SPACEFLIGHT SAFETY HANDBOOK FOR SATELLITE OPERATORS · 2020-02-17 · SPACEFLIGHT SAFETY HANDBOOK...

SPACEFLIGHTSAFETYHANDBOOKFORSATELLITEOPERATORS

VERSION 1.4 FEBRUARY 2019

18SPCSProcessesforOn‐OrbitConjunctionAssessment&CollisionAvoidance

18thSpaceControlSquadronJointForceSpaceComponentCommandVandenbergAirForceBase,California,USATel+1‐805‐605‐3533www.space‐track.org

TABLE OF CONTENTS

Contents

RevisionHistory _________________________________________________________________________________________ 1

USSTRATCOMSSASharingProgram___________________________________________________________________ 2

ProgramDescription______________________________________________________________________________________________2

Space‐Track.org____________________________________________________________________________________________________2

SpaceflightSafetyProcess_______________________________________________________________________________ 3

On‐OrbitConjunctionAssessment(CA)_________________________________________________________________________3

HACvsHAC_____________________________________________________________________________________________________4

O/OEphemerisvsHAC(EPHvsHAC)________________________________________________________________________4

O/OEphemerisvsO/OEphemeris(EPHvsEPH)___________________________________________________________5

On‐OrbitCollisionAvoidance(COLA)____________________________________________________________________________5

BasicEmergencyCAandCOLAServices ________________________________________________________________________5

AdvancedCAandCOLAServices_________________________________________________________________________________6

ScreeningSchedule _______________________________________________________________________________________________7

ScreeningVolumes________________________________________________________________________________________________7

ReportingCriteria_________________________________________________________________________________________________9

Notifications_____________________________________________________________________________________________________10

BasicEmergencyCANotifications_____________________________________________________________________________10

AdvancedCANotifications_____________________________________________________________________________________11

LaunchConjunctionAssessment_______________________________________________________________________________11

EarlyOrbitConjunctionAssessment __________________________________________________________________________12

End‐of‐Life/DisposalSupport__________________________________________________________________________________12

DeorbitSupport_________________________________________________________________________________________________12

Tasking___________________________________________________________________________________________________________13

EphemerisFormats_____________________________________________________________________________________14

Overview_________________________________________________________________________________________________________14

NASAEphemerisFormat_______________________________________________________________________________________15

UTCEphemerisFormat_________________________________________________________________________________________16

GenericOn‐OrbitEphemerisFormat__________________________________________________________________________17

TABLE OF CONTENTS

ModifiedITCEphemerisFormat_______________________________________________________________________________18

OrbitalEphemerisMessage(OEM)Format___________________________________________________________________19

HowtoNameEphemerisFiles_________________________________________________________________________22

Overview_________________________________________________________________________________________________________22

FileNameExamples ____________________________________________________________________________________________23

HowtoSubmitEphemerisFiles_______________________________________________________________________24

Overview_________________________________________________________________________________________________________24

SubmittingEphEmerisThroughSpace‐Track.org_____________________________________________________________25

ManualUploadofEphemerisFiles_____________________________________________________________________________25

APIUploadofEphemerisFiles_________________________________________________________________________________26

Email_____________________________________________________________________________________________________________26

GuidanceforHigh‐InterestEvents_____________________________________________________________________________26

ManeuverNotificationFormat_________________________________________________________________________27

Overview_________________________________________________________________________________________________________27

Format___________________________________________________________________________________________________________27

HowtoNameManeuverNotifications ________________________________________________________________32

Overview_________________________________________________________________________________________________________32

Examples_________________________________________________________________________________________________________32

HowtoSubmitManeuverNotifications_______________________________________________________________33

Overview_________________________________________________________________________________________________________33

SubmittingMANEUVERSNotificationsThroughSpace‐Track.org___________________________________________34

ManualUploadofManeuverNotifications____________________________________________________________________34

APIUploadofManeuverNotifications_________________________________________________________________________35

Email_____________________________________________________________________________________________________________35

ContactUs_______________________________________________________________________________________________36

18SPCSOrbitalSafetyCrew____________________________________________________________________________________36

18SPCSSSASharingTeam_____________________________________________________________________________________36

Space‐Track.orgAdministrationTeam_________________________________________________________________________36

AnnexA:18SPCSProbabilityofCollision ____________________________________________________________37

How18SPCSCalculatesProbabilityofCollision _____________________________________________________________37

TABLE OF CONTENTS

ObjectSize_______________________________________________________________________________________________________38

PositionandVelocityVectors___________________________________________________________________________________39

CovarianceandErrorEllipsoids _______________________________________________________________________________40

Assumptions_____________________________________________________________________________________________________41

Computation_____________________________________________________________________________________________________42

Notes_____________________________________________________________________________________________________________43

Bibliography_____________________________________________________________________________________________________43

REVISION HISTORY

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RevisionHistory

Version Description of Changes Date

1.1 1.CorrectedNORAD_CAT_IDinManeuverNotificationformat,pg.252.AdditionofcontactinformationforSpace‐Track.org,pg.32

12Aug16

1.2 1.Definitionsofellipsoidandcovariancescreenings,pg.5‐6 27Aug161.3 1.Screeningschedule,pg.4

2.CorrectiontoBasicReportingCriteriaforLEOCDMs,pg.43.AddeddecimalnotationrequirementtoOEM,pg.184.Added“USER_DEFINED_MAN_ID”tomaneuvernotification,pg.275.Addedadditionalvaluesto“USER_DEFINED_MAN_PURPOSE,”pg.286.Changed“SOLVED”to“DETERMINED”for“USER_DEFINED_MAN_STATUS,”pg.287.Under“HowtoNameManeuverNotifications”update“FileNames”andadded“FileVerification,”pg.298.Updated“SubmittingManeuverNotifications”toexplainthateachorganizationhastheirownmaneuverfolder,pg.319.Added“UpdatingManeuverNotifications,”pg.31

3Jan17

1.4 1.ReplacedJSpOCandJFCCSPACEreferenceswith18SPCS,CSpOCorJFSCC,throughoutthedocumentasneeded2.AddedSSASharingProgramdescription,pg.23.AddedHACvsHAC,O/OEphemerisvsHAC,andO/OEphemerisvsO/OEphemerisdescriptions,pg.4‐54.AddedPDT/PSTtimestoScreeningSchedule,pg.75.OEMstateandcovariancecomponentsmustbeindecimalnotation,pg.216.USER_DEFINED_MAN_IDaddedtomaneuvernotification,pg.307.AddedadditionaloptionstoUSER_DEFINED_MAN_PURPOSE,pg.308.AddedAnnexA:How18SPCSCalculatesProbabilityofCollision

Feb19

USSTRATCOM SSA SHARING PROGRAM

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USSTRATCOMSSASharingProgram

PROGRAM DESCRIPTION

UnitesStatesStrategicCommand(USSTRATCOM),astheU.S.governmententityresponsibleforspacesituationalawareness(SSA),iscommittedtopromotingasafe,stable,sustainableandsecurespaceenvironmentthroughthesharingofSSAinformation.In2009,USSTRATCOMinitiatedtheSSASharingProgramtoprovideno‐costbasic,emergency,andadvancedSSAdataandservicesthatcoverthefullscopeofsupportneededthroughthelifetimeofasatellitefrompre‐launchpreparationtoend‐of‐lifedisposal.Servicesincludelaunchconjunctionassessment(CA),launchsupportandearlyorbitdetermination,on‐orbitCAandcollisionavoidance(COLA),anomalysupport,end‐of‐lifeanddisposalsupport,deorbitsupport,andreentryassessment.Theseservicesareavailabletoalloperatorsofactivespacecraftatthebasicemergencylevel,andattheadvancedlevelforentitieswhohavesignedSSASharingAgreementswithUSSTRATCOM.BasicSSAdataisdisseminatedtotheworldthroughthewebsitewww.space‐track.org,whichisaccessibletoanyonewitharegistereduseraccount.USSTRATCOMhasdelegatedmanagementoftheSSASharingProgramtotheJointFunctionalSpaceComponentCommand(JFSCC),formerlyJointFunctionalComponentCommandforSpace(JFCCSPACE).WhileJFSCCoverseestheSSASharingprogramasawhole,theyhavedirectedthe18thSpaceControlSquadron(18SPCS)toexecuteday‐to‐daySSAdatasharingandsupport,andactasthedirectinterfacebetweensatelliteoperatorsandtheU.S.DepartmentofDefense.18SPCSperformsthespacesurveillancemissionfortheU.S.AirForce,andprovidesfoundationalSSAfortheU.S.government,aswellasglobalspaceentitiesthroughtheSSASharingProgram.ThisrolewasperformedbytheJointSpaceOperationsCenter(JSpOC)untilJuly2016,atwhichtime18SPCSwasactivatedandassumedallspacesurveillancefunctions.InJuly2018,theJSpOCbecametheCombinedSpaceOperationsCenter(CSpOC.)18SPCSiscurrentlyco‐locatedwiththeCSpOCatVandenbergAirForceBase,butnowhastheseparateanddistinctmissionofperformingspacesurveillance,maintainingthespacecatalog,andprovidingfoundationalSSAtoU.S.andglobalpartners.

SPACE-TRACK.ORG

Space‐Track.org(www.space‐track.org)isapublicwebsiteownedbyUSSTRATCOM,managedbyJFSCC,andpopulatedby18SPCS.ItistheprimarymethodofsharingSSAinformationwiththeglobalspacecommunityatthebasic,emergency,andadvancedlevels.AllsatelliteoperatorsshouldregisterforauseraccountinordertoaccessthefullscopeofSSAdataandservicesavailablethroughtheSSASharingProgram.

SPACEFLIGHT SAFETY PROCESS

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SpaceflightSafetyProcess

ON-ORBIT CONJUNCTION ASSESSMENT (CA)

TheJFSCCConjunctionAssessment(CA)process,executedby18SPCS,identifiescloseapproachesbetweenactivesatellitesandothercatalogedspaceobjects.ItbeginswithsatelliteobservationsfromtheU.S.SpaceSurveillanceNetwork(SSN),whichincludesavarietyofsensorsthroughouttheworldthatdetect,track,catalogandidentifyman‐madeobjectsorbitingEarth.SSNobservationsareinputintothe18SPCSmissionsystem,whichimplements“SpecialPerturbations”(SP)orbitpropagationtheory.Orbitdetermination(OD)isperformedautomaticallymultipletimesperdaytodeterminethepositionandvelocityofeachobject,whichisthenupdatedinthe18SPCSHighAccuracyCatalog(HAC).The18SPCSOrbitalSafetyTeamusestheHACtoscreentheorbitaltrajectoriesofallactivesatellitesagainsttherestofthecatalog,whichincludesotheractivesatellites.

Eachactivesatellitescreenedisreferredtoasaprimaryobject,andalloftheotherobjectsintheHACarereferredtoassecondaryobjects.Secondaryobjectsincludenon‐active(dead)payloads,rocketbodies,anddebris,aswellasanalystobjectsthatarenotinthepubliccatalog.18SPCSperformsthreetypesofscreenings:

1. HACvsHAC:SPcatalogdataforactivesatellitesscreenedagainstSPcatalogdataforallcatalogedspaceobjects

2. EphemerisvsHAC:Ephemerisprovidedbythesatelliteowner/operator(O/O)screenedagainsttheSPcatalogdataforallcatalogedspaceobjects

3. EphemerisvsEphemeris:EphemerisprovidedbythesatelliteO/OscreenedagainstephemerisprovidedbyotherO/O’s


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18SPCSperformsscreeningsforeveryactivesatelliteandowner/operator(O/O)ephemerisaminimumofonceevery24hoursinaccordancewiththeprocessinthefollowingdiagram:

Figure1:ConjunctionAssessmentScreeningProcess

HAC vs HAC

ThescreeningprocessstartswiththeupdateoforbitalinformationfortheentirecatalogbasedondatafromtheSSNsensorsandupdatedDynamicCalibrationAtmosphere(DCA)coefficients,whichisthenpropagated7daysfornear‐earthobjectsand10daysfordeep‐spaceobjects.Theseupdatestakeplaceevery8hours.Usingthisdata,18SPCSperformsaninitialscreeningofallactivesatellitesagainsttherestofthecatalogtoidentifyconjunctioncandidateswithinscreeningcriteria.Thesecandidatesarereevaluatedby18SPCSOrbitalSafetyAnalysts(OSAs)toensurethatallofthemostcurrentobservationsareincorporatedintotheOD.18SPCSthenconductsarefinementscreeningtoupdatetheconjunctionestimatesofallofthecandidatesidentifiedintheinitialscreening.Iftheparametersoftheconjunctionfallwithincriteriathatidentifiesacloseapproach,18SPCSwillnotifytheO/Oofthesatellite(s)inaccordancewiththecriteriaandmethodslistedinReportingCriteria.BasedonthequalityoftheOD,18SPCSmayalsoincreasesensortaskingontheprimaryorsecondaryobjects.

O/O Ephemeris vs HAC (EPH vs HAC)

AllO/O’smaysubmitpredictiveephemeristo18SPCSforscreeningagainsttheHAC.O/OsmustsubmitephemerisasOperationalorSpecial.An“Operational”filerepresentswherethesatelliteisplannedtogoandmayormaynotincludemaneuvers.Thesefilestypicallyincorporatenormalstation‐keepsorrepresentafreefallorbit.Ifmaneuversareincluded,18SPCSassumesthesewilldefinitelyoccur.Otherwise,thefilesshouldbesubmittedasSpecial.Operationalfileswillbescreenedaccordingtoaroutineschedule.A


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“Special”filerepresentswherethesatelliteisplannedtogoifamaneuverisbeingconsideredforanon‐routineevent,suchascollisionavoidanceorrelocation.A“Special”fileisusedforplanningpurposesonly,unlessthemaneuverbecomesoperational,inwhichcaseitmustberesubmittedas“Operational.”All“Special”fileswillbetreatedashigh‐interestandscreenedassoonaspossibleafterreceiptbythe18SPCSOrbitalSafetyCrew.Multiplespecialfilesmaybesubmittedforthesamesatellite,butonlyoneoperationalfileatatimecanbesubmittedpersatellite.Resultswillbeprovidedinaccordancewithreportingcriteria.

O/O Ephemeris vs O/O Ephemeris (EPH vs EPH)

AlloperationalO/OephemeriswillbescreenedagainstallotheroperationalO/Oephemerisonascheduledbasis.O/OephemerislabeledasspecialwillnotbescreenedagainstO/Oephemeris.Resultswillbeprovidedinaccordancewithreportingcriteria.OperationalO/O-providedephemerisisscreenedthroughitsduration,meaningthat18SPCSwillretainitinthesystemandcontinuallyscreenitevery8hoursuntilthereislessthan8hoursofpropagatedephemerisfornear‐earthobjectsorlessthan12hoursfordeep‐spaceobjectsremaininginthefile.ThisisonlyapplicabletoEPHvsEPHscreenings.

ON-ORBIT COLLISION AVOIDANCE (COLA)

Inresponsetoacloseapproach,anO/Omayconsiderperformingcollisionavoidance(COLA),whichistheprocessofplanningandpossiblyexecutingamaneuvertomitigatetheriskofcollision.BasedoninformationreceivedintheCAnotifications,orfromotherdatasources,theO/Odecideswhetherornottoperformcollisionavoidancebymaneuveringtheirsatellite.IftheO/OdecidestoconsiderCOLAactions,theymaysend18SPCStheirpredictiveephemerisdata(inapprovedformatsandcoordinatesystems)which18SPCSwillthenscreenagainsttheHACandprovidenotificationswithinreportingcriteriatotheO/Osothattheymaydecidehowtoproceed.Thisexchangeofdatamaycontinueuntilthetimeofclosestapproach(TCA),afterwhich18SPCSwillresumeroutinescreeningoftheactivesatelliteinaccordancewiththescreeningschedule.IftheydonotperformCOLA,18SPCSwillcontinuetomonitortheconjunctionandprovideupdatesbasedondatafromtheSSNuntiltheTCAhaspassed.

BASIC EMERGENCY CA AND COLA SERVICES

18SPCSperformsCAandprovidesCOLAsupportforallsatelliteoperatorsofactivespacecraftasanemergencyservicetoensurespaceflightsafety.Alloperatorsareeligibleforthisbasicemergencyservice,whichincludesthefollowingusingbasicscreeningvolumesandreportingcriteria:

1. HACvsHACscreenings2. OperationalO/OEphemerisvsHACscreenings3. OperationalO/OEphemerisvsOperationalO/OEphemerisscreenings4. SpecialO/OEphemerisvsHACscreenings

4.1 O/OEphemerisversussecondaryobject,or4.2 O/OEphemerisversusfullcatalog

5. HAC‘1‐versus‐1’requests:screeningoneprimaryobjectagainstonesecondaryusingHACdata


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ADVANCED CA AND COLA SERVICES

O/OswhohavesignedanSSASharingAgreementwithUSSTRATCOMmayrequestAdvancedCAandCOLAthroughtheOrbitalDataRequest(ODR)process.(Visithttps://www.space‐track.org/documentation#/odrformoreinformation.)AdvancedCAandCOLAsupportincludesthefollowingusingadvancedscreeningvolumesandreportingcriteria:

1. HACvsHACscreenings2. OperationalO/OEphemerisvsHACscreenings3. OperationalO/OEphemerisvsOperationalO/OEphemerisscreenings4. SpecialO/OEphemerisvsHACscreenings

4.1 Ephemerisversussecondaryobject,or4.2 Ephemerisversusfullcatalog

5. HAC‘1‐versus‐1’requests:screeningoneprimaryobjectagainstonesecondaryusingHACdata6. AccesstopropagatedSPephemeris(withoutcovariance)forallobjectsintheunclassifiedspace

catalog7. AccesstoSPstatevectors(withoutcovariance)forallobjectsintheunclassifiedcatalog


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SCREENING SCHEDULE

Thefilenamedictateshow18SPCSwillprocessephemerisfiles.IftheO/Odesignatesitas“Special”18SPCSwillprocessitashigh‐interestandscreenitassoonaspossible.Ifitisdesignatedas“Operational”18SPCSwillscreenitaccordingtothescheduleinTable1:ScreeningSchedule.Resultsfor“Special”ephemerisareprovidedwithin4hoursofreceivingtheephemeris,and“Operational”resultsareprovidedwithin8hoursofthescheduledscreeningtimes.

Table 1: Screening Schedule

1330–2130UTC(PacificDaylightTime),1430–2230UTC(PacificStandardTime)NearEarthHACGeosynchronous Earth Orbit (GEO)HighAccuracyCatalog(HAC)(forsatellitesfrom0to180longitude)NearEarthEphemeris[includesallephemerissubmittedbetween0530–1330UTC(PDT),0630‐1430UTC(PST)]DeepSpaceEphemeris[includesallephemerissubmittedbetween0530–1330UTC(PDT),0630‐1430UTC(PST)]Owner/OperatorEphemerisvsOwner/OperatorEphemeris2130–0530UTC(PacificDaylightTime),2230–0630UTC(PacificStandardTime)NearEarthHACHighlyEllipticalOrbit(HEO)/MediumEarthOrbit(MEO)HACUnnamedNearEarthHACUnnamedDeepSpaceHACNearEarthEphemeris[includesallephemerissubmittedbetween1330–2130UTC(PDT),1430‐2230UTC(PST)]DeepSpaceEphemeris[includesallephemerissubmittedbetween1330–2130UTC(PDT),1430‐2230UTC(PST)]Owner/OperatorEphemerisvsOwner/OperatorEphemeris0530–1330UTC(PacificDaylightTime),0630–2230UTC(PacificStandardTime)NearEarthHACGEOHAC(forsatellitesfrom180to359.99longitude)NearEarthEphemeris[includesallephemerissubmittedbetween2130–0530UTC(PDT),2230‐0630(PST)]DeepSpaceEphemeris(includesallephemerissubmittedbetween2130–0530UTC(PDT),2230‐0630(PST)]Owner/OperatorEphemerisvsOwner/OperatorEphemeris

SCREENING VOLUMES

Thefollowingtablesdefinethescreeningvolumesanddurations18SPCSusesforbasicCA,advancedCA,andephemerisscreenings.18SPCSconductstwotypesofscreenings:ellipsoidandcovariance.Bothscreeningsusecovariance‐based“uncertaintyvolumes”aroundthesatellitesandthephysicalsizeofthesatellitesthemselves(referredtoas“exclusionvolumes”).Thesamebasicmathematicalapproachisused


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forbothellipsoidandcovarianceoptions,althoughprobabilityofcollision(Pc)isonlycomputedforthecovarianceoption.Fortheellipsoidscreeningoption,theuncertaintyvolumeisstaticthroughoutthescreeningprocess,whereascovariancescreeningsemployajointuncertaintyvolumethatisbasedontime‐varyingcovariancevaluesfortheprimaryandsecondaryobjectsatthetimeofclosestapproach.PleasereferenceAnnexAformoreinformationonhow18SPCScalculatesprobabilityofcollision.

Table 2: Basic HAC Screening Volumes

Screening OrbitRegimeDefinition Propagation Radial In-Track Cross-Track

GEOBasicEllipsoid

1300min<Period<1800minEccentricity<0.25&Inclination<35º

10days 10km 10km 10km

HEOBasicEllipsoid

Perigee>2000km&Eccentricity>0.25


MEOBasicEllipsoid

600min<Period<800minEccentricity<0.25


LEOBasicCovariance

Perigee≤2000kmEccentricity<0.25

7days 1km 1km 1km

Table 3: Advanced HAC Screening Volumes


GEOAdvancedEllipsoid

1300min<Period<1800minEccentricity<0.25&Inclination<35º


HEOAdvancedEllipsoid



MEOAdvancedEllipsoid

600min<Period<800minEccentricity<0.25


LEO4AdvancedCovariance

1200km<Perigee≤2000kmEccentricity<0.25

7days 0.5km 2km 2km



7days 0.5km 12km 10km



7days 0.5km 28km 29km



7days 2km 44km 51km

Table 4: Ephemeris Screening Volumes


DeepSpaceEllipsoid

Period>225min 10days 20km 20km 20km

NearEarthCovariance

Period<225min 7days 2km 25km 25km


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REPORTING CRITERIA

ThefollowingtablesdefinethereportingcriteriausedforbasicandadvancedCAandCOLA.TheadvancedserviceprovidesmoredatafurtherinadvanceofTCAandmayberequestedthroughtheOrbitalDataRequest(ODR)process.ThesetablesapplytobothHighAccuracyCatalog(HAC)andephemerisscreenings.“Deepspace”includesGeosynchronousEarthOrbit(GEO),HighlyEllipticalOrbit(HEO)andMediumEarthOrbit(MEO).“Nearearth”includesallLowEarthOrbit(LEO)regimes.

Table 5: Basic Reporting Criteria

ReportingCriteria:

Space-TrackCriteria EmergencyCriteriaEmergency

PhoneCallCriteria

NotificationMethod:

ConjunctionDataMessage(CDM)

ConjunctionDataMessage(CDM)&CloseApproachNotification(CAN)email

CDM,CANemail&phonecall

DeepSpace:HAC&Ephemeris

TCA≤10daysandOverallmiss≤10km


TCA≤3daysandOverallmiss≤500m

NearEarth:HAC&

Ephemeris

TCA≤3daysandOverallmiss≤1kmandProbabilityofCollision≥e^‐4



Table 6: Advanced Reporting Criteria

ReportingCriteria:

Space-TrackCriteria EmergencyCriteriaEmergency

PhoneCallCriteria

NotificationMethod:

CDM CDM&CANemailCDM,CANemail&phone

call

DeepSpace,HAC

AllresultswithinAdvancedCAscreeningvolumes


TCA≤3daysandOverallmiss≤500m

DeepSpace,Ephemeris



N/A

LEO4,HAC




LEO3,HAC




LEO2,HAC




LEO1,HAC




NearEarth,Ephemeris



N/A


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NOTIFICATIONS

O/Oswillreceiveavarietyofemailnotificationsdependingonthescreeningperformedandresultsproduced.AlloftheseemailsaregeneratedandtransmittedbytheSpace‐Trackwebsite,basedonfilesthat18SPCSOrbitalSafetyCrewhasuploadedtothewebsite.Themessagesare:

• CloseApproachNotification(CAN)email:sentforeverypredictionthatmeetsemergencyreportablecriteria.Includesprimary,secondary,missdistances,TCA,andfornearearthevents,probabilityofcollision(Pc).OnlysenttoemailaddressesthattheO/OhasdesignatedtoreceiveCANs.(Controlledby18SPCS–[email protected].)

• CDMemail:notifiesO/OsthatnewCDMsareavailableonSpace‐Trackfortheirorganization.Alluserswith‘CDMNotification’permissionsreceivethisemail.(Controlledbyeachorganization’sPrimaryRepresentativeor18SPCSthroughtheOperatorPanelonSpace‐Track.org.)

• Negativeresultsemail:Confirmsthataspecificephemerisfilehasbeenscreenedandstatesthattherearenoresultswithintheephemerisscreeningvolume.

• Expandedresultsemail:Confirmsthataspecificephemerisfilehasbeenscreened,andthatthereareresultsthatarewithintheephemerisscreeningvolumes,butnotwithinemergencyreportablecriteria.O/OswhodonothavetheadvancedCAservicemayreceivethisnotification.

Thereareslightdifferencesbetweenthenotificationssentforbasicandadvancedservices.ThefollowingfiguresshowwhichmessagesshouldbeexpectedfordifferentscenarioswithintheCAandCOLAprocess.

BASIC EMERGENCY CA NOTIFICATIONS

Figure2:BasicEmergencyCANotifications

18SPCSScreening


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ADVANCED CA NOTIFICATIONS

Figure3:AdvancedCANotifications

LAUNCH CONJUNCTION ASSESSMENT

18SPCSalsoprovideslaunchconjunctionassessment(CA),whichidentifiespotentialconjunctionsthatmayresultinacollisionbetweenlaunchingobjectsandon‐orbitobjects.UsingO/O‐providedinformation,18SPCSscreensthelaunchvehicleagainstthespacecatalogandidentifiesperiodsduringthelaunchwindowwhichmayputtherocketandpayloadatincreasedriskforcollision.Formoreinformationonthisservice,referencetheLaunchConjunctionAssessmentHandbook.

18SPCSScreening


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EARLY ORBIT CONJUNCTION ASSESSMENT

EarlyorbitCAincludesthescreeningofO/O‐providedephemerisagainstthecatalogtofacilitatethesafemaneuveringofanewlylaunchedobjectintoitsfinalorbit.18SPCSwillscreentheephemerisusingearlyorbitscreeningvolumesandreportallresultsinaccordancewithadvancedreportingcriteria.TheO/Omayprovidemorethanoneephemerisfileforeachmaneuvertoallowanalysisofmultiplescenarios,buttheymustbelabeled“Special.”ToreceiveearlyorbitCAsupporttheO/Oshould:

Pre‐launch:o SubmitanODRrequestingearlyorbitCA,specifyingthepreferredscreeningvolumeo RegisterforauseraccountonSpace‐Track.orgo Providetheearlyorbitmaneuverplano Sendinpre‐launchephemerisfortesting,ifdesired

Post‐launch:o Provideephemerisinanapprovedformatandusingthecorrectfilenameformatforeach

screeningo Provideanupdatedmaneuverplan,asneeded

Table 7: Early Orbit Screening Volumes


DeepSpaceEllipsoid



NearEarthCovariance


7days 2km 44km 51km

END-OF-LIFE/DISPOSAL SUPPORT

IfanO/Odecidestomoveasatellitetoaless‐populatedorbitattheendofitslifetime,18SPCSwillassisttheO/Obyscreeningmaneuverephemerisandprovidingresults.AllO/O‐providedephemeriswillbescreenedusingstandardephemerisscreeningvolumes,andresultswillbeprovidedinaccordancewithbasicoradvancedreportingcriteria.Toreceiveend‐of‐life/disposalCAtheO/Oshould:

SubmitanOrbitalDataRequest(ODR)ifallresultswithinadvancedreportingcriteriaaredesired(noODRisrequiredforbasicreportingcriteria)

Providethemaneuverplan Provideephemerisinanapprovedformatandusingthecorrectfilenameformatforeach

screening

DEORBIT SUPPORT

AdeorbitisthecontrolledreentryofasatelliteintotheEarth’satmosphere.IfanO/Odecidestodeorbitasatelliteorrocketstagethroughaseriesofmaneuvers,18SPCScanprovideCAscreenings,aswellascoordinatewithNASAtoensurethedeorbitingspacecraftsafelydescendsthroughtheInternationalSpace


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Station’s(ISS)orbit.Afterthespacecraftcompletesitsmaneuvers,18SPCScanconfirmfinalreentry.Toreceivethisservice,theO/Oshould:

SubmitanODRifallresultswithinadvancedreportingcriteriaaredesired(noODRisrequiredforbasicreportingcriteria)

Providethemaneuverplan Provideephemerisinanapprovedformatandusingthecorrectfilenameformatforeach

screening

TASKING

Incertainsituations,moreobservationsonanobjectwillimprovethequalityoftheconjunctionprediction.Tocollectmoreobservations,18SPCScanincreasethetaskingpriorityandrevisitfrequencyassignedtosensors.18SPCShasembeddedthisintheroutinescreeningcycle,asseeninFigure1.Ifanobjectmeetsanyofthefollowingcriteria,itstaskingwillbeautomaticallyincreased:

Nottrackedwithin72hours,or Lessthan20observationsincurrentDifferentialCorrection(DC),or FewerthantwosensorscontributingtocurrentDC,or Atleastonecomponentofcovarianceover100,000m2

Foremergencyreportableevents,theO/Omaycontact18SPCStoconfirmthatbothobjectsintheconjunctionaretaskedappropriatelybyemailing18SPCS.doo.orbitalsafety@us.af.mil.

EPHEMERIS FORMATS

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EphemerisFormats

OVERVIEW

Satelliteoperatorsmayprovideephemerisinvariousformats,whichwillbeconvertedintoformatsthatcanbeprocessedby18SPCS’ASW(AstrodynamicsSupportWorkstation)/SuperComputationofMissBetweenOrbits(COMBO)software.Operatorsmayelecttoincludecovariancedatainordertoaccommodateprobabilityofcollisioncalculations.

Allephemerisformatsmustbeinthemeanequator/meanequinox(MEME)J2000.0framewithpositionalvalues(x,y,z)giveninkilometerunitsandvelocityvalues(dX,dY,dZ)giveninunitsofkilometers/second.Ifcovariancevaluesareprovided,theymustbeindecimalnotationandtheUVWorbitalreferenceframe.

Allephemerisformatsmayincludeinformationalheaderlinespriortothedatalineswhichprovidedate,time,andstatevector(positionandvelocity)values.Eachformatdiffersinthenumberandcontentofheaderlines,formatofthedateandtimevalues,anddifferentprecision(numberofdecimalplaces)forthestatevectorcomponents.

18SPCS’conversionutilitysoftwareconvertsallephemerisformatstotrueequator/meanequinox(TEME)ofDateforepochJ2000.Themostrecenttimeconstantsdatafilewithinthe18SPCSmissionsystemcorrectsforleapseconds.

TheNASAandUTCformatsincludepositionandvelocityonly.18SPCSacceptsthreeformatswithcovariance:

• GeneralOn‐Orbit(GOO)mayinclude3x3(6lowertriangularmatrix)positioncovariancedatavalues,

• ModifiedITCmayinclude6x6(21lowertriangularmatrix)positionandvelocitycovariancedatavalues

• OrbitalEphemerisMessage(OEM)format,bothwithandwithoutcovariancedata,whichincludesacapabilityforprocessing6x6(21lowertriangularmatrix)positionandvelocitycovariancedatavalues.VisittheConsultativeCommitteeforSpaceDataSystems(CCSDS)websiteformoreinformationontheOEM:https://public.ccsds.org/Pubs/502x0b2c1.pdf

EPHEMERIS FORMATS

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NASA EPHEMERIS FORMAT

TheNASAephemerisdatafileformatcontainsnodescriptiveheaderlinesprecedingtheephemerisdata.EachdatalinecontainsadateformattedasYYDOY(onlytwodigitsofyearandDayofYear(DOY)inplaceofmonthandday),atimestampformattedasHHMMSS.SSS,andthestatevectorposition(x,y,z)andvelocity(dx,dy,dy)values.

Figure4:NASAEphemerisFormatExample

18SPCS’conversionutilityseparatestheentriesoneachephemerisdatalinewhereveroneormorespace(i.e.““)charactersarefound.Foreachdatalinethisprocessingresultsinacombineddate/timefield(i.e.“yyDOYhhmmss.sss”)andeachstatevectorcomponent(i.e.position(x,y,z)andvelocity(i.e.dx,dy,dz))value.Thisephemerisformatdoesnotincludecovariancevalues.

EPHEMERIS FORMATS

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UTC EPHEMERIS FORMAT

TheUTCephemerisdatafileformatcontains21descriptiveandheaderlinesprecedingtheephemerisdata.EachdatalinecontainsadateformattedasYYYY/MM/DD,atimestampformattedasHH:MM:SS.SSS,andthestatevectorposition(x,y,z)andvelocity(dx,dy,dy)values.

Figure5:UTCEphemerisFormatExample

Theconversionutilityignoresthe21headeranddescriptivelinesandseparatestheentriesoneachephemerisdatalinewhereveroneormorespace(i.e.““)charactersarefound.Foreachdatalinethisprocessingresultsinadatefield(i.e.“yyyy/mm/dd”),atimefield(i.e.“hh:mm:ss.sss”)andeachstatevectorcomponent(i.e.position(x,y,z)andvelocity(i.e.dx,dy,dz))value.

Thedatefieldisseparatedintomonth,day,andyearvalueswhereveraforwardslash(i.e.“/”)characterisfound;andthetimefieldisseparatedintohour,minute,andsecondvalueswhereveracolon(i.e.“:”)characterisfound.TheDayofYear(DOY)valueiscalculatedfromthemonthandday,takingintoaccountanyadjustmentneededforaleapyear.

Thisephemerisformatdoesnotincludecovariancevalues.

EPHEMERIS FORMATS

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GENERIC ON-ORBIT EPHEMERIS FORMAT

TheGenericOn‐Orbitephemerisdatafileformatistheonlyformatthatisrequiredtocontainpositioncovariancedatainadditiontoephemerisdata.Itcontainsfourdescriptiveandheaderlines,wherethefourthindicatestheorbitalreferenceframeofthecovariancedata.Thedatasectionconsistsof,foreachephemerisdatapoint,onelinewhichcontainstheepochdateandtimeandstatevector,followedbyonelineholdingthesixvaluesofthe3x3lowertriangularpositioncovariancematrix.

Figure6:GenericOn-OrbitDataExample

Theconversionutilitymakesarudimentarycheckthatthecorrectnumberofdatalinesarepresent.Thenumberofheaderlinesissubtractedfromthenumberoflinesintheephemerisdatafile;iftheresultisnotdivisibleby2(astatevectorlineplusacovariancedataline)awarningmessageisoutput,buttheconversioncontinues.

Theconversionutilityignorestheheaderanddescriptivelinesexceptforthecovarianceorbitframereferencevalue.Itthenreadsthedatalinesingroupsoftwoandseparatestheentriesoneachdatalinewhereveroneormorespace(i.e.““)charactersarefound.Foreachgroupoftwodatalinesthisprocessingresultsinadate/timefield(i.e.“yyyDOYhhmmss.sss”),eachstatevectorcomponent(i.e.position(x,y,z)andvelocity(i.e.dx,dy,dz))valueand6covariancevalues.

Thecombineddate/timefieldisseparatedintoyear,DayofYear,hour,minute,andsecondvalues.

EPHEMERIS FORMATS

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MODIFIED ITC EPHEMERIS FORMAT

TheModifiedITCephemerisdatafileformatistheonlyformatthatisrequiredtocontainbothpositionalandvelocitycovariancedatainadditiontoephemerisdata.Itcontainsfourdescriptiveandheaderlines,wherethefourthindicatestheorbitalreferenceframeofthecovariancedata.Thedatasectionconsistsof,foreachephemerisdatapoint,onelinewhichcontainstheepochdateandtimeandstatevector,followedbythreelinesholdingthetwenty‐onevaluesofthe6x6lowertriangularpositionandvelocitycovariancematrix.

Figure7:ModifiedITCDataExample

Theconversionutilitymakesarudimentarycheckthatthecorrectnumberofdatalinesarepresent.Thenumberofheaderlinesissubtractedfromthenumberoflinesintheephemerisdatafile;iftheresultisnotdivisibleby2(astatevectorlineplusacovariancedataline)awarningmessageisoutput,buttheconversioncontinues.

Thepreprocessingscriptignorestheheaderanddescriptivelinesexceptforthecovarianceorbitframereferencevalue.Itthenreadsthedatalinesingroupsoffourandseparatestheentriesoneachdatalinewhereveroneormorespace(i.e.““)charactersarefound.Foreachgroupoffourdatalinesthisprocessingresultsinadate/timefield(i.e.“yyyDOYhhmmss.sss”),eachstatevectorcomponent(i.e.position(x,y,z)andvelocity(i.e.dx,dy,dz))valueand21covariancevalues.

Thecombineddate/timefieldisseparatedintoyear,DayofYear,hour,minute,andsecondvalues.

EPHEMERIS FORMATS

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ORBITAL EPHEMERIS MESSAGE (OEM) FORMAT

TheOEMephemerisdatafileformatcontainsavariablenumberofoptionaldescriptivelinesinterspersedwithblocksofephemerisandoptionallycovariancedata.NotethatblocksofcovariancedatamaycontainmorethanonesetofcovariancedataseparatedbyEPOCHkeywords.

Foracompletediscussionofthisephemerisdataformatreferto:“OrbitEphemerisMessage(OEM)”,OrbitDataMessages,ConsultativeCommitteeforSpaceDataSystems(CCSDS),RecommendedStandardCCSDS502.0‐B‐2,BlueBook,November2009.

Figure8:OEM(withoutcovariance)Example

EPHEMERIS FORMATS

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Figure9:OEM(withcovariance)Example

Theconversionutilityremovesalllinesthatareblankorcontainthekeywords(definedintheformatdocument)CCSDS_OEM_VERS,CENTER_NAME,COMMENT,COV_REF_FRAME,CREATION_DATE,INTERPOLATION,INTERPOLATION_DEGREE,META_START,META_STOP,OBJECT_ID,OBJECT_NAME,ORIGINATOR,REF_FRAME,START_TIME,STOP_TIME,TIME_SYSTEM,USEABLE_START_TIME,orUSEABLE_STOP_TIME.Thisreducesthedatafiletolinescontainingblocksofephemerisdataandoptionalblocksofcovariancedata.ThecovariancedatablocksaredelimitedbylinescontainingthekeywordsCOVARIANCE_STARTandCOVARIANCE_STOP.Therecanbemorethanonesectionofcovariancedata

EPHEMERIS FORMATS

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withinablock,eachdelimitedbyanEPOCHline.Thedate/timevaluesoftheoptionalcovariancedataontheEPOCHlinesarenotguaranteedbytheformatstandardtomatchanyephemerisdatadate/timevalues.

Thevaluesoneachephemerisdatalineareseparatedoutwhereveroneormorespace(i.e.““)charactersarefound.

Foreachephemerisdataline,furtherprocessingresultsindate/timefields(i.e.“yyyy‐mm‐dd”and“hh:mm:ss.sss”)andeachstatevectorcomponent(i.e.position(x,y,z)andvelocity(i.e.dx,dy,dz))value.Foreachoptionalblockofcovariancedata,processingresultsindate/timefields(i.e.“yyyy‐mm‐dd”and“hh:mm:ss.sss”)andthe21covariancevalues.NotethatstatevectorandcovariancecomponentsmustbeindecimalnotationasseeninFigure9.

Thedatefieldisseparatedintomonth,day,andyear,valueswhereveradash(i.e.“‐”)characterisfound;andthetimefieldisseparatedintohour,minute,andsecondvalueswhereveracolon(i.e.“:”)characterisfound.TheDayofYear(DOY)valueiscalculatedfromthemonthandday,takingintoaccountanyadjustmentneededforaleapyear.

Theconversionutilitythenmatchesbydateandtimethecovariancedata,ifitexists,toalineofephemerisdata.Ifthedateandtimeexactlymatches,thecovariancedataisappendedtotheephemerisstatevectorforthatdateandtime.Thecovariancedataisdiscardedifthedateandtimedonotmatchforasectionofephemerisdata.Ephemerisdatalinesthatdonothavedateandtimematchesofcovariancedatawillhavetwenty‐onezero(i.e.“0.0”)valuesfollowingthestatevector.

HOW TO NAME EPHEMERIS FILES

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HowtoNameEphemerisFiles

OVERVIEW

18SPCSrequiresthatsatelliteoperatorsnametheirfilesfollowingaspecificformat,whichdictateshowtheephemerisfilewillbeprocessed.IftheO/OsubmitstheirfilesthroughthewebsiteSpace‐Track.org,Space‐Trackwillverifythefileisnamedcorrectlyandreturnerrormessagesifitisnot.Thefilenameformatis:

<DataType>_<Catalog#>_<CommonName>_<DayTimeGroup>_<Operational/Special>_<MetaData>_<classification>.<FileExtension>

Table 8: Ephemeris File Name Convention

Data Field Description Normative Value (N) or Example (E)

Obligatory

DataType Thetypeofdatabeingprovided. (N)MEME(unlessotherwisedirectedby18SPCS)Thisentrymustbeinuppercaseletters

Yes

Catalog# The5‐digitdesignatorfortheobjectasdefinedinUSSTRATCOM’ssatellitecataloghttps://www.space‐track.org/#/catalog

Iftheobjectisnotcataloged,use:

UCN##fornear‐earthobjects

UCD##fordeep‐spaceobjects

Substitute##with01,02,03etc.uptothetotalnumberoffilessubmitted

(E)25544

(E)00900

(E)UCN01

(E)UCD02

Yes

CommonName Thespacecraftnamefortheobject.RecommendusingthecommonnamelistedinUSSTRATCOM’ssatellitecataloghttps://www.space‐track.org/#/catalog

Iftheobjectisnotcataloged,usethecommonnamedesignatedbythesatelliteoperator

(E)ISS(ZARYA)(E)CALSHERE1

Yes

DayTimeGroup Starttimeoftheephemeris,usingthree‐digitdayofyearandHH:MMinUTC,whereHHisthehourinthe24‐hoursystem

(E)3001224=Oct25,2015at12:24pmUTC

Yes

Continuedonnextpage

HOW TO NAME EPHEMERIS FILES

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Table 8: Ephemeris File Name Convention, cont.

Data Field Description Normative Value (N) or Example (E)

Obligatory

Operational/Special

An“Operational”filerepresentswherethesatelliteisplannedtogoandmayormaynotincludemaneuvers

Thesefilestypicallyincorporatenormalstation‐keeps,orrepresentafreefallorbit

Ifmaneuversareincluded,18SPCSassumesthesewilldefinitelyoccur.Otherwise,thefilesshouldbesubmittedasSpecial.

A“Special”filerepresentswherethesatelliteisplannedtogoifaspecialmaneuverisexecuted

A“Special”fileisusedforplanningpurposesonly,unlessthemaneuverbecomesoperational,inwhichcaseitmustberesubmittedas“Operational”

All“Special”fileswillbetreatedashigh‐interest

*MultipleSpecialfilesmaybesubmittedforthesamesatellite,butonlyoneOperationalfileatatimecanbesubmittedpersatellite*

(N)Operational

(N)Special

Yes

MetaData Additionalinformationprovidedatthediscretionofthesatelliteoperator.Oftenusedtoindicatemaneuverstatus,ormaneuveroptionplan.IfthereisnoadditionalinformationtoprovidetheremustbeadoubleunderscorebetweenOperational/SpecialandClassification

(E)nomnvr

(E)stationkeepingmnvr

(E)burn02

(E)__

Yes

Classification Thesecurityclassificationofthedata.Onlyunclassifiedfileswillbeacceptedthroughwww.space‐track.org

(E)unclassified

(E)UNCLASSIFIED

(E)Unclassified

Yes

FileExtension Indicatesthetypeoffile (N).txt(unlessotherwisedirectedby18SPCS)

Yes

FILE NAME EXAMPLES

• MEME_25544_ISS_1651200_operational__unclassified.txt

• MEME_25544_ISS(ZARYA)_1651200_operational_nomnvr_UNCLASSIFIED.txt

• MEME_25544_ISS_1651200_special_mnvr01_Unclassified.txt

• MEME_UCN01_SPACESAT1_1651200_special_burn01_Unclassified.txt

HOW TO SUBMIT EPHEMERIS FILES

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HowtoSubmitEphemerisFiles

OVERVIEW

Ephemerisfilesmaybesubmittedinthreeways:

1. ThroughthewebsiteSpace‐Track.org(preferred):

1.1 Manually,or1.2 UsingApplicationProgrammingInterface(API)

2. Byemailtothe18SPCSOrbitalSafetyCrewat18SPCS.doo.orbitalsafety@us.af.mil

Space‐Track.orgisthemostefficientandsecuremethodofsubmittingephemerisfilesonarecurringbasis.Tosetupaccess,contactthe18SPCSSSASharingTeamat18SPCS.doo.ssasharing@us.af.milor+1‐805‐606‐2675.TheSSASharingTeamwillrequestthefollowinginformation:

1. ThetitleofyourCDMorganization(theorganizationnameunderwhichyouaccessCDMsontheOperatorPanel)

2. TheSpace‐Trackusernamesofthepersonnelwhowilluploadephemerisforyoursatelliteconstellation

TheSSASharingCellwillcreateapermissions‐basedephemerisfolderforyourorganizationthatwillonlybeaccessedby18SPCSandthepersonnelyouauthorize.Youwillhaveupload,download,anddeletepermissionstothefolder,whichyouwillaccessthroughtheFilesPanel:

Figure10:FilesPanelonSpace-Track.org


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SUBMITTING EPHEMERIS THROUGH SPACE-TRACK.ORG

OnceyouhaveanephemerisfolderontheFilesPanel,youwillalsohaveaccesstothe‘Download’and‘Upload’tabs.Onthe‘Upload’tabyoucanuploadavarietyoffilestothefoldersyouhavepermissionsto.Allofthesefolderswilldisplayinthe‘Destination’dropdownmenu:

Figure11:FilesPanelUploadTab-Ephemeris

MANUAL UPLOAD OF EPHEMERIS FILES

1. LogintoSpace‐Track.org,clickon‘Files’atthetopofthepage,andthenclickonthe‘Upload’tab2. Click‘ChooseFiles’andselectyourephemerisfiles

2.1 Maximumnumberoffilesinasingleuploadis1002.2 Maximumtotalsizeperuploadis60MB2.3 Filesmustbenamedinaccordancewithguidelinesin‘HowtoNameEphemerisFiles’

3. Usingthe‘Destination’dropdownmenu,selectthe‘/Ephemeris/’folderforyourorganization4. Click‘Upload’touploadthefiles5. Afterasuccessfulupload,thefileswilldisplayin‘MyUploads(Last30Days)’Table


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6. The18SPCSOrbitalSafetyCrewwillreceiveanotificationthatyouhaveuploadednewephemerisfilesforscreening.However,iftheephemerisis‘Special’orinresponsetoahigh‐interestevent,pleasephoneoremailtheOrbitalSafetyCrewtonotifythemthatyouneedanimmediateresponse:+1‐805‐605‐[email protected].

API UPLOAD OF EPHEMERIS FILES

PleasevisitSpace‐Track.org’s‘API’and‘HowTo’pagesforthemostup‐to‐dateguidance(includingusagelimitations)onAPIcapabilities:https://www.space‐track.org/documentation#apiandhttps://www.space‐track.org/documentation#/howtoSimilartomanualuploads:

1. Filesmustbenamedinaccordancewithguidelinesin‘HowtoNameEphemerisFiles’2. Afterasuccessfulupload,thefileswilldisplayin‘MyUploads(Last30Days)’Table3. The18SPCSOrbitalSafetyCrewwillreceiveanotificationthatyouhaveuploadednewephemerisfiles

forscreening.However,iftheephemerisis‘Special’orinresponsetoahigh‐interestevent,pleasephoneoremailtheOrbitalSafetyCrewtonotifythemthatyouneedanimmediateresponse:+1‐805‐605‐[email protected].

EMAIL

IfyourorganizationdoesnothaveanephemerisfolderontheFilesPanel,youmaysendyourephemerisbyemailtothe18SPCSOrbitalSafetyCrewat18SPCS.doo.orbitalsafety@us.af.mil.Please:

1. Namefilesinaccordancewithguidelinesin‘HowtoNameEphemerisFiles’2. Sendonly.txtfiles‐.zipfilescannotbeaccepted

GUIDANCE FOR HIGH-INTEREST EVENTS

Afteryousubmityourephemeris,pleasesendanemailorphonethe18SPCSOrbitalSafetyCrew([email protected]+1‐805‐605‐3533)toprovidethefollowinginformationregardingthehigh‐interestrequest:

1. Theconjunctioneventthatyouareconcernedabout(providesecondaryobject,CDMIDnumber,and/ortimeofclosestapproach)

2. Theobject(s)youneedyourephemerisscreenedagainst(forexample,oneobject,ashortlistofobjects,orthewholecatalog)

3. ReferenceyourapprovedOrbitalDataRequest,ifapplicable4. ConfirmationthatyoudoordonothaveanSSASharingAgreementwithUSSTRATCOM(Formore

informationonSSASharingAgreements,pleasevisithttps://www.space‐track.org/documentation#/odr)

MANEUVER NOTIFICATION FORMAT

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ManeuverNotificationFormat

OVERVIEW

18SPCSrespectfullyrequeststhatsatelliteoperatorsnotify18SPCSofplanned,confirmed,andcancelledmaneuvers.Thisallows18SPCStomaintainamoreaccuratecatalog,anticipatespaceflightsafetyrisks,andprovidemorerelevantconjunctionnotifications.18SPCShaschosentheOrbitalParameterMessage(OPM)asthemostappropriatemethodofexchangingmaneuverinformation.However,tomakeitapplicableto18SPCSoperationsandreportingprocedures,18SPCShasmademodificationstothemessage,whicharedetailedinthefollowingManeuverNotificationFormat.GuidelinesthataddtoordeviatefromthedirectionprovidedintheCCSDSOrbitDataMessageBlueBookareindicatedinredtext.ToaccesstheBlueBookvisit:https://public.ccsds.org/Pubs/502x0b2c1.pdf

FORMAT

TheOPMshallberepresentedasacombinationofthefollowing:(1)aheader,(2)metadata(dataaboutdata),(3)data,and(4)optionalcomments.Space‐TrackonlyacceptsKVN(Key/ValueNotation).

ThefollowingtablesspecifyforeachsectionoftheOPM:(1)thekeywordtobeused,(2)ashortdescriptionoftheitem,(3)examplesofallowedvaluesortheunitsofthetime,whenapplicable,and(4)whethertheitemisobligatoryoroptional.

Table 9: OPM Header

Keyword Description Example ObligatoryCCSDS_OPM_VERS Formatversionintheformof

‘x.y’,where‘y’isincrementedforcorrectionsandminorchanges,and‘x’isincrementedformajorchanges.

2.0 Yes

COMMENT ReferenceODMBlueBook6.7

Thisisacomment No

CREATION_DATE Filecreationdate/timeinUTC

2015‐07‐04T12:00:00

Yes

ORIGINATOR Creatingagencyoroperator.MustbethesametitleastheCDMaccountonSpace‐Track.org

CNES,JAXA,ESA,IRIDIUM

Yes

USER_DEFINED_RELEASABILITY Acceptablevalues:PUBLIC,PRIVATEPUBLIC:OperatoragreestosharefilewithallothersatelliteoperatorswhohaveaccesstoSpace‐Track’sOperatorPanel

PUBLIC No.Ifnotprovided,willdefaulttoPRIVATE


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PRIVATE:Operatorsagreestosharefilewith18SPCSonly

USER_DEFINED_CLASSIFICATION Acceptablevalue:UNCLASSIFIED18SPCSwillonlyacceptunclassifiedfiles.

UNCLASSIFIED Yes

TABLE 10: OPM METADATA

Keyword Description Example ObligatoryCOMMENT ReferenceOrbitalDataMessageBlue

Book6.7Thisisacomment No

OBJECT_NAME Spacecraftnameforwhichtheorbitstateormaneuverinformationisprovided.RecommendusingcommonnamesintheSatelliteCatalogonwww.space‐track.org

ISS(ZARYA) Yes

OBJECT_ID Objectidentifieroftheobjectforwhichtheorbitstateisprovided.UseinternationaldesignatoraspublishedonSpace‐Track.orgusingformatYYYY‐NNNP{PP},where:YYYY=Yearoflaunch.NNN=ThreedigitserialnumberoflaunchinyearYYYY(withleadingzeros).P{PP}=Atleastonecapitalletterfortheidentificationofthepartbroughtintospacebythelaunch.

1998‐067A Yes

USER_DEFINED_NORAD_CAT_ID 5‐digitsatellitecatalognumberassignedbyUSSTRATCOM.

25544 No

CENTER_NAME Originofreferenceframe Earth Yes

REF_FRAME NameofreferenceframeinwhichthestatevectorandoptionalKeplerianelementdataaregiven

MEME2000InternationalTerrestrialReferenceFrame (ITRF)ICRFTODTDR

Yes,ifstatevectorincluded

REF_FRAME_EPOCH Epochofreferenceframe,ifnotintrinsictothedefinitionofthereferenceframe.

2001‐11‐06T11:17:33 No

TIME_SYSTEM Timesystemusedforstatevector,maneuver,andcovariancedata.MustbeUTC.

UTC Yes

TABLE 11: OPM DATA – STATE VECTOR COMPONENTS IN SPECIFIED COORDINATE SYSTEM

Keyword Description Units ObligatoryCOMMENT ReferenceOrbitalDataMessageBlueBook6.7 N/A No


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EPOCH EpochofstatevectorinUTC N/A NoX PositionvectorX‐component km NoY PositionvectorY‐component km NoZ PositionvectorZ‐component km NoX_DOT VelocityvectorX‐component km/s NoY_DOT VelocityvectorY‐component km/s NoZ_DOT VelocityvectorZ‐component km/s No

TABLE 12: OPM DATA – Osculating Keplerian Elements in Specified Reference Frame

Keyword Description Units ObligatoryCOMMENT ReferenceOrbitalDataMessageBlueBook6.7 n/a NoSEMI_MAJOR_AXIS Semi‐majoraxis km NoECCENTRICITY Eccentricity n/a NoINCLINATION Inclination deg NoRA_OF_ASC_NODE Rightascensionoftheascendingnode deg NoARG_OF_PERICENTER Argumentofpericenter deg NoMEAN_ANOMALY Meananomaly deg NoGM Gravitationalcoefficient(GravitationalConstantxCentral

Mass)km**3/s**2 No

TABLE 13: OPM DATA – SPACECRAFT PARAMETERS

Keyword Description Units ObligatoryCOMMENT ReferenceOrbitalDataMessageBlueBook6.7 n/a NoMASS Spacecraftmass kg NoSOLAR_RAD_AREA SolarRadiationPressureArea(AR) m**2 NoSOLAR_RAD_COEFF SolarRadiationPressureCoefficient(CR) n/a NoDRAG_AREA DragArea(AD) m**2 NoDRAG_COEFF DragCoefficient(CD) n/a No

TABLE 14: OPM DATA – POSITION/VELOCITY COVARIANCE MATRIX

Keyword Description Units ObligatoryCOMMENT ReferenceOrbitalDataMessageBlueBook6.7 n/a NoCOV_REF_FRAME Coordinatesystemforcovariancematrix n/a NoCX_X Covariancematrix[1,1] km**2 NoCY_X Covariancematrix[2,1] km**2 NoCY_Y Covariancematrix[2,2] km**2 NoCZ_X Covariancematrix[3,1] km**2 NoCZ_Y Covariancematrix[3,2] km**2 NoCZ_Z Covariancematrix[3,3] km**2 NoCX_DOT_X Covariancematrix[4,1] km**2/s NoCX_DOT_Y Covariancematrix[4,2] km**2/s NoCX_DOT_Z Covariancematrix[4,3] km**2/s NoCX_DOT_X_DOT Covariancematrix[4,4] km**2/s**2 NoCY_DOT_X Covariancematrix[5,1] km**2/s No


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CY_DOT_Y Covariancematrix[5,2] km**2/s No

TABLE 14: OPM DATA – POSITION/VELOCITY COVARIANCE MATRIX, CONTINUED

Keyword Description Units ObligatoryCY_DOT_Z Covariancematrix[5,3] km**2/s NoCY_DOT_X_DOT Covariancematrix[5,4] km**2/s**2 NoCY_DOT_Y_DOT Covariancematrix[5,5] km**2/s**2 NoCZ_DOT_X Covariancematrix[6,1] km**2/s NoCZ_DOT_Y Covariancematrix[6,2] km**2/s NoCZ_DOT_Z Covariancematrix[6,3] km**2/s NoCZ_DOT_X_DOT Covariancematrix[6,4] km**2/s**2 NoCZ_DOT_Y_DOT Covariancematrix[6,5] km**2/s**2 NoCZ_DOT_Z_DOT Covariancematrix[6,6] km**2/s**2 No

Table 15: OPM DATA – Maneuver Parameters

Keyword Description Units ObligatoryCOMMENT ReferenceODMBlueBook6.7 n/a NoMAN_EPOCH_IGNITION Epochofignition.ReferenceODMBlueBook6.5.9 UTC YesMAN_DURATION Maneuverduration(If=0,impulsivemaneuver) s YesMAN_DELTA_MASS Masschangeduringmaneuver(valueis<0) kg NoMAN_REF_FRAME Coordinatesystemforvelocityincrementvector(value

mustbeselectedfromOPMBlueBookannexA)n/a Yes

MAN_DV_1 1stcomponentofthevelocityincrement km/s YesMAN_DV_2 2ndcomponentofthevelocityincrement km/s YesMAN_DV_3 3rdcomponentofthevelocityincrement km/s YesUSER_DEFINED_MAN_ID AuniqueIDappliedtothespecificmaneuverbythe

operator.UniqueIDsmaybe1‐30alphanumericcharacters,dashes,orunderscores

n/a No

USER_DEFINED_DV_OVERALL

Ifthisfieldisnotpopulated,Space‐TrackwillcalculateitbasedoncomponentDVs

km/s No

USER_DEFINED_EPHEM Nameofthecorrespondingephemerisfile,iftheoperatorhassubmittedephemeristo18SPCSforscreening

n/a No

USER_DEFINED_MAN_PURPOSE

Specifiesthepurposeofthemaneuver.Acceptablevalues:AEROBRAKE(Aerobraking)ATT_ADJUST(Attitudeadjust)COLA(Collisionavoidance)DEORBIT(Deorbit)DISPOSAL(Disposal)FLYBY_TARG(Flybytargeting)LEOP(Launch&EarlyOrbit)MNVR_CLEANUP(Maneuvercleanup)MASS_ADJUST(Massadjust)TRIM(Orbittrim)

n/a Yes


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OTHER(Other)PER_RED(Periodreduction)RELOCATION(Relocation)SCI_OBJ(Scienceobjective)SPIN_RATE_ADJUST(Spinrateadjust)SK(Station‐keeping)TRAJ_CORR(Trajectorycorrection)

USER_DEFINED_TCA ForCOLAmaneuvers,providetheTimeofClosestApproach(TCA)inUTCofthecorrespondingconjunctionevent.RecommendusingtheTCAintheConjunctionDataMessage.

UTC YesforCOLAmaneuver,Noforallothers

USER_DEFINED_MAN_STATUS

Acceptablevalues:PREDICTED,DETERMINED,CANCELLED.PREDICTED=plannedmaneuverDETERMINED=confirmedmaneuverCANCELLED=cancelledmaneuver

n/a Yes

HOW TO NAME MANEUVER NOTIFICATIONS

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HowtoNameManeuverNotifications

OVERVIEW

TosimplifytheprocessforO/Os,Space‐TrackwillgeneratethefilenameforeachmaneuvernotificationusinginformationprovidedwithintheOPM,specificallytheRELEASABILITY,OBJECT_ID,ORIGINATOR,andCLASSIFICATIONfields.Inthefilename,theO/Oonlyneedstoincludetheirorganizationname(mustbethesameastheirCDMaccountonSpace‐Track)andauniqueidentifier,suchasasequencenumberortimestamp,followedbythe.opmextension:

<CDMOrganization>_<uniqueidentifier>_<metadata>.opm

EXAMPLES

AslongastheobligatoryfieldsarecorrectlyincludedwithintheOPM,theoperatormaynametheirfileanythingtheywantaslongasthefirsttwofieldsaretheCDMOrganizationandauniqueidentifier:

• NASA_1234_ISS.opm

• NASA_1235_25544.opm

• NASA_165120000.opm

• NASA_165120001_25544COLA.opm

HOW TO SUBMIT MANEUVER NOTIFICATIONS

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HowtoSubmitManeuverNotifications

OVERVIEW

Maneuvernotificationsmaybesubmittedinthreeways:

1. ThroughthewebsiteSpace‐Track.org(preferred)

1.1 Manually,or1.2 UsingApplicationProgrammingInterface(API)

2. Byemailtothe18SPCSOrbitalSafetyCrewat18SPCS.doo.orbitalsafety@us.af.mil

Space‐Track.orgisthemostefficientandsecuremethodofsubmittingmaneuvernotificationsonarecurringbasis.AlloperatorswhohaveaccesstotheOperatorPanelwillautomaticallybegivenpermissionstouploadmaneuvernotificationsforthesatellitesassignedtotheirCDMaccount.TosetupaCDMaccount,contactthe18SPCSSSASharingTeamat18spcs.doo.ssasharing@us.af.milor+1‐805‐606‐2675.

OnceanactiveCDMaccountisestablished,theoperator(s)assignedtotheaccountwillhaveaccesstotheOperatorandFilesPanels:

Figure12:FilesPanelonSpace-Track.org


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SUBMITTING MANEUVERS NOTIFICATIONS THROUGH SPACE-TRACK.ORG

OntheFilesPanel,youwillhaveaccesstothe‘Download’and‘Upload’tabs.Onthe‘Upload’tabyoucanuploadavarietyoffilestothefoldersyouhavepermissionsto.Allofthesefolderswilldisplayinthe‘Destination’dropdownmenu:

Figure13:FilesPanelUploadTab-Maneuvers

MANUAL UPLOAD OF MANEUVER NOTIFICATIONS

1. LogintoSpace‐Track.org,clickon‘Files’atthetopofthepage,andthenclickonthe‘Upload’tab2. Click‘ChooseFiles’andselectyourOPMmaneuvernotificationfiles

2.1 Maximumnumberoffilesinasingleuploadis1002.2 Maximumtotalsizeperuploadis60MB2.3 Filesmustbeformattedinaccordancewith‘ManeuverNotificationFormat’

3. Usingthe‘Destination’dropdownmenu,selectthe‘/Maneuvers/YourOrganization’folder.(Note:Tosetupamaneuverfolder,contactthe18SPCSSSASharingTeamat18SPCS.doo.ssasharing@us.af.milor+1‐805‐606‐2675.)


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4. Click‘Upload’touploadthefiles.5. Afterasuccessfulupload,thefileswilldisplayin‘MyUploads(Last30Days)’Table6. The18SPCSOrbitalSafetyCrewwillreceiveanotificationthatyouhaveuploadednewmaneuver

information.

API UPLOAD OF MANEUVER NOTIFICATIONS

PleasevisitSpace‐Track.org’s‘API’and‘HowTo’pagesforthemostup‐to‐dateguidance(includingusagelimitations)onAPIcapabilities:https://www.space‐track.org/documentation#apiandhttps://www.spacetrack.org/documentation#/howtoSimilartomanualuploads:

1. Filesmustbeformattedinaccordancewith‘ManeuverNotificationFormat’2. Afterasuccessfulupload,thefileswilldisplayin‘MyUploads(Last30Days)’Table3. The18SPCSOrbitalSafetyCrewwillreceiveanotificationthatyouhaveprovidednewmaneuver

information.However,ifthemaneuverrequiresattentionasahigh‐interestevent,pleasephoneoremailtheOrbitalSafetyCrew:+1‐805‐605‐[email protected].

EMAIL

Ifyourorganizationdoesn’thaveaccesstotheOperatorandFilesPanels,youmaysendyourmaneuvernotificationsbyemailtothe18SPCSOrbitalSafetyCrewat18SPCS.doo.orbitalsafety@us.af.mil.Please:

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CONTACT US

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ContactUs

18 SPCS ORBITAL SAFETY CREW

Forquestionsonconjunctionassessmentandcollisionavoidancesupport,pleasecontactthe18SPCSOrbitalSafetyCrewbyemailat18SPCS.doo.orbitalsafety@us.af.milorbyphoneat+1‐805‐605‐3533.Examplesofsupportinclude:

Questionsaboutscreeningresults Screeningsforhigh‐interestevents MissingCDMs

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ForquestionsaboutspecializedSSAsupportordataproducts,pleasecontactthe18SPCSSSASharingTeambyemailat18SPCS.doo.ssasharing@us.af.milorbyphoneat+1‐805‐606‐2675.Examplesinclude:

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APIqueries Automation Websiteperformance

ANNEX A: 18 SPCS PROBABILITY OF COLLISION

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AnnexA:18SPCSProbabilityofCollision

FigureX:GeometryofaConjunction

HOW 18 SPCS CALCULATES PROBABILITY OF COLLISION

For a conjunction1 between two objects in earth orbit, the 18th Space Control Squadron (18 SPCS) can

compute and report a value commonly referred to as probability of collision (Pc2). This value can be

used by a satellite owner/operator (O/O) as a warning and may motivate further study of the

encounter on their part3.

1 Also referred to as a “close approach” 2 Also PoC, POC 3 When an assumption is made in the calculation of Pc, it’s made on the side of overestimating the Pc, in other

words, being conservative. In this way O/O’s are made aware of all potential encounters. The first place where

this conservatism appears is in what is actually calculated. What is computed is not probability of collision but

rather the probability that two objects are less than a specified distance apart at their time of closest approach

(TCA). This is due to several factors, 18 SPCS not knowing the exact size, shape and orientation of both objects

at TCA being one of the major ones. So the approach is to determine the longest distance the two satellite’s

centers of mass can be apart and still have the two satellites touch. This defines the “specified distance apart”

that is used in the Pc calculation. Note that if the two satellites are not spheres, then a simple change in

orientation means that they may not touch and no collision would occur. So the Pc value calculated always has

this factor of conservatism built in.


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The data at the time of closest approach (TCA) required to compute Pc includes:

The size of the primary object

The size of the secondary object

The inertial position and velocity vectors of the primary object

The inertial position and velocity vectors of the secondary object

The 3-dimensional position covariance of the primary object

The 3-dimensional position covariance of the secondary object

If available and reliable these data items are provided in the Conjunction Data Message (CDM).

OBJECT SIZE

The Pc calculation requires an estimate of the sizes of the primary and secondary objects. Because of

a dimensionality reduction of the problem (to be described subsequently), the combined sizes of the

two objects ultimately will be represented as an area in a projected plane—the “conjunction plane.”

The typical procedure is to circumscribe the primary and secondary objects each by a sphere, add the

two sphere radii to generate a supervening sphere that can contain both circumscribing spheres, and

project this supervening sphere into the conjunction plane as a circle.

The size of the objects4 is called AREA_PC in the CDM and given in square meters. The CDM Blue

Book5 states:

AREA PC: The actual area of the object (m**2). The area could be known by the

owner/operator of the satellite or defined by using a Radar Cross Section (RCS) as in the case

of debris. If the value of the area is unknown or not available, “0.0” may be displayed. This

parameter can be useful for calculation collision probability.

SuperCOMBO6 is used to identify conjunctions in 18 SPCS. It has three modes: standoff radius,

ellipsoid, and covariance. Pc is computed in covariance mode. It requires that the sizes of both objects

be input by the operator. Size is specified (in meters) by the Radius of Exclusion Volume; as stated

4The primary and secondary objects are referred to as OBJECT1 and OBJECT2 in the CDM.5 Conjunction Data Message Recommended Standard CCSDS 508.0-B-1 BLUE BOOK, June 2013 6 Super Computation of Miss Between Orbits. SuperCOMBO is an application program in the Astrodynamics Support Workstation (ASW). It is based on the Air Force Space Command (AFSPC) Astrodynamics Standards program COMBO.


Page39

above, it represents the minimum radius of a sphere that can contain the object7 8. SuperCOMBO can

also assign specific values or use an RCS catalog to establish size.

Caution is advised if RCS is used to establish size. For many reasons RCS may not represent the

physical size of an object, and certainly not its shape or orientation at TCA.

POSITION AND VELOCITY VECTORS

The position and velocity vectors9 of the primary and secondary objects at TCA are given in the CDM

in the fields X, Y, Z, X_DOT, Y_DOT, and Z_DOT. Position units are km and velocity units are km/sec.

They are normally referenced to the International Terrestrial Reference Frame (ITRF), but other

coordinate frames may be specified in accordance with the CDM Blue Book. The velocity vectors are

not explicitly used in the computation of Pc. However, they are required in order to establish the point

and time of closest approach, to perform needed coordinate transformations/rotations to prepare the

data properly for the Pc calculation, and to determine encounter geometry at TCA.

In 18 SPCS the position and velocity of the objects at TCA are normally computed by interpolating

between ephemeris points10. The ephemeris points are obtained by propagating forward in time the

object’s special perturbations (SP) state vector at epoch11. The state vector at epoch is computed

using an orbit determination method based on minimum variance differential correction (DC).

In some cases ephemeris points of an object are provided to 18 SPCS by the satellite

owner/operator12. If so, these are used in lieu of, or in addition to, internal ephemeris.

While not explicitly input to compute Pc, the position and velocity of the secondary relative to the

primary at TCA are included in the CDM. They are given in Radial, Transverse, and Normal (RTN)

7 If an O/O reports object size to 18 SPCS in m**2 then that should be the area of a circle that completely “covers” the object when viewed from any angle. 18 SPCS would then compute the Radius of Exclusion Volume

from this area as (O/O-reported-size divided by )**0.5. 8 Pre-assigned default values for payloads and platforms (5 meters), rocket bodies and unknown objects (3 meters) and debris (1 meter) were determined through a study of sizes of objects in the space object catalog and are normally used. Plans are being developed to allow O/O’s to provide a more accurate value of this parameter for use in CA screening in the future.9 Together, often referred to as the state vector 10 An ephemeris is a time ordered listing of an object’s position and velocity vectors (that may include covariance). 11 Or general perturbations (GP) element set (ELSET) in the absence of an SP state vector12 In J2000 mean equator mean equinox coordinates


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coordinates of the primary object13. Also provided are overall MISS_DISTANCE and RELATIVE_SPEED.

Relative position and velocity offer a more intuitive view of the conjunction14 and tell the user if the

secondary is moving fast or slow relative to the primary at TCA.

COVARIANCE AND ERROR ELLIPSOIDS

The covariance matrix, provided separately for the primary and secondary object (as described in

Annex C of CCSDS 508.0-B-1), is routinely furnished in 6 x 6 form, even though only the position

portion is needed for the Pc calculation as described in this memorandum. A covariance matrix

characterizes the uncertainty in a satellite state vector, similar to the way that variance, the square of

the standard deviation, is a metric for assessing the spread of student test scores about the mean. An

example of a position covariance, using the notation of the CCSDS standard, is given as the symmetric

matrix below. The diagonal elements represent the variance in each of the components (R, T, and N),

and the off-diagonal terms give the covariance between the two named components (the product of

the two components’ standard deviations and their correlation coefficient):

NCNTCNRCN

TCNTCTRCT

RCNRCTRCR

Cov

___

___

___

The covariance matrix for each object is referenced to its own RTN coordinate frame. For each object

matrix components are computed using 5-point Lagrange interpolation of the covariance in the

ephemeris file produced by the 18 SPCS15.

An owner/operator may also send 18 SPCS an ephemeris that includes covariance16. In these cases the

covariance values in the CDM are interpolated from the ephemeris (with covariance) provided by the

owner/operator.

13 Also called the Radial-Intrack-Crosstrack (RIC) coordinate frame or the UVW system 14For example, at TCA the secondary relative to the primary is at “eleven o’clock high.”15 Covariance at TCA quantifies uncertainty in the state vector at TCA. Covariance at epoch quantifies uncertainty

at epoch. Covariance at epoch in essence maps the observed-minus-computed residuals in the observations

used in the DC to the uncertainty in the state vector produced by the DC.

16 Owner/operators provide ephemeris with covariance in three formats: (1) the Generic On-Orbit (GOO) (2) Modified ITC, or Orbital Ephemeris Message (OEM)


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The one-standard-deviation17 three dimensional (3D) error ellipsoid may be used to visualize the

“size” of the position uncertainty at TCA. Diagonalization of the 3X3 covariance matrix yields the size,

shape, and orientation of the error ellipsoid18. Often, because of drag, the largest uncertainty will be

in the direction of the satellite’s motion relative to the atmosphere19.

ASSUMPTIONS

When computing Pc at 18 SPCS several assumptions are made. These include:

Object sizes are known or can otherwise be assigned an upper bound.

The conjunction is “hyperkinetic, meaning that the conjunction duration is very short; this allows the additional simplifying assumption that the relative motion between objects is rectilinear throughout the encounter.

Gaussian theory and statistics apply.

Covariance for both objects is known and constant throughout the encounter.

Primary and secondary errors are independent allowing “combined” covariance to be the simple sum of the individual covariances (in a common frame), resulting in the “joint” covariance.

The covariance is not “too large.”

The covariance is not “too small20.”

Technical papers have been written to address the computation and/or validity of Pc when one or

more of these assumptions come into question. If at TCA (1) covariance is too small, (2) covariance is

too large, or (3) the relative speed of the secondary relative to the primary is too small, then Pc is not

computed in SuperCOMBO. The values for covariance “too large” and relative speed “too small” are

user-settable.

Another concern is covariance realism: does the covariance used in the computation of Pc truly reflect

the uncertainty in the state vectors at TCA? Because quantitative studies have shown that covariance

is often underestimated, empirical techniques have been devised to scale or otherwise inflate the

covariance – make it larger – to compensate for the underestimation.

171 sigma or 118 The square root of each of the eigenvalues of the covariance matrix gives the sizes of the ellipsoid axes. The eigenvectors provide the orientation of the ellipsoid axes relative to the object’s RTN coordinate frame. 19A very rough estimate of the sizes of the three axes of the error ellipsoid can be obtained by taking the square root of each of the diagonal elements CR_R, CT_T, and CN_N.20Thistestisneededtotrapzerocovariance.


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COMPUTATION

Computation of Pc takes place in the collision plane21. This is the plane perpendicular to the relative

velocity vector at TCA. This reduces the mathematics from 3D to 2D, and the kinematics from dynamic

to static. In 2D the equation used to compute Pc is:

222

)()(2

1exp

|)(|2

1 12/1

dyx

S/PT

S/P dxdyCCDet

PoC rrrr

In this double integral equation:

= 3.141592653589793. C is the 2X2 projection of the combined 3X3 covariance at TCA onto the collision plane22.

Det(C) is the determinant of C.

d is the sum of the two object sizes23.

“exp” is the exponential function, i.e., e to the power in parentheses with e = 2.718281828459045.

r = (x, y)T is any point in the collision plane such that x2 + y2 d2.

rS/P = (rS/P, 0)T is the position of the secondary relative to the primary along the x-axis in the collision plane.

C-1 is the inverse of C.

This equation integrates the 2D Gaussian probability density function (centered on the secondary

object) over the circle of radius d (centered on the primary object24).

21 Also called the encounter plane22 The combined covariance is the sum of the covariance matrices corresponding to each object (both referenced to the same coordinate frame). The applicability of Gaussian statistics allows for combining two covariance matrices into one using term-by-term summation. 23 The sum of the radii of exclusion volumes for the two objects 24Withtheprimaryobjectlocatedattheoriginofa2Dcoordinateframeinthecollisionplane


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Methodologies, coordinate transformations, assumptions, combining Gaussian distributions,

limitations, and techniques used to facilitate computation of Pc are discussed at length in technical

papers listed in the bibliography25.

NOTES

18 SPCS uses error functions (ERF) for computing the double integration in the Pc equation. In

addition 18 SPCS performs integration over a square circumscribing the circle of radius d. This square

is aligned with the axes of the combined 2D probability density function in the collision plane. This

simplifies the computation of Pc but gives a very slightly larger – but safe – value.

For these and other reasons agencies may compute slightly different values for Pc for the same

conjunction. If the difference is large however further investigation is warranted26.

BIBLIOGRAPHY

Several excellent references for understanding the computation of Pc are listed at the Space Assigned

Number Authority (SANA) registry at http://sanaregistry.org/r/cdm_cpm/cdm_cpm.html.

In addition the following references are valuable because in the 1990s Doctor Foster matured much of

the theory and methodology used by NASA to compute Pc for Space Shuttle and International Space

Station (ISS) safety of flight analysis. A version of the NASA algorithm for computing Pc was integrated

into the ASW program SuperCOMBO in 2000. Certain theoretical issues were modified and expanded

25 The computation of Pc is a dynamic 3D problem: the two objects are moving in 3-dimensions. However, it can

be simplified by reducing the dynamic 3D problem to a static 2D problem. This is accomplished by performing

calculations in the 2D collision plane. Consider the two error ellipsoids of the primary and the secondary objects

at the instant of TCA as they “punch holes” in the collision plane that’s perpendicular to the relative velocity

vector. The problem of computing Pc immediately collapses to a static 2D problem in the collision plane. The

ellipsoids become ellipses in the collision plane representing the 1 “contour lines” of the two-dimensional

Gaussian probability density functions (pdf) of the objects. One can think of the 2D Gaussian (“bell-shaped”)

probability density functions for the primary and the secondary as being “little hills” sitting over their respective

satellite positions at TCA in the collision plane. In effect, the ellipse for one object is but one of many nested

“Russian Doll” contour lines going out to infinity in the 2D collision plane for the Gaussian pdf, just as in 3D the

error ellipsoid is but one of an infinite number of nested Russian dolls. These two “little hills” combine into one

hill – one pdf – centered over the secondary for the actual computation of the Pc.

26In 2000 AFSPC/A9 numerically validated the computation of Pc in SuperCOMBO.


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by F.C. Chan in his important monograph Spacecraft Collision Probability (El Segundo, CA: Aerospace

Press, 2008).

Foster, J.L., and Frisbee, J.H., Comparison of the Exclusion Volume and Probability Threshold Methods

for Debris Avoidance for the STS Orbiter and International Space Station. NASA /TP-2007-214751.

Houston, Texas: NASA Johnson Space Flight Center, May 2007 (written in October 1999).

Foster, J.L., and Wortham, M.B, ISS Debris Avoidance Maneuver Threshold Analysis. NASA /TP-2007-

214752. Houston, Texas: NASA Johnson Space Flight Center, May 2007 (written in October 1999).

Foster, J.L., The Analytic Basis for Debris Avoidance Operations for the International Space Station.

Proceedings of the Third European Conference on Space Debris, p. 441-445, Darmstadt, Germany, 19 -

21 March 2001.

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