

Missileinventory

Engageability

Informationexchange

requirement

PPLI NPG

Platformstatus

message

SurveillanceNPG

Trackmessage

Engagement CoordinationNPG

ECmessage

Engagementstatus

message

Target positionand velocity

Missileinventory

Engageability

Trackdata

Missileinventory

Ideal case:

Unconstrainedinformationexchange

Unconstrainedbandwidth

Update rates user-specified

Operational case:

Informationexchangelimited

Constrainedbandwidth

Update ratescomply withexisting messagestandards



BRIANL.HOLUBisamemberoftheSeniorProfessionalStaffintheAirDefenseSystemsEngineeringGroup.HereceivedaB.S.ingeneralengineeringin1982andanM.S.indigitalsystemsin1985fromLoyolaCollege.HeiscurrentlyworkingonaSystemsEngineeringCertificate fromtheJHUWhitingSchoolofEngineering.Before joiningAPLin1999,Mr.Holubworked forWestinghouseElectricCorp.(nowNorthropGrumman),wherehefocusedonsystemsengineeringinsupportofC2functionsforairdefense,airtrafficcontrol,andlawenforcementproducts.SincejoiningAPL,hehassupportedNavyBMC4IeffortsfocusingonTBMDandNTWsystems.HehasbeenamemberofIEEEfor13yearsandamemberoftheInterna-tionalCouncilofSystemsEngineering(INCOSE)[email protected].

REBECCAJ.GASSLERgraduated fromVirginiaTechwithaB.S. in aerospaceengineering.SheiscurrentlypursuinganM.S.ininformationsystemsandtechnol-ogyfromTheJohnsHopkinsUniversity.ShejoinedtheAirDefenseSystemsEngi-neeringGroupofADSDinJune2000andhasdedicatedmostofhertimetoworkingonACES.Ms.Gassler’sprofessional interests concernmodeling, simulation,andanalysisofcurrentandfuturetheater-andcampaign-leveloperations.Here-mailaddressisrebecca.gassler@jhuapl.edu.

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