International Laser Ranging Service (ILRS)

ReportoftheInternationalAssociationofGeodesy2015-2017─Travauxdel’AssociationInternationaledeGéodésie2015-2017

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InternationalLaserRangingService(ILRS)https://ilrs.gsfc.nasa.gov

E.C.Pavlis1,M.R.Pearlman2,C.E.Noll3,L.Combrinck4,G.Bianco61JointCenterforEarthSystemsTechnology,UMBCandNASAGSFC,Baltimore,MD21250,

USA2Harvard-SmithsonianCenterforAstrophysics(CfA),Cambridge,MAUSA02138,USA

3NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA4HartebeesthoekRadioAstronomyObservatory,Krugersdorp,SOUTHAFRICA

6AgenziaSpazialeItaliana,CGS,Matera,ITALY

OverviewThe ILRS is the international source that provides Satellite Laser Ranging (SLR) andLunar Laser Ranging (LLR) observation data and data products for scientific andengineeringprogramswith themain focusonEarthandLunarapplications.Thebasicobservables are the precise two-way time-of-flight of ultra-short laser pulses fromground stations to retroreflector arrays on satellites and the Moon and the one-waytime-of-flightmeasurements to space-borne receivers (transponders). These data setsaremadeavailabletothecommunitythroughtheCDDISandtheEDCarchives,andarealso used by the ILRS to generate fundamental data products, including: accuratesatelliteephemerides,Earthorientationparameters,three-dimensionalcoordinatesandvelocities of the ILRS tracking stations, time-varying geocenter coordinates, static andtime-varying coefficients of the Earth’s gravity field, fundamental physical constants,lunarephemeridesandlibrations,andlunarorientationparameters.SLR is one of the four space geodetic techniques (alongwithVLBI, GNSS, andDORIS)whoseobservations are thebasis for thedevelopment of the InternationalTerrestrialReferenceFrame(ITRF),whichismaintainedbytheIERS.SLRdefinestheoriginofthereference frame, the Earth center-of-mass and, along with VLBI, its scale. The ILRSgeneratesdailyastandardproductofstationpositionsandEarthorientationbasedonthe analysis of the data collected over the previous sevendays, for submission to theIERS, and produces LAGEOS/Etalon combination solutions for maintenance andimprovementof the InternationalTerrestrialReferenceFrame.The latestrequirementis to improve the reference frame to anaccuracyof 1mmaccuracyand0.1mm/yearstability, a factor of 10–20 improvement over the current product. To address thisrequirement, the SLR community will need to significantly improve the quantity andquality of ranging to the geodetic constellation (LAGEOS-1, LAGEOS-2, and LARES) tosupportthedefinitionofthereferenceframe,andtotheGNSSconstellationstosupporttheglobaldistributionofthereferenceframe.TheILRSparticipatesintheGlobalGeodeticObservingSystem(GGOS)organizedundertheIAGtointegrateandhelpcoordinatetheServiceactivitiesandplans.

ILRSStructureTheILRSOrganization(seeFigure1)includesthefollowingpermanentcomponents:

• Networkoftrackingstations• OperationsCenters• GlobalDataCenters• AnalysisandAssociateAnalysisCenters


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• CentralBureau• GoverningBoard• StandingCommittees(SCs)

o Analysiso DataFormatsandProcedureso Missionso NetworksandEngineeringo Transponders

• StudyGroups(SGs)andBoardso LaserRangingtoGNSSs/cExperiment(LARGE)o QualityControlBoardo SoftwareStudyGroupo SpaceDebrisStudyGroup

Figure1.TheorganizationoftheInternationalLaserRangingService(ILRS).

The role of these components and their inter-relationship is presented on the ILRSwebsite(https://ilrs.gsfc.nasa.gov/about/organization/index.html).The Governing Board (GB) is responsible for the general direction of the service. Itdefines official ILRS policy and products, determines satellite-tracking priorities,developsstandardsandprocedures,andinteractswithotherservicesandorganizations.ThemembersofthecurrentGoverningBoard,selectedandelectedforatwo-yearterm,arelistedinTable1.TheCentralBureau (CB) is responsible for thedaily coordinationandmanagementofthe ILRS in a manner consistent with the directives and policies established by theGoverningBoard.TheprimaryfunctionsoftheCBaretofacilitatecommunicationsand


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information transferwithin the ILRSandbetween the ILRSand the external scientificcommunity,coordinateILRSactivities,maintainalistofsatellitesapprovedfortrackingsupport and their priorities, promote compliance to ILRSnetwork standards,monitornetwork operations and quality assurance of data, maintain ILRS documentation anddatabases,producereportsasrequired,andorganizemeetingsandworkshops.TheCBoperates the communication center for the ILRS. The CB performs a long-termcoordinationandcommunicationroletoensurethatILRSparticipantscontributetotheServiceinaconsistentandcontinuousmannerandthattheyadheretoILRSstandards.PermanentStandingCommittees(SCs)andtemporaryStudyGroups(SGs)providetheexpertisenecessarytomaketechnicaldecisions,toplanprogrammaticcoursesofaction,andareresponsibleforreviewingandapprovingthecontentoftechnicalandscientificdatabasesmaintainedbytheCentralBureau.AllGBmembersserveonatleastoneofthefiveSCs,ledbyaChairandCo-Chair(seeTable1).TheSCscontinuetoattracttalentedpeople from the general ILRSmembership who contributed greatly to the success oftheseefforts.

Table1.ILRSGoverningBoard(asofMay2017)

Name Position CountryJamesBennett Appointed,WPLTN AustraliaGiuseppeBianco Appointed,EUROLAS,GoverningBoardChair ItalyLudwigCombrinck Elected,LunarRepresentative SouthAfricaUrsHugentobler Ex-Officio,RepresentativeofIAGCommission1 Germany

GeorgKirchner Appointed,EUROLAS,NetworksandEngineeringStandingCommitteeCo-Chair Austria

VincenzaLuceri Elected,AnalysisRepresentative,AnalysisStandingCommitteeDeputyChair Italy

DavidMcCormick Appointed,NASA USAJanMcGarry Appointed,NASA,TransponderStandingCommitteeCo-Chair USA

HorstMueller Elected,DataCentersRepresentative,DataFormatsandProceduresStandingCommitteeChair Germany

CareyNoll Ex-Officio,Secretary,ILRSCentralBureau USAToshimichiOtsubo Appointed,WPLTN,MissionsStandingCommitteeChair Japan

ErricosPavlis Elected,AnalysisRepresentative,AnalysisStandingCommitteeChair USA

MichaelPearlman Ex-Officio,Director,ILRSCentralBureau USAUlrichSchreiber Elected,At-Large,TransponderStandingCommitteeChair GermanyDanielaThaller Appointed,IERSRepresentativetoILRS Germany

MattWilkinson Elected,At-Large,NetworksandEngineeringStandingCommitteeChair UK

TBN Appointed,At-Large TBN Appointed,At-Large

FormerGoverningBoardMembersduring2015-2017WuBin Appointed,WPLTN China

JürgenMüller Elected,LunarRepresentative Germany

DataProductsThe main ILRS analysis products consist of SINEX files of weekly-averaged stationcoordinatesanddailyEarthOrientationParameters(x-pole,y-poleandexcesslength-of-day—LOD)estimatedfrom7-dayarcsofSLRtrackingofthetwoLAGEOSandtwoEtalonsatellites.AsofMay1,2012,theofficialILRSAnalysisproduct isdeliveredonaDAILY


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basisbyslidingthe7-dayperiodcoveredbythearcbyonedayforwardeveryday.Thisallows the ILRS to respond to twomain users of its products: the ITRS CombinationCenters and the IERS EOP Prediction Service at USNO. The former requires a singleanalysisperweek;thelatterhoweverrequiresas“fresh”EOPestimatesaspossible,thatthe “sliding”dailyanalysis readilyprovides.Two typesofproductsaredistributed foreach7-dayperiod:alooselyconstrainedestimationofcoordinatesandEOPandanEOPsolution,derivedfromthepreviousoneandconstrainedtoanITRF,whichbeginningonJune1,2017,isITRF2014.OfficialILRSAnalysisCenters(ACs)andCombinationCenters(CCs) generate these products with individual and combined solutions respectively.BoththeindividualandcombinedsolutionsfollowstrictstandardsagreeduponwithintheILRSAnalysisStandingCommittee(ASC)toprovidehighqualityproductsconsistentwith the IERSConventions.Thisdescription refers to the status asofMay2017.EachofficialILRSsolutionisobtainedthroughthecombinationofsolutionssubmittedbytheofficialILRSAnalysisCenters:

• ASI,AgenziaSpazialeItaliana• BKG,BundesamtfürKartographieundGeodäsie• DGFI,DeutschesGeodätischesForschungsInstitut• ESA,EuropeanSpaceAgency• GFZ,GeoForschungsZentrumPotsdam• GRGS,ObservatoiredeCoted’Azur• JCET, Joint Center for Earth SystemsTechnology andGoddard Space Flight

Center• NSGF,NERCSpaceGeodesyFacility

Since2016, the ILRShasreleasedanadditionaloperationalproductonaweeklybasisthrough a pilot project. These official products are precision orbits in standard SP3cformattedfilesforthefoursatellitetargets(LAGEOS-1,-2,andEtalon-1,-2).FollowingtheadoptionofITRF2014,theASCplanstoissueanextendedversionofthereferenceframe,theSLRF2017,whichwillincludesometwo-dozenadditionalSLRsitesthatwerenotpartof ITRF2014.TheASCwill re-analyzeallof thedata receivedsince1983,usingthenewITRFandnewmodels foran improvedstandardproduct thatareconsistent with the currently released operational products. The ILRS products areavailable,viaftpfromtheofficialILRSDataCentersCDDIS/NASAGoddardSpaceFlightCenter and EDC/TUM/DGFI: ftp://cddis.nasa.gov/slr/products/pos+eop andftp://edc.dgfi.tum.de/pub/slr/products/pos+eop).The individual ILRSAC andCCproduct contributions aswell as the combinations aremonitoredonadailybasisingraphicalandstatisticalpresentationofthesetimeseriesthroughadedicatedportalhostedbytheJCETACat:http://geodesy.jcet.umbc.edu/ILRS_AWG_MONITORING/The main focus of the Analysis SC activities over the past two years was theimprovementofmodelingused in thereductionof theSLRdataandgenerationof theofficialproductsforthedevelopmentofITRF2014,(Lucerietal.,2014).Inparticular,allACsmademajor efforts to complywith the adopted analysis standards and the IERSConventions2010,theconsistentmodelingof lowdegreetime-varyinggravitationandtherealisticmodelingofthemeanpoleincomputingthepoletideeffects(Pavlisetal.,2014).SincethedeliveryofthepreliminaryandfinalversionsofITRF2014,theASChasfocusedonevaluatingthemandprovidingfeedbacktoITRSforadjustmentsthatledtothe finally adopted version. The efforts to identify, quantify and contain systematic


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errors in the SLR data have continued with many new initiatives that ILRS feelsnecessaryinordertoimprovedataquality.Itisrecognizedthatpracticesthatwilllimitormitigatetheeffectofsystematicerrorsinthe ILRS data, improve the final products through realistic description of geophysicalprocesses,andstrengthenthequalityoftheproductsincludebutarenotlimitedtousingLARESasanadditionalaccuratetargetindevelopingtheofficialproducts(Pavlisetal.,2015).Inadditiontothatthough,anewstudygroup,theILRS“QualityControlBoard”,withmembers from all areas of expertisewithin the service, has been established togenerate tools and procedures that will help the station engineers identify withconfidence and as quickly as possible, issues with their data, before they get too fardowntheproductionline.MoredetailsontheinitialresultsfromthisnewinitiativearegivenunderthesectionfortheILRSASC.Currently,theLLRgroupisintheprocessofdevelopingauniquedatasetofallavailableLLRdatainthenewlyadoptedCRDformat,inordertobetterservethecommunityandtoconformwiththeILRSstandards.

SatelliteLaserRanging

ILRSNetworkThepresent ILRSnetwork includes over forty stations in24 countries (see Figure2);someofthesestationsareundergoingrefurbishmentandupgrade.Duringthelastfiveyears, new stations joined the ILRS network in Badary, Baikonur, Irkutsk, Svetloe,Zelenchukskaya,(Russia),Sejong(Korea),andBrasilia(Brazil)filling-inveryimportantgeographicgaps.TheRussiangroupshaveadvancedtheideaofplacingtwoSLRstationsat critical locations to help address the tracking load. They have co-located an SLRstationwith theNASAMOBLAS atHartebeesthoek (SouthAfrica) and have offered toplaceasysteminTahitico-locatedwiththeNASAsystemsthere.TheRussiansarealsoplanning the installation of a new system in Ensenada (Mexico). The TIGO system,operational in Concepción (Chile) since2002,was closed in 2014 and relocated to LaPlata (Argentina); operations are expected to resume in late 2017. New stations,underwayatPonmundiandMt.Abu(India)andinMetsahövi(Finland)areexpectedtobe operational in late 2017 or early 2018. A new SLR station is planned for Yebes(Spain).TheNASASpaceGeodesyProject(SGP)isplanningforconstructionofuptotennextgenerationSLRsystemsaspartofcoresites;thefirstofthosesystemsareplannedfordeploymentatMcDonald,TX,Haleakala,HIandGSFCinthe2019–2021timeframe.Afourth is being built in cooperation with the NMA for Ny Ålesund (Norway). Severalsystems are planned to replace current legacy systems. Large gaps are still veryprominent in Africa and South America and discussions are underway with severalgroupsinthehopeofaddressingthisshortcoming.Stations designated as operational havemet theminimum ILRS qualification for dataquantity and quality. In 2015, the ILRS Governing Board approved a new ILRS PassPerformance Standard of 3500 passes per year as an interim step toward a morecomprehensivelong-termstrategy:

• 2passesperweekoneachLEOsatellite(2300LEOpassesperyear)• 4passesperweekoneachLAGEOSsatellite(600LAGEOSpassesperyear)• 2passesperweekoneachHEOsatellite(>3000HEOpassesperyear)


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Ingeneral,stationscontinuetoimprovetheirperformance.Severalstationsdominatedthe network in the last three years,with the Yarragadee, Changchun andMt. Stromlostations being the strongest performers. The next group of stations with impressivecontributions included Herstmonceux, Graz, Wettzell, Greenbelt, Zimmerwald,MonumentPeak,Matera,andPotsdam.Duringthetwelve-monthperiodfromApril2016toMarch2017,thirteenstationsmettheupdatedILRSminimumrequirementfortotalnumbersofpassestracked(seeFigure3).

Figure2.ILRSnetwork(asofMay2017).

As shown in Table 2, several stations are now operating with kHz lasers and fastdetectors,therebyincreasingdatayieldandallowingthemtobemoreproductivewithpass interleaving, a critical step as the number of satellites being trackedwith SLR isincreasingdramatically.Somestationshavedemonstratedmmprecisionnormalpoints,afundamentalsteptowardaddressingthenewreferenceframerequirements.

SatelliteMissionsTheILRSiscurrentlytrackingoverninetyartificialsatellitesincludingpassivegeodetic(geodynamics) satellites, Earth remote sensing satellites, navigation satellites, andengineering missions (see Figure 4). The large list of satellites is saturating somestationsthatarenotfullymannedandstrategiesarebeingexaminedtotrytomaximizestation data value. The stations with lunar capability are also tracking the lunarreflectors.Inresponsetothislargerosterofsatellites,aswellasforsupportoftandemmissions(e.g.,GRACE-A/-B,TanDEM-X/TerraSAR-X)andgeneraloverlappingschedules,moststationsintheILRSnetworkaretrackingsatelliteswithinterleavingprocedures.The ILRS assigns satellite priorities in an attempt to maximize data yield on the fullsatellite complex while at the same time placing greatest emphasis on the mostimmediate data needs. Priorities provide guidelines for the network stations, butstationsmay occasionally deviate from the priorities to support regional activities ornational initiatives and to expand tracking coverage in regionswithmultiple stations.GeneraltrackingprioritiesaresetbytheGoverningBoard,basedonapplicationtotheCentral Bureau and recommendation of the Missions Standing Committee (seehttps://ilrs.gsfc.nasa.gov/missions/mission_operations/priorities/index.html).


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Table2.High-RepetitionRateILRSStations(asofMay2017)

SiteName StationRepetitionRate(Hz)

Altay 1879 300Arkhyz 1886 300Badary 1890 300Baikonur 1887 300Beijing 7249 1000Brasilia 7407 300

Changchun 7237 1000Graz 7839 2000

Herstmonceux 7840 2000Irkutsk 1891 300

Komsomolsk 1868 300Kunming 7820 1000LaPlata 7405 100

Mendeleevo 1874 300MountStromlo 7849 100

Potsdam 7841 2000Sejong 7394 5000Shanghai 7821 1000Svetloe 1888 300

Wettzell(SOS) 7827 1000Zelenchukskaya 1889 300Zimmerwald 7810 110

Figure3.ILRSnetworkperformance(totalpasses),April2016throughMarch2017.


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Figure4.Thepast,current,andfutureILRSsatellitetrackinglist(asofMay2017).

MissionsareaddedtotheILRStrackingrosterasnewsatellitesarelaunchedandasnewrequirementsareadopted;missions for completedprogramsare removed (seeFigure4).SignificanteffortwasspentbytheILRSCBonrestrictedtrackingproceduresfortheSentinel-3AandLomonosovmissionstoensurethatonlyauthorizedstationsrangedtothe satellites and did so only during authorized time periods to avoid any damage tovulnerableonboardinstrumentation.TheILRScontinuestotrackseveralsatellites(e.g.,Envisat, TOPEX/Poseidon, ETS-8) considered “space debris” to provide ephemeridesandorientationdatatohelpwithtrajectory/safetyplanning.The tracking approval process begins with the submission of a Missions SupportRequest Form, which is accessible through the ILRS website(https://ilrs.cddis.eosdis.nasa.gov/docs/2016/ilrsmsr_1604.pdf).TheformprovidestheILRSwiththefollowinginformation:adescriptionofthemissionobjectives, mission requirements including any tracking restrictions, responsibleindividuals and contact information, timeline, satellite subsystems, and details of theretroreflector array and its placement on the satellite; amission concurrence sectiongrantstheILRSstationspermissiontoperformlaserrangingtothesatellite.Thisformalsooutlinestheearlystagesofintensivesupportthatmayberequiredduringtheinitialorbitalacquisitionandstabilizationandspacecraftcheckoutphases.Alistofupcomingspace missions that have requested ILRS tracking support is summarized in Table 2alongwiththeirsponsors,intendedapplication,andprojectedlaunchdates.


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Table2.RecentlyLaunchedandUpcomingMissions(asofMay2017)

SatelliteName Sponsor Purpose LaunchDateRecentlyLaunched

Compass(5new,9totalsatellites) ChineseDefenseMinistry Positioning,

navigation,timing 2007-present

Galileo(13new,18totalsatellites) ESA Positioning,

navigation,timing 2011-present

GLONASS(2new,24totalsatellites)

RussianFederationMinistryofDefense

Positioning,navigation,timing 1989-present

IRNSS(3new,6totalsatellites) ISRO Positioning,

navigation,timing 2013-2016

Jason-3 CNES,NASA,Eumetsat,NOAA Oceanography Jan-2016

Lomonosov*

ScobeltsynInstituteofNuclearPhysics,

LomonosovMoscowStateUniversity

Upperatmosphericresearch Apr-2016

PN-1A BACC Preciseorbitdetermination Sep-2015

Sentinel-3A* ESA,Eumetsat Marineobservation Feb-2016ApprovedbyILRSforFutureSLRTracking

APOD/PN-1B,-1C,-1D BeijingAerospaceControlCenter Engineering 2015

COSMIC-2 UCARAtmospheric

research,validationofGNSSorbits

2017

LightSail-B PlanetarySociety Engineering 2017NISAR NASA Earthsensing 2020

QZS-2,-3,-4 CabinetOffice,GovernmentofJapan

Positioning,navigation 2017

Sentinel-3B ESA.Eumetsat Oceanography 2017FutureSatelliteswithRetroreflectors

GPS-III U.S.DoD,DoT Positioning,navigation,timing 2019

HY-2B,-2C,-2D CNES,CNSA Earthobservation 2017-2019

ICESat-2 NASA Icesheetmassbalance,sealevel 2018

Sentinel-6 ESA.Eumetsat,NASA,NOAA OceanAltimetry 2020

SWOT NASA,CNES SARaltimeter 2020

Note:*denotesrestrictedtrackingmission;onlyauthorizedstationsperformlaserrangingtothesatellite

Duringthisreportingperiod,overtwentyGNSSsatellitesfromfourconstellationswereaddedtotheILRSprioritylist.Inaddition,andasshowninTable2,fourothersatellites,including two restricted trackingmissions,were launched and supported by the ILRSnetwork. The ILRS tracking roster presently includes six GLONASS satellites, nineCompass satellites, eighteen Galileo satellites, five IRNSS satellites, and one QZSSsatellite.Followingdiscussionsat the2012 ILRSTechnicalWorkshop, “Satellite,Lunarand Planetary Laser Ranging: Characterizing the Space Segment,” in Frascati, Italy inNovember 2012, and agreements that were approved by the ILRS and GGOS afterdeliberationswithinthe“LAserRangingtoGNSSs/cExperiment(LARGE)”StudyGroupmeetinginApril2014,severalstationsroutinelytracksegmentsofpassesofall24activeGLONASS satellites and beyond. The newer “high” satellites are using retroreflectorarrays that satisfy the ILRS standard. As a result, stations are having greater rangingsuccess.


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Recently, the ILRS has become more involved in supporting Precision Time Transfer by laser ranging. The ILRS network is currently tracking the Jason-2 satellite (launched in 2008) which includes the Time Transfer by Laser Link (T2L2) instrument. The T2L2 instrument records the time of arrival of laser pulses. It provides a means to synchronize the clocks of the ILRS stations, as well as to characterize the performance of the DORIS Ultra Stable Oscillator (USO) onboard the Jason-2 spacecraft. The data from T2L2, as well as other information, have been used to derive a detailed model of the DORIS USO behavior, including direct modeling of radiation effects, passage through the South Atlantic Anomaly (SAA) and natural aging of the oscillator. Applying this USO model it is possible to synchronize the clocks used in the Laser Ranging station to the same international time scale (UTC) at around 5 ns accuracy. The analysis of the T2L2 data has revealed that many stations exhibit a bias w.r.t. to UTC, sometimes as high as a few microseconds (for example the prolific Yarragadee station had a long-term evolving bias of 1 microsec throughout 2013). While the ILRS requests that stations maintain their timing within 200 ns, the data from T2L2 reveal a source of error that has not previously been considered. Time biases at the level of less than 1 microsec are hard to resolve from the orbit determination analysis, so the data from T2L2 will allow us to characterize station timing behavior and examine its impact on the reference frame and ILRS products. The T2L2 project team led by Dr. Pierre Exertier (Grasse SLR observatory) have provided “corrected” SLR data to the ILRS analysis centers, based on analysis of data from T2L2.

A precise clock in space provides a worldwide access to high performance groundclocks.HereSLRplaysanimportantrole.Itprovidesaccuraterangeandtimebetweenclearly defined reference points on ground and in space. This represents a two-waymeasurementtechnique,themainingredientofthe“EinsteinSynchronization”process,the only technique that can compare remote clockswith high accuracy.The European Space Agency (ESA) is developing the Atomic Clock Ensemble (ACES) experiment for flight on the International Space Station (ISS). The ELT (European Laser Timing) follows in the path of T2L2. The goal is to demonstrate an accuracy of time transfer at the level of 50 ps, with a perspective of 25 ps. The ELT payload consists of a corner cube retroreflector a SPAD detector, and an event timer. ELT will provide an alternative to time transfer via microwave link (MWL) and will provide superior accuracy.

LunarLaserRanging(LLR)NetworkTheLLRresultsareconsideredamongthemostimportantsciencereturnoftheApolloera. Of all the active ILRS observatories there are currently only four which aretechnicallyequippedtotrackretro-reflectorarrays locatedonthesurfaceof theMoonoronspacecraftorbitingaroundtheMoon.In2016,onlytwoLunarLaserRanging(LLR)sitescollectedrangingdatatotheMoon:theObservatoiredelaCôted’Azur,France(63NP’sat532nmand756at1064nm(infrared))andtheAPOLLOsiteinNewMexico,USA(273NPs).Sincethelatterpartof2014,theObservatoiredelaCôted’AzurLLRstationhasbeenabletorangewithinfraredwavelength(1064nm),thesedataareavailableattheir website (http://www.geoazur.fr) and are included in the statistics shown inFigures5to7.Mostof theObservatoirede laCôted’AzurLLRdataarenowobtainedusinginfraredinsteadofgreen(532nm)laserpulses.Unfortunately, no NPs have been obtained from theMcDonald Observatory in Texas,USA. This means, a time series of LLR tracking at McDonald, which has run for fourdecades, has been interrupted. This discontinuation will adversely affect researchutilizing LLR data as theMcDonald station has been amajor contributor to LLR dataoveralongperiodoftime.


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TheLLRmeasurementstatisticsfor2016(Figure5)showsthatabout75%ofthedatahavebeencollectedat theFrenchMeOsitenearGrasseandabout25%of thedataatAPOLLO.Figure6illustratesthestatisticsfortheobservedretro-reflectorarrays,wheremuchbettercoverageofallreflectorswasachievedthaninpreviousyears.Nevertheless,most of the datawas obtained on the big Apollo 15 reflector array (48%), while theotherfourreflectorsprovideddataatthe12-14%level.Figure7presentstheentireLLRdatasetfrom1970to2016displayingtheamountofdatacollectedbyeachoftheactiveLLRsitesineachyear.Thetotaldatayieldoverthisperiodisabout22,000NPs,recentlyaveraging about 600NPs per year. At theObservatoire de Paris, an “assisting tool” isavailabletosupportlunartrackingbyprovidingpredictionsoffutureLLRobservationsaswellasavalidationofpastLLRnormalpoints.Thistoolandfurtherinformationcanbe accessed via the ILRS website athttps://ilrs.gsfc.nasa.gov/science/scienceContributions/lunar.html.LLR data analysis is carried out by a fewmajor LLR analysis centers: Jet PropulsionLaboratory(JPL),Pasadena,USA;Center forAstrophysics(CfA),Cambridge,USA;ParisObservatory Lunar Analysis Center (POLAC), Paris, France; Institute of Geodesy (IfE),UniversityofHannover,Germany.Inthelastfewyears,theNationalInstituteforNuclearPhysics (INFN), Frascati, Italy, and the Graduate University for Advanced Studies(SOKENDAI), Tokyo, Japan, have also increased their analysis activities. The six LLRanalysis centers focus on different research topics (such as relativity, lunar interior,etc.).SomeinteresttowardsthisendhasalsobeenshownbytheHartebeesthoekRadioAstronomyObservatory(SouthAfrica)whereanex-ObservatoiredelaCôted’Azur1-maperturetelescopeisbeingpreparedforLLRuse.Inaddition,variousresearchprojectshavebeensuccessfullyruncombiningLLR,GRAIL,andLROdata.OnegeneralobjectiveofLLRanalysisistoimproveaccuracyfromthecurrentcmtothemmlevel.Thevariousanalysiscenterscontinuetheircomparisoninitiativetomutuallyimprove the various reduction codes. Recent activities also include comprehensivesimulations to show the potential benefit of improved tracking with additionalobservatoriesand/ortonewreflectors.Aboveall,LLRremainsoneofthebesttoolstosupportlunarscience,tostudytheEarth-Moondynamicsand to testGeneralRelativity in thesolarsystem(Mülleretal.2014).LLRanalysissteadilyreducesthemarginsforapossibleviolationofEinstein’stheoryofrelativityandimpressivelyunderpinsitsvalidity–nowinthe100thyearofitsexistence.

Figure5.Observatorystatisticsin2016. Figure6.Reflectorstatisticsin2016.


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Figure7.DatayieldoftheglobalLLRnetworkofstations(throughtheendof2016).

RecentActivities

GeneralThe ILRSGoverningBoardapprovedanupdate to the ILRSTermsofReference (ToR)(https://ilrs.gsfc.nasa.gov/about/termsofref.html) in mid-2016; the IAG accepted therevisionandthenewToRwasadoptedinNovember2016.ThemostsignificantchangetotheILRSToRwastheadditionoftwoAt-LargememberstotheILRSGBwhowillbeappointedby theGB.Other changes addressed the addition of newSCs and clarifyingterminology.

StandingCommitteeandStudyGroupProgressAll ILRS standing committees held meetings during ILRS workshops held during thereporting period (2015 ILRS Technical Workshop in Matera Italy and the 20thInternationalWorkshoponLaserRanging inPotsdamGermany).TheAnalysisSCheldadditionalmeetingsduringthe2015,2016,and2017EGUGeneralAssembliesinViennaAustria.AnalysisStandingCommittee(ASC)

InadditiontotheproductionoftheofficialILRSASCproducts,theASCfocusedontwoPilot Projects (PP) during the reporting period: onewas a continuation of the orbitalproduct PP,which in early 2016 evolved into a bona fide official product as reportedearlier under the “Data Products” section. The other PPwas agreed at the ILRSTech.Workshop inMatera, Italy, and the purpose of that effortwas to develop an efficientanalysisprocedurethatwillmonitorthelong-termperformanceofsystematicerrorsatstations.Atestperiodoffouryears(2005toendof2008)wasselectedasthevalidationof the procedure and the products of the contributing ACs. Of the eight ACs, sixcontributed to thePP. In the initial phase a combinationof all available contributionswasperformedandthecomparisonof the individualestimates to thecombinedresultwasusedtovalidatethecontributingseries.InthenextphaseofthePPtheprocedurewill be implemented as a standard product and the ASC will develop guidelines foridentifying likely errors and notify the affected stations. This step is expected to becompletedbymid-2017.AsubsequentPPwillintroduceLARESasthefifthtargettobe


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usedforthedevelopmentoftheofficialILRSproductsandatthesametime,thedeliveryofweeklyaveragedlow-degreesphericalharmoniccoefficientsofthegravitationalfieldmodel.ThisPPisexpectedtobecompletedbytheendof2017.Theco-chairsoftheASCareleadingasguest-editorsthepublicationofaspecialissueoftheJournalofGeodesy,dedicated on Laser Ranging. The call-for-submissions resulted in over forty proposedcontributionsandtheprocessiscurrentlyinthestageofthereviewoftheabstractsbythe guest-editors and the editor in chief todecidewhichof thesewill be accepted forpublicationinthespecialissue.DataFormatsandProceduresStandingCommittee(DFPSC)

Discussions at DFPSC splinter meetings proposedmodifications to the ILRS standardCPFandCRDformatsthatareneededtofine-tunehandlingofseveralissues,includingtransponder missions, day wrap-around, and the need for more lunar and softwareancillarydata;astudygroupwithintheSCwillworkonthesechanges.Anewprocedurewas formulated and distributed to ILRS stations for handling the December 31 2016leapsecond;thisprocedure,whichmainlydirectedstationstostoptrackingduringtheleapsecond,wasmoderatelysuccessfulandwillbereiteratedforfutureoccurrencesofleapseconds.MissionsStandingCommittee(MSC)

TheMSC,workingwiththeILRSCB,completedarevisiontotheILRSMissionsSupportRequestform.Thisformisthevehicleusedbymissionsponsorstoprovideinformationrequired by the ILRS to enable the ILRS to determine if future laser ranging to thesatelliteiswarranted.TheformprovideskeycontactsandparameterstoallowtheILRSto use the SLR data in the development of science data products and to provide themissions with SLR data that supports their goals. The MSC also reviewed submittedrequestformsandprovidedrecommendationsandfeedbacktotheCBandGBforfuturemission support. Missions approved during the reporting period include: COSMIC-2,LightSail-B,Lomonosov,NISAR,andSentinel-3A/-3B.NetworksandEngineeringStandingCommittee(NESC)

TheNESCdevelopedaprocedurethatcouldbeusedbyILRSstationstotakeaseriesofmeasurementsof theirsystem’sbeamdivergence;measuring laserbeam intensitiesatsatellite heights can inform missions of any potential hazard to sensitive on-boardequipment. Results of the measurements submitted by participating stations werediscussedat the2016workshop inPotsdam.TheSCcontinuestoreviewthe ILRSSiteLog format and consider modifications that would benefit the ILRS and missionoperators.TheNESCchairestablishedadiscussionforumtostrengthentheconnection,communication, and collaboration between international colleagues and share theexperienceandknowledge intheILRStoaddressproblemscommontostations inthenetwork.TheNESCdistributedaquestionnairetothestationsaboutwhatisneededtoaddress systematics; results from the questionnaire were presented at the NESCmeetinginPotsdam.TranspondersStandingCommittee(TSC)

Currently, themain focus for the TSC is on highly accurate time transfer, particularlyELT forACES (expected launch in2018)on the International Space Station.The SC isworkingwithstationstoimplementrequirementsforthemission.Duringitsmeetings,the SC alsodiscussed commonview time transfer and cross system rangingvia spacedebristargets forthedirectdetectionofthe laserreturnanddiffuselyscatteredsignalfromthepartnerstation.ExperimentsareunderwaybetweenWettzellandGraz.


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QualityControlBoard(QCB)

The ILRSQuality ControlBoardwas organized at the 19th InternationalWorkshoponLaserRangingtoaddressSLRsystemsbiasesandotherdataissuesthathavedegradedtheILRSdataandtheirderivedproducts.TheboardisajointactivityundertheASCandthe NESC andmeets by telecon on amonthly basis. Current activities include resultsfromtheASC’s“StationSystematicErrorsPilotProject”anddevelopmentoftoolsforthestationstoviewsystemperformanceandexaminesystematicerrors.Theplanistohavetheseasweb-baseddiagnostic toolsby the latterpartof2017.SeveralAC’shavebeenroutinelyexaminingtheincomingSLRdataandprovidingrapidfeedbacktothestationsonsuspectperformance.TheBoardisalsoexaminingtoolsandproceduresthatwouldenhancedatascrutinyatthestations.SoftwareStudyGroup(SSG)

TheSSGworkstoidentifyexistingsoftwareofusetoILRSstations.TheSSGhasworkedwiththeILRSCBtoprovidelinkstothesesoftwarepackagesontheILRSwebsite.Asetof lunarprediction, filtering,andnormalpointingsoftware isworking itswaythroughtheopen-sourcingprocessatNASAGSFC.SpaceDebrisStudyGroup(SDSG)

TheSDSGwasformedin2014tocoordinateandassiststationsinlaserrangingtospacedebris targets.TheSGalsoactsasan interfacebetweenthe ILRSand thespacedebrisactivitieswithinESA.Earlyon,theSGorganizedseveralcampaignsonTOPEX,Envisat,andotherSD targets.Over the last threeyears, thenumberof stations tracking spacedebrishasincreasedsignificantly.Measurementsinmulti-static/bi-colordebrisrangingmeasurements are being taken to uncooperative targets. “Stare andChase” is anothermethod for tracking uncooperative targets and has also been successfully tested.Significantresultshavebeenseenforscience,POD,attitudemotion,pre-entrydata,andotherapplications.

MissionCampaignsLARGE

Duringthe18th InternationalWorkshoponLaserRanginginJapaninNovember2013,theILRSagreedtoexpandtheILRSnetworksupportforGNSSconstellations.TheILRSand GGOS formed a joint study group, the LAser Ranging to GNSS s/c Experiment(LARGE)todefineanoperationalGNSStrackingstrategytoimprovetheILRSresponsetoGNSSuserrequirementsandtoclarifyoutstandingILRSandIGSissueswiththeGNSSsatellitesandgroundstations.Thesatelliteconstellationsofinterestwithretroreflectorarrays include GLONASS, BeiDou (Compass), Galileo, GPS, IRNSS, and QZSS. TheGLONASS constellation is fully populated. BeiDou and Galileo constellations are inprocess.GPSsatelliteswithlaserretroreflectorarrayswillbeginlaunchinginthe2019timeframe.Whencompleted,thefullGNSScomplexshouldreachover100satellites.Several GNSS tracking campaigns have been held since 2014, adjusting priorities tofocusonasubsetof theGNSSsatellites.Some improvementhasbeenseen,but itmayalso have been the result of stations becoming more familiar with GNSS tracking.Subsequentsessionsexaminedtheresultsinmoredetail(numberofsegmentsperpass,numberofnormalpointsper segment, “location”within thepass toacquire thedata).Moredetailscanbefoundinaposterpresentedatthe2015ILRSTechnicalWorkshopinMatera(Nolletal.,2015).TheILRSisnowinamodeofrunningthree-monthsessionsonselectedsatellitesineachoftheGNSSconstellations.


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GREAT

MonthlycampaignscontinueonGalileo-201withGalileo-202asabackup,tostudythebehavior of on-board clocks and the gravitational redshift predicted by GeneralRelativity. Launch problems placed in elliptical orbits which induce a periodicmodulationofthegravitationalredshiftattheorbitalfrequency.Sincethesespacecrafthaveatomicclockswithgoodstability(apassivehydrogenmaserclockonGalileo-201and a rubidium clock on Galileo-202), a test of the variation of the redshift can beperformedandanaccumulatedrelativisticeffectcanbedeterminedoverthelongterm.In response to our Galileo mission request, the ILRS conducted monthly, week-longcampaigns for a period of one year in support of the ESA funded experiment, GREAT(Galileo gravitational Redshift Experiment with eccentric sATellites). Stations wereasked toobserveeachpass, samplingdata fromthebeginning to theend,withoneortwonormalpoints,maximumoffiveminutesinduration,aboutevery50minutes.TheILRSandthecommunityawaitresultsfromtheexperiment.In addition to the LARGE and GREAT efforts, the ILRS has supported several othertrackingcampaigns,includingtheIRNSSconstellationatgeosynchronousorbits.

ILRSMeetingsThe ILRS organizes yearlyworkshops, the biannual InternationalWorkshop on LaserRangingandthenILRSTechnicalWorkshops,orientedtowardSLRpractitioners,ontheyearsbetween.MeetingsoftheGoverningBoardandstandingcommitteesaretypicallyheldinconjunctionwiththeseILRSworkshops.AsummaryofrecentandplannedILRSmeetingsisshowninTable3.Minutesandpresentationsfromtheworkshopsandthesesplinter meetings are available from the ILRS website(https://ilrs.gsfc.nasa.gov/about/reports/workshop/index.html andhttps://ilrs.gsfc.nasa.gov/about/reports/meeting_reports.html).The ILRS also conducts meetings of the Central Bureau on a monthly basis. Thesemeetings review network station operation and performance, as well as coordinatesupport of upcomingmissions,monitoring andmanaging the ILRS infrastructure, andfuturedirectionsandactivities.InMay2016, the ILRS celebrated forty years of supporting LAGEOS; the satellitewaslaunchedonMay04,1976.Toacknowledge theanniversary, theNASASpaceGeodesyProgram sponsored a symposium at NASA GSFC with several talks from speakersinvolved in the program over the last forty years. Links to information about thesymposium aswell as general information about LAGEOS, is available at thewebsite:https://lageos.cddis.eosdis.nasa.gov/Celebrating_40_years_of_LAGEOS.html. Similarly, 30yearsofAjisaitrackingwascelebratedonAugust13,1986.The ILRS co-sponsored several workshops over the last three years. The 2015 ILRSTechnicalWorkshopwasheldinOctober2015inMateraItaly;thethemeofthefocusedworkshop was “Network Performance and Future Expectations for ILRS Support ofGNSS,TimeTransfer,andSpaceDebrisTracking”andaddressthetopicsthatimpactthequality of the data products and operations. Abstracts, presentations, posters, andpapers from the workshop are online at the workshop’s website:https://cddis.nasa.gov/2015_Technical_Workshop/.InOctober2016,theHelmholzCenterPotsdamoftheGFZGermanResearchCentreforGeosciencesorganizedandhostedthe20thInternationalWorkshoponLaserRanginginPotsdam,Germany.Over170attendeesparticipatedintheworkshop.Thethemeforthis


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workshop, "The Path Toward the Next Generation Laser Ranging Network" allowedattendeestopresentideasforfutureadvancesinSLRtechnologyandscience;workshopmaterials, abstracts, presentations, posters, and papers, are available at themeeting’swebsite https://cddis.nasa.gov/lw20. This workshop continued the “station clinic”sessionconcepttoaddressstationoperationstopics;ILRSexpertsmetinsmallgroupsofstation engineers and operators to provide solutions to common station problems,informationtomaintainstationstability,andguidelinesforinteractingwiththeanalystsindeterminingstationbiases.

Table3.RecentILRSMeetings(asofMay2017)

Timeframe Location MeetingApril2015 Vienna,Austria ILRSAnalysisStandingCommitteemeetingOctober2015 Matera,Italy 2015ILRSTechnicalWorkshop

ILRSGoverningBoardmeetingILRSStandingCommitteemeetings

October2016 Potsdam,Germany 20thInternationalWorkshoponLaserRangingILRSGoverningBoardmeeting

ILRSStandingCommitteemeetingsApril2016 Vienna,Austria ILRSAnalysisStandingCommitteemeetingApril2017 Vienna,Austria ILRSAnalysisStandingCommitteemeetingOctober2017 Riga,Latvia 2017ILRSTechnicalWorkshop

ILRSGoverningBoardmeetingILRSStandingCommitteemeetings

PublicationsDetailed reports frompastmeetings canbe foundon the ILRSwebsite. ILRSBiannualReports summarize activities within the service over the period since the previousrelease.TheyareavailableashardcopyfromtheCBoronlineattheILRSwebsite.Thelatest volume is the eighth published report for the ILRS and concentrated onachievementsandworkinprogressratherthanILRSorganizationalelements.However,this report, the 2009-2010 ILRS Report, published in late 2012, was the last editionproducedbytheILRSduetotheextensiveamountofworkrequiredtogeneratethesedocuments. The ILRS CB is currently looking into issuing these reports in a morestreamlinedfashion.The ILRSCentralBureaucontinues tomaintain the ILRSwebsite, installedonaCDDISwebserver at NASA GSFC. The website, https://ilrs.gsfc.nasa.gov, is updated severaltimesperweekasrequired.Abibliographyof laserrangingpublicationsismaintainedonthiswebsite.ILRSAnalysisCenter reports and inputs areusedby theCentralBureau for reviewofstation performance and to provide feedback to the stationswhen necessary. Specialweekly reports on on-going campaigns are issued by email. The CB also generatesmonthlyandquarterlyPerformanceReportCardsandpoststhemontheILRSwebsite(https://ilrs.gsfc.nasa.gov/network/system_performance/index.html).TheseReportCardsevaluate data quantity, data quality, and operational compliance for each trackingstation relative to ILRS minimum performance standards. These results includeindependent assessments of station performance from several of the ILRSanalysis/associate analysis centers. The statistics are presented in tabular form bystation and sorted by total passes in descending order. Plots of data volume (passes,normal points, and minutes of data) and RMS (LAGEOS, Starlette, calibration) are


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created from this information and available on the ILRS website. Plots, updatedfrequently, ofmultiple satellite normal point RMS and number of full-rate points pernormalpointasafunctionoflocaltimeandrangehavebeenaddedtotheILRSwebsitestationpages.

OtherActivitiesInApril2013,theILRSwasacceptedasanetworkmemberoftheInternationalCouncilforScience(ICSU)WorldDataSystem(WDS).TheWDSstrivestoenableopenandlong-termaccesstomultidisciplinaryscientificdata,dataservices,productsandinformation.TheWDSworkstoensure long-termstewardshipofdataanddataservicestoaglobalscientificusercommunity.TheILRSisanetworkmemberoftheWDS,representingitstwodata centers and coordinating their activitieswithin theWDS. TheWDS requeststhat allmembers present a report every two years at itsmember’s forum. In 2016, aposter (Noll and Pearlman, 2016) reviewing the ILRS was presented at the WDSMember’sForuminDenverCO.

IssuesandChallengesSeveral challenges are on the horizon for the ILRS as it moves forward. Many gapsremain in the ILRSnetwork’s geographic coverage, primarily in LatinAmerica,Africa,andOceania.TheILRSnetworkconsistsofamixofnewandoldtechnologiesandlevelsof financial support and there is a lack of standardization in system hardware andoperations.Thenumberofsatellite targets for the ILRSnetwork trackingcontinues toincrease and because of this increase, there is a need to implement more effectivetrackingstrategies.Furthermore,thereisaneedtobemoreselectiveonthetimespenton each target. Data quality issues continue to affect the ILRS products; efforts areunderwaytodetectandreducesystematicerrors.

ReferencesLuceri,V.,E.C.Pavlis,B.Pace,D.König,M.Kuzmicz-Cieslak,G.Bianco,(2014),“Overview

of the ILRScontribution to thedevelopmentof ITRF2013”, IAGGeodesySymposia,ProceedingsofREFAG2014,T.vanDam(Ed.),Springer.

Müller,J.,L.Biskupek,F.Hofmann,E.Mai,(2014)“LunarLaserRangingandRelativity”.Book chapter in “Frontiers of Relativistic Celestial Mechanics”, vol. 2 (ed. by S.Kopeikin),deGruyter,p.99-146,2014.

Noll, C., M. Pearlman, M. Torrence, (2015) “Summary of Results from ILRS GNSSTracking Campaigns”, presented at 2015 ILRS Technical Workshop, Matera, Italy,October 26-30, 2015. (https://cddis.nasa.gov/2015_Technical_Workshop/docs/posters/P1.8_Noll_poster.pdf).

Noll,C.,M.Pearlman,(2016)“InternationalLaserRangingService:Supportinggeodeticand geophysical research and applications through Satellite Laser Ranging”,presented at 2016WDSMembers' Forum, Denver, Colorado, September 11, 2016.(https://ilrs.gsfc.nasa.gov/docs/2016/ILRS_WDS_Poster_v4.pdf).

Pavlis, E. C., V. Luceri, M. Kuzmicz-Cieslak, D. König, G. Bianco, (2014), “ModelingimprovementsintheILRSreprocessingforITRF2013”,presentedattheIAGGeodesySymposium REFAG2014, 12-17 October, 2014, Luxembourg,(http://geophy.uni.lu/users/tonie.vandam/REFAG2014/SESS_II_Techniques/Pavlis_.pdf).


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Pavlis, E. C., G. Sindoni, Ciufolini, I. A. Paolozzi, (2015) “Contribution of LARES andgeodeticsatellitesonEnvironmentalmonitoring”,Proceedingsof15thInter.Conf.onEnvironment and Electrical Engineering, 10-13 June, 2015, Rome, Italy,(https://easychair.org/conferences/submission.cgi?submission=2290027;a=8709951).

Documents

International Laser Ranging Service (ILRS)