Observing and understanding the Earth system variations from space geodesy

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JournalofGeodynamicsxxx (2013) xxx–xxx

ContentslistsavailableatScienceDirect

Journal of Geodynamics

jo u r n al ho me p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j o g

Observing and understanding the Earth system variations from space geodesy

Shuanggen Jin

^a,∗

, Tonie van Dam

^b

, Shimon Wdowinski

^c

aShanghaiAstronomicalObservatory,ChineseAcademyofSciences,Shanghai200030,China

bUniversityofLuxembourg,LuxembourgL-1359,Luxembourg

cUniversityofMiami,Miami,FL33149,USA

a r t i c l e i n f o

Articlehistory:

Received4July2013

Receivedinrevisedform5August2013 Accepted6August2013

Available online xxx

Keywords:

Spacegeodesy Earthsystem Interaction Climatechange

a b s t r a c t

TheinteractionandcouplingoftheEarthsystemcomponentsthatincludetheatmosphere,hydrosphere, cryosphere,lithosphere,andotherfluidsinEarth’sinterior,influencetheEarth’sshape,gravityfieldand itsrotation(thethreepillarsofgeodesy).Theeffectsofglobalclimatechange,suchassealevelrise, glaciermelting,andgeoharzards,alsoaffecttheseobservables.However,observationsandmodelsof Earth’ssystemchangehavelargeuncertaintiesduetothelackofdirecthightemporal–spatialmea- surements.Nowadays,spacegeodetictechniques,particularlyGNSS,VLBI,SLR,DORIS,InSAR,satellite gravimetryandaltimetryprovideauniqueopportunitytomonitorand,therefore,understandthepro- cessesandfeedbackmechanismsoftheEarthsystemwithhighresolutionandprecision.Inthispaper, thestatusofcurrentspacegeodetictechniques,somerecentobservations,andinterpretationsofthose observationsintermsoftheEarthsystemarepresented.Theseresultsincludetheroleofspacegeo- detictechniques,atmospheric–ionosphericsounding,oceanmonitoring,hydrologicsensing,cryosphere mapping,crustaldeformationandloadingdisplacements,gravityfield,geocentermotion,Earth’soblatenessvariations,Earthrotationandatmospheric-solidearthcoupling,etc.Theremainingquestionsand challengesregardingobservingandunderstandingtheEarthsystemarediscussed.

1. Introduction

TheEarthsystemvariescontinuouslyduetointeractionsand couplingbetweenitsmainfluidcomponentsthatincludetheatmo- sphere,thehydrosphere,thecryosphere,thelithosphere,andthe Earth’sinterior.Massredistributionsdrivenbygeodynamicpro- cessesintheEarthsystemaffecttheEarth’sshape, gravityfield androtation.However,thesystemandcomponentsarealsobeing affectedbyglobalclimatechange.TheIntergovernmentalPanelon ClimateChange(IPCC)ofUnitedNationsreportedthattheglobal climatehasbeenwarmingsincethemiddleofthe20thcentury due toincreasesin atmospheric greenhousegasconcentrations (PachauriandReisinger,2007).Toconfirmandassessthefuture effects ofglobalwarming,stillrequires additionalobservations.

However,traditionalmeteorologicalsensorssufferfromanumber oflimitations.Inadditiontobeinghighlylabor-intensivetodeploy

∗Corresponding author at: Shanghai Astronomical Observatory, Chinese AcademyofSciences,Shanghai200030,China.Tel.:+862134775292;

fax:+862164384618.

E-mailaddresses:sgjin@shao.ac.cn,sg.jin@yahoo.com(S.Jin),

tonie.vandam@uni.lu(T.vanDam),shimonw@rsmas.miami.edu(S.Wdowinski).

theyalsohavealow-spatialresolution,e.g.,lowtemporalresolu- tionradiosondeballoonsthatarelaunchedtwiceperday(Jinand Luo,2009;Jinetal.,2009).Anotherexampleisionosphericmon- itoring,inwhichtraditionalinstrumentsareveryexpensiveand provideonlylowspatialsampling.Furthermore,ionosondescan- notmeasurethetopsideionosphereandsometimessufferfrom absorptionduringstorms,whereasIncoherentScatterRadar(ISR) hasgeographicallimitations,andevenlow-Earth-orbitingsatellite (altitudesof400–800km)observationscannotprovideinformation onthewholeionosphere(Jinetal.,2006).

Oceanshavebeenobservedbydriftingbuoysandtidegauges (TG)foralmosttwocenturies(Barnett,1984).Meansealevelrise wasobservedusingTGthrough the20thcentury.Sea-levelrise wasthoughttobedrivenbyastericcomponentduetothethermal expansionoftheoceansandanon-stericcomponentduetofresh waterinputfromthemeltingofcontinentalicesheetsandglaciers (Holgateand Woodworth,2004;Churchand White,2006;Feng etal.,2013),thataffectourlivingenvironments,marineecosys- tems,coastsandmarshes.However,TGprovidestherelativesea level variationswith respecttotheland. Furthermore,TGdata arepointmeasurementsandareprovidedatlowspatialresolu- tions.Wecannotquantifythestericcontributiontothesealevel budgetduetothelackofglobaloceantemperatureandsalinity 0264-3707/$–seefrontmatter© 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.jog.2013.08.001

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marinemagneticanomalieslocatedonbothsidesofthemid-ocean ridgesandfromtheazimuthof transform-faults(attimescales ofmillionsofyears),aswellasfromhistoric earthquakesource parameters.Anotherissueisthatitisdifficulttomonitorpresent- dayintraplatecrustaldeformationsinsufficientdetails(JinandZhu, 2002).Forexample,currentdeformationinEastAsiaisdistributed overabroadareaextendingfromTibetinthesouthtotheKuril- Japantrench intheeastwithsomemicro-plates,suchasSouth ChinaandpossiblytheAmurianplate,embeddedinthedeform- ingzone(Jinetal.,2007).Becauseofthesparseseismicityinthe regionandthelackofcleargeographicalboundariesinthebroad platedeformationzones,ithasbeendifficulttodescribeaccurately thetectonickinematicsandplateboundariesactinghere.Earth- quakesandvolcanoesfrequentlyoccurworldwide,withnotable casesincluding: theMw=9.1 SumatraEarthquake in 2004;the Mw=8.1WenchuanEarthquakein2008;theMw=8.0ChileEarth- quakein2010;andtheMw=9.0TohokuEarthquakein2011.These earthquakestookthousandsoflivesandgeneratedhugetsunamis.

Seismometersaroundtheglobecanestimatethenatureofearth- quakes,butthedetailsofrapidruptureareusuallyobscuredbythe lackofnear-ﬁeldnear-real-timeobservations.Inaddition,observ- ingandmodelingEarth’sinterioractivitiesarestillchallengingdue tothecomplexmasstransportandotherphysicalprocessesacting withintheEarthsystem(Jinetal.,2010a;JinandFeng,2013).

Therefore,itishardtoobserveandmodelEarthsystemandenvi- ronmentchanges.Today,Earthobservationsfromspaceprovidea uniqueopportunitytomonitorvariationsinthevariouscompo- nentsoftheEarthsystem.Observationsincludedatafromthedense distributionofgroundglobalpositioningsystem(GPS)stations;

highresolutionspace-borneGPSradiooccultation;long-termVery LongBaselineInterferometry(VLBI);SatelliteLaserRanging(SLR);

DopplerOrbitographyandRadiopositioningIntegratedbySatellite (DORIS);InterferometricSyntheticApertureRadar(InSAR);Light DetectionandRanging (LiDAR);satelliteradar andlaseraltime- try(Abshireet al., 2005);and satellite gravimetry (particularly CHAMP/GRACE/GOCE)(e.g.,Tapley etal., 2004).Theseobserva- tionsallowustomeasureandmonitorsmalldisplacementsofthe Earth’ssurfaceandmasstransportwithhighprecisionandhigh spatial–temporalresolutions.Thesedataprovideauniqueoppor- tunitytoinvestigatemasstransportassociatedwithgeodynamics, naturalhazards,andclimatechangeandtobetterunderstandthe processesthemselvesandtheirinteractionswithintheEarthsys- tem.Inthispaper,thedifferentspacegeodetictechniquesandEarth systeminteractionsareintroduced.Recentobservationsandmod- elingresultsfromspacegeodesythatallowustoinferchangesin theatmosphere,oceans,hydrosphere,cryosphere,lithosphere,and theEarth’sdeepinteriorarepresented.

2. Spacegeodetictechniquesandobservations

The word “geodesy” etymologically comes from the Greek

“geôdaisia”: “dividing the Earth”. Today, geodesy is deﬁned as measuringtheEarth’ssize,shape,orientation,gravitationalﬁeld and their variations with time using geodetic techniques, e.g., Arcmeasurements(historic),Triangulation,Trilateration,Travers- ing,Leveling,Zenithorverticalanglesmeasurementandground gravimetry. However, thesetraditional geodetictechniques are

techniquesarebrieﬂyintroduced.

2.1. GNSS

GlobalNavigationSatelliteSystem(GNSS)isarecenttermused todescribethevarioussatellitenavigationsystems,suchasGPS, GLONASS,BeidouandGalileo.TheGlobalPositioningSystem(GPS) withitsunprecedentedprecision,hasprovidedgreatcontributions tonavigation,positioning,timingandscientiﬁcquestionsrelated to precise positioningon Earth’s surface, since it became fully operationalin1994(Jinetal.,2011a).Today,anumberofEarth system science questions have been successfully investigated, includingtheestablishmentofahighprecisionInternationalTer- restrialReferenceFrame(ITRF),Earthrotation,geocentermotion, time-variabilityofthegravityﬁeld,orbitdeterminationaswellas remotesensing oftheatmosphere,hydrology andoceans.With thedevelopmentofthenextgenerationofmulti-frequencyand multi-systemGNSSconstellations,includingtheU.S.’smodernized GPS-IIFandplannedGPS-III,Russia’srestoredGLONASS,thecoming EuropeanUnion’sGALILEOsystem,andChina’sBeidou/COMPASS system,aswellasanumberofregionalsystemsincludingJapan’s Quasi-ZenithSatelliteSystem(QZSS)andIndia’sRegionalNaviga- tionSatelliteSystems(IRNSS),moreapplicationsandopportunities willberealizedtoexploretheEarthsystemusinggroundandspace borneGNSS.

2.2. VLBI

Very-long-baseline interferometry, VLBI, plays a key role in space geodesy by receiving natural quasars signals at two or more Earth-based radio telescopes. VLBI is particularly impor- tant in establishingthe InternationalCelestial ReferenceFrame (ICRF).WiththeglobalVLBItrackingnetwork,VLBIcanbeused tomonitorplatemotion, crustaldeformation,polarmotionand length-of-dayaswellasbeingusedfordeepspaceexploration.

The “VLBI2010: Current and FutureRequirements for Geodetic VLBISystems”providedapathtoanext-generationsystemwith unprecedentednewcapabilitiesand accuracies,including1mm positionand 0.1mm/year velocity measurement accuracy, con- tinuous observationsforstationpositions andEarthorientation parameters.Additionalapplicationswillbedevelopedinthenear future(Titov,2010).Forexample,theChineseVLBINetwork(CVN) hasbeenupgraded(e.g.,twoVLBIradiotelescopesin Shanghai andUrumqi,andseveralproposednewVLBI2010-typesystem)and newapplicationssuchassuccessfullytrackingtheChina’sChang’E- 1/2lunarexplorationprobes(Weietal.,2013).

2.3. SLR/LLR

SatelliteLaserRanging(SLR)systemspreciselydeterminethe distanceofthesatellitesabovetheEarth’sgeocenterbymeasuring thetimetosendandreceiveshortlaserpulses.Thesedatahave beenwidelyusedinpreciseorbitdetermination(POD)ofartiﬁcial satellitesandstationmotions.Inaddition,sincethelow-earth-orbit (LEO)laserrangingsatellitesaresensitivetothelow-degreegravity ﬁeld,SLRisconsideredthegold-standardinmonitoringgeocenter andC₂₀(ChengandTapley,2004;Jinetal.,2011b).LunarLaser Ranging(LLR)canmeasurethedistancebetweenthemoonand

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S.Jinetal./JournalofGeodynamicsxxx (2013) xxx–xxx 3

Fig.1.Mainspacegeodetictechniques.

Earth,whichallowsustocheckcertaingeneralrelativityprincipals andexplorerotationalpropertiesofthemoon.

2.4. DORIS

Doppler Orbitography and Radio positioning Integrated by Satellite (DORIS) is a technique whereby satellites receive the broadcastedwavelengthsignaltransmittedbygroundstations.As thesatellitemoves,thereceivedfrequencyisshifted.Thesatellite velocityandpositionarelinkedbytheKeplerianequationofits orbit,sotheDopplershiftallowingustodeterminethestationto satellitedistance.DORIScandeterminetheorbitofthetransmit- tingsatellites,Earthrotationandstationcoordinatesthatcanbe usedindifferentgeodeticapplications(seeWillisetal.,2010).

2.5. InSAR

InterferometricSyntheticApertureRadar(InSAR)canbeused monitorsurfacedeformationordeterminedigitalelevationmodel (DEM)usingdifferencesbetweentwoormoresyntheticaperture radar(SAR)images(MassonnetandFeigl,1998).Thetechniquecan

potentiallymeasuredisplacementoftheEarth’ssurfaceovertime spansofdaystoyearswithcentimeterlevelprecision.Recently, manyvariedapplicationsofInSARhavebeenpublished,e.g.,moni- toringearthquakes,volcanoes,landslides,hydrologicalcycle,ocean environment,andglaciermovement.Inaddition,troposphericand ionospheric delayscanbeextracted usingtheInSARtechnique.

Thesedatacanbeusedinmeteorologyandspaceweather.

2.6. Satellitealtimetry

Satellitealtimetricobservationshavepromotedandadvanced oceanographicresearchsincetheﬁrstmissionTopex/Poseidonin 1993aswellaslaterGEOSATFollow-On(GFO),ERS-2,Jason-1/2, andEnvisat.Anumberofresultshavebeenobtainedwithalmost coveringthewholeEarthatcentimeteraccuracy,e.g.,themeansea surfaceheight(MSSH),sealevelchangeandoceancirculationas wellasElNinoeventsmonitoring(Fengetal.,2013).Inaddition, laseraltimeteralsocanmonitorglaciermelting.Forexample,the GeoscienceLaserAltimeterSystem(GLAS)onbardICESatcanmon- itortheicesheetmassbalances(Zwallyetal.,2005;Abshireetal., 2005).

Table1

Rolesandusesofeachspacegeodetictechnique.

Parameters GNSS VLBI DORIS SLR LLR Altimetry InSAR/LiDAR Gravimetry

ICRF √

ITRF √ √ √ √ √

(√

) (√

)

Polarmotion √ √ √ √ √

(√ )

Nutation (√

) √

(√

) √

UT1 √

Lengthofday √ √ √ √ √

(√ )

Geocenter √ √ √

(√

) √

Gravityﬁeld √ √ √ √

(√

) √ √

Platemotion √ √ √ √

(√

) (√

) √ √

LEOorbits √ √ √ √ √ √

Navigation/orbit √ √ √ √ √ √

Timingandclock √ √

(√ )

Ionosphere √ √ √ √ √

Troposphere √ √ √ √ √

Hydrology √ √ √ √

Oceans √ √ √ √

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Fig.2.Earthsystemcomponentsandinteraction.

2.7. Satellitegravimetry

With the recent development of the low-earth orbit (LEO) satellitegravimetry,theprecisionandtemporalresolutionofthe Earth’sgravity ﬁeld model hasbeengreatly improved.Satellite gravimetry is a successful innovation and breakthrough in the ﬁeldofgeodesy,following theGlobalPositioning System(GPS).

Unlikethetraditionalterrestrialgravitymeasurements,themost advanced SST(Satellite-to-SatelliteTracking)and SGG (Satellite GravityGradiometry)techniquesareusedtoestimatetheglobal high-precision gravity ﬁeld and its variations (Knudsen et al., 2011). Satellite-to-Satellite Tracking technique includes theso- calledhigh–lowsatellite-to-satellitetracking(hl-SST)andlow–low satellite-to-satellitetracking(ll-SST)(Wolff,1969),whichcanpre- ciselydeterminethevariation rateofthedistancebetweentwo satellites.Satellitegravimetry,particularlyrecentGravityRecovery andClimateExperiment(GRACE),canmonitorthemasstransport and redistribution in theEarthsystem,which has beenwidely appliedingeodesy,oceanography,hydrology,icemasschange,geo- dynamicsandgeophysics(Wahretal.,1998;Tapleyetal.,2004;

Famigliettietal.,2011;JinandFeng,2013).

3. Spacegeodesyandearthsysteminteraction

TheEarthsystemisverycomplexduetotheinteractionand couplingofitscomponents(Fig.2).For example,meltingconti- nentalglacierswillresultinrisingsealevels,tectonicactivitiesand reservoiraccumulatedwatermayleadtohazards,e.g.,earthquakes, tsunamis, landslides and mud-rock ﬂows and the atmospheric pressureloadingwillchangetheshape,geocenter,oblatenessand rotationoftheEarth,etc.

Today space geodetic techniques with high precision and temporal–spatial resolution can measure and monitor such changes within the Earth system, including Earth rotation, Earth’s oblateness,geocenter,geometry and deformation, gravity ﬁeld, atmosphere, oceans, hydrosphere, cryosphere, crust, ocean/atmospheric/polar tides, ocean/atmospheric/hydrological loading, tectonic motions, volcanoes, earthquakes, tsunamis, glacieristostaticadjustment(GIA)andlandslides,etc.Fig.3shows theroleofspacegeodesyinobservingandunderstandingtheEarth system.Thereferenceframeis a benchmarkfor Earthrotation, geometryanddeformationandgravityﬁeldmeasurementsthatcan bepreciselydeterminedusingspacegeodesy.Earthsystemchanges

… Terrestrial Techniques

Leveling Tide Gauge Gravimetry Tiangulation

…

Tides Loadings Geocenter motions

Tectonic motions Volcanoes Earthquakes

Tsunamis GIA Landslides

….

Gravity Field Reference

Frame

Fig.3. Spacegeodesyandearthsysteminteraction.

affecttheEarth’srotation,geometryanddeformationandgravity ﬁeldaswellasreferenceframe.

4. ObservingandunderstandingEarthsystem 4.1. Atmosphericsounding

Spacetechniques’radiosignalspropagatethroughtheEarth’s neutral atmosphereand ionosphere.Thisresultsinlengthening the geometricpath of the ray, usually referred to as the “tro- posphericdelayandionosphericdelay.”Thesedelaysareoneof majorerrorsourcesforspacetechniques.Today,thespacetech- niquessuchasGPS,VLBI,InSAR,DORISandaltimetryareableto measuresuchdelaysandcorrespondingprecipitablewatervapor (PWV)and ionospherictotalelectroncontent(TEC)witha high resolutionandprecision.Theseproductshavebeenwidelyapplied in meteorology, climatology, numerical weather models,atmo- sphericscienceand spaceweather(e.g.,Jinetal.,2006;Jinand Park,2007;Jinetal.,2008,2009;JinandLuo,2009).Forexample, Fig.4showsthelong-termverticalTECvariationsfromGPSatthe

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 0

10 20 30 40 50

VTEC (TECU)

VTEC at (Lat: -87.5^o; Lon: 120^o)

Time (year)

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 0

100 200 300

Flux (10-22 WM-2 Hz-1)

Time (year) 10.7 cm Solar Flux

Fig.4. VerticalTEC(VTEC)timeseriesat(120E^◦,−87.5S^◦)and10.7cmSolarFlux.

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point(120E^◦,−87.5S^◦),whicharemainlyaffectedbysolaractivity.

Additionally,thespace-borneGPSradiooccultation(RO)missions canestimateatmosphericandionosphericproducts,includingthe pressure,temperature,watervapor,totalelectroncontent(TEC) andelectrondensityaswellastheirvariationcharacteristics,e.g., theUS/ArgentinaSAC-C,GermanCHAMP(CHAllengingMinisatel- litePayload),US/GermanyGRACE(GravityRecoveryandClimate Experiment),Taiwan/USFORMOSAT-3/COSMIC(FORMOsaSATel- litemission–3/ConstellationObservingSystemforMeteorology, IonosphereandClimate)satellites,theGermanTerraSAR-Xsatel- litesandtheEuropeanMetOp(Schmidtetal.,2010;Jinetal.,2011a).

4.2. Oceanmonitoring

Theoceanshavebeenwellmonitoredbysatellitealtimetryfor abouttwodecades.Thesedataprovideinformationonthemean seasurfaceheight(MSSH),oceancirculation,tidesandsealevel change.Thesealevelchangeincludesthestericandnon-stericsea changes.Whilethestericsealevelchange,i.e.,thermalexpansion, iswellestimatedbyArgoobservations,thenon-stericcomponent, i.e.,eustaticsealevelchange,relatedtomasschangesdrivenbythe additionofwatertotheoceansfromthemeltingofcontinentalice sheetsandfreshwaterinriversandlakes(Fengetal.,2013)ismore difﬁculttoestimate.Thedifﬁcultyisduetothehighlyuncertain estimatesoftheAntarcticandGreenlandmassandlandreservoirs.

Thesatellite-basedGRACEobservationsprovideauniqueoppor- tunitytodirectlymeasuretheglobaloceanandcontinentalwater masschange.For example,Fig.5 showsthesealevelchangein cm/yearfromsatellitealtimetry,Argoand GRACE(2002–2011), whichgenerallyquantifythetotalsealevelchangebudget.

In addition, GNSS-Reflectometry (GNSS-R) is a new innova- tiveandpromisingapproachinoceanremotesensing.Inaddition, thetechniqueposesmanypotentialadvantages(Jinetal.,2011a), includingprimarilyglobalcoverageandlong-termsatellitemission lifetime.Inthelastfewyears,severalexperimentswereundertaken andnumerousadvancementswererealized.ThefirstGPSsignals reflectedfromtheseasurfaceinsidetropicalcycloneswereana- lyzed,andthewindspeedresultswereobtained(Katzbergetal., 2001).Currently,the GPS reflectedsignals from theoceansur- facecanmeasureseasurfaceheightwiththeachievableaccuracy (Martin-Neiraetal.,2001;KatzbergandDunion,2009).

4.3. Hydrologicsensing

The power level of the GPS reflected signal from the land containsinformationaboutthesoilmoisture,dielectricconstant, surface roughness,and possiblevegetative cover ofthe reflect- ingsurface. Katzberg et al.(2006) obtainedthesoil reflectivity anddielectricconstantusingaGPSreflectometerinstalledonan HC130aircraftduringtheSoilMoistureExperiment2002(SMEX02) nearAmes,Iowa,which wereconsistentwithresultsfoundfor other microwave techniques operating at L-band. In addition, themulti-pathfromgroundGPSnetworksispossiblyrelatedto thenear-surfacesoilmoisture.Larsonetal.(2008)foundnearly consistent fluctuations in near-surface soil moisture from the ground-basedobservationsofGPSmulti-path.TheyfoundGPSesti- matesofsoilmoisturewerecomparabletoestimatesinthetop5cm ofsoilmeasuredfromconventionalsensors.

Satellitegravimetry, in particularGRACE, candeterminethe monthlyterrestrialwaterstoragechange.Fig.6showsthetrendof terrestrialwaterstoragechangeintheworldbasedonGRACEdata (August2002–February2011),whichreﬂectgroundwaterdeple- tion,droughtandﬂoods.Forexample,theterrestrialwaterstorage isfallingbyabout−15.5mm/yearinthenorthwestregionofIndia duetogroundwaterdepletion(Rodelletal.,2009).TheAmazon basininSouthAmericanhasincreasedratesofterrestrialwater

Fig.5. Sealevelchangeincm/year fromsatellite altimetry,ArgoandGRACE (2002–2011).

storagereserveswithabout20.5mm/yearduetorecentﬂoods.In theLaPlataregionofSouthAmerica,theterrestrialwaterstorage isfallingatarateof−9.8mm/yearduetorecentdroughts.

4.4. WetlandmentoringbyInSAR

WetlandInSARisauniqueapplicationoftheInSARtechnique, becauseunlikeallotherapplicationsthatdetectdisplacementsof solidsurfaces,wetlandInSARdetectschangesofaquaticsurfaces.

Themethodworks,becausetheradarpulseisbackscatteredtwice

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Fig.6.ThetrendofterrestrialwaterstoragefromGRACE(2002–2011).

(“double-bounce”–Richardsetal.,1987)fromthewatersurface andvegetation.Althoughmostvegetationscatteringtheoriessug- gestthattheshortwavelengthradar signal(X-andC-band),as wellasthecross-polarizationsignal,interactsmainlywithupper levelsofthevegetationand,hence,shouldbesuitableforrepeat passinterferometry,theanalysisofalldatatypeswithdifferent polarizationshaveindicatedthesuitabilityofallinSARdatatothe wetlandapplication (Honget al., 2010a;Hongand Wdowinski, 2011).Howeverthedataquality,ascalculatedfrominterferometric coherence,stronglydependsonthetimespanbetweentheInSAR acquisitions.Shortwavelengthsignals(X-andC-band)maintain coherencewhentheacquisitiontimespanisintherangeof1–70 days,dependingontheacquisitionparametersandthevegetation type,whereasthelongerwavelengthL-bandcanmaintaincoher- enceuptothreeyearsinawoodywetlandenvironment(Kimetal., 2013).

WetlandInSARworks verywellin wetlands andfloodplains enablinghighspatialresolutiondetectionofwaterlevelchanges with5–10cm levelaccuracy(Wdowinskiet al.,2004,2008).By combiningtheInSARobservationswithground-measuredstage (waterlevel)data,itispossibletogeneratehighspatialresolution mapsofabsolutewaterlevelsandtheirchangesovertime(Hong etal.,2010b).WetlandInSARobservationsareusefulforwetland monitoringby waterauthorities, characterizationoftidal flush- ingzone(Wdowinskietal.,2013),andconstraintsofhighspatial resolutionsurfaceflowmodel.

4.5. Cryospheremapping

Glacierscanalsobemonitoredbyradarremote-sensing(e.g., InSAR).However,coverageislimitedandcostsarehigh.Astheice andsnowthicknessarerelatedtotheamplitudeofthereflected signalasafunctionoftheincidenceangleorrelativeamplitudes betweendifferentpolarizations,thesnowandicethicknesscan beretrievedfromtheGPSreflectedsignals.Komjathyetal.(2000) derivedtheconditionof seaandfresh-water ice aswellasthe freeze/thawstateoffrozengroundfromaircraftexperimentswith GPSreflectionsovertheArcticseaiceandicepacknearBarrow, Alaska,USA.Inaddition,thechangeinsnowdepthisalsorelated tothecorrespondingmulti-pathmodulationoftheground-based GPSsignal,whichcanbemonitoredbyGPS(Jacobson,2010).In addition,GRACEcandeterminewelltheglacierandice-sheetmass

variations.Forexample,Fig.6showsrapidmeltingofglaciersin Greenland,WestAntarcticaandAlaska.

4.6. Crustaldeformationmonitoring

Spacegeodetictechniques,particularlythedenserGPSobser- vations,providenewtoolstomonitorandmodelthepresent-day crustaldeformationassociatedwithhazards.Forexample,EastAsia islocatedinacomplexconvergentzonewithseveralplates,e.g., Pacific,NorthAmerican,Eurasian, andPhilippineSeaplates(Jin andZhu,2003).SubductionofthePhilippineSeaandPacificplates andexpulsionofEurasianplatewithIndianplatecollisionmak- ingthisregiononeofthemostseismicallyactiveanddeforming regionsintheworld.CurrentdeformationinEastAsiaisdistributed overabroadareaextendingfromTibetinthesouthtotheBaikal RiftzoneinthenorthandtheKuril-Japantrenchintheeast.The interplatedeformationandinteractionbetweentheblocksarevery complicatedandactive.Becauseoflowseismicityanduncertainties inthegeographicalboundaryoffaults,itisdifficulttoaccurately determinethecrustaldeformationanddescribethetectonicfea- turesand evolution ofthedeformation beltsinEastAsia.With moreGPSobservationsinEastAsia,thesystemwillprovidenew waystoclearlymonitorthedetailedcrustaldeformation,e.g.,the nationalprojects“CrustalMovementObservationNetworkofChina (CMONC)withmore than1000GPSsitesand theJapanese GPS EarthObservationNetwork(GEONET)withmorethan1000con- tinuousGPSsites.ThesedenseGPSobservationswillobtainamore accurateestimateofcrustaldeformationinEastAsia.Fig.7shows crustaldeformationratesintheEurasiaplate(EU)fixedreference framewitherrorellipsesin95%confidencelimits.Thepossibilityof micro-platemotionindependentoftheEurasianplatewastested andthecharacteristicsoftectonicactivitiesweregivenusingGPS derivedvelocities(Jinetal.,2007).

4.7. Geophysicalloading

ContinuousGPSobservationshavesigniﬁcantseasonalchanges, whicharemostlyexcitedbytheredistributionofthesurfaceﬂuid mass(includingatmosphere,ocean,andcontinentalwater)(van DamandWahr,1987;vanDametal.,2001;Dongetal.,2002).How- ever,variousempiricalmodelshavelargerdifferences,particularly hydrologicalmodelsduemainlytothelackofacomprehensive

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Fig.7. CrustaldeformationratesintheEurasiaplate(EU)ﬁxedreferenceframewith errorellipsesin95%conﬁdencelimits.

globalnetworkforroutinemonitoringoftheappropriatehydrolog- icalparameters.Thesatellitemission,GRACE,hasprovidedglobal surfaceﬂuidmasstransportformorethanadecade.Thevertical loadingdisplacementsfromGPS,GRACEandgeophysicalmodels arecomputedandcompared,andatmostglobalsites,therootmean square(RMS)ofGPScoordinatetimeseriesisreducedafterremov- ingtheGRACEandmodelsestimates,andtheannualvariationsof theGPSheightatmostsitesagreewellwithGRACEestimatesin theamplitudeandphase(Fig.8)(Zhangetal.,2012).Forexample, Fig.9showstheverticaldisplacementtimeseriesfromGPS,GRACE andgeophysicalmodelsatBRAZsite.Itindicatesthatthenonlinear seasonalGPSverticaldisplacementvariationsaremainlycaused bythegeophysicalloadingeffects.However,atsomesites,partic- ularlyintheAntarctica,someoceancoasts,smallpeninsulasand Europe,discrepancyhasbeenfoundbetweentheGPSandGRACE estimates(e.g.,vanDametal.,2007).Theremainingdisagreement maybeduetotheGPStechnicalerrorsandGRACEaccuracy(e.g.,

2002 2003 2004 2005 2006 2007 2008 2009 -20

-15 -10 -5 0 5 10 15 20 25

Time(year)

Vertical displacement residuals(mm)

BRAZ GPS

GRACE MODEL

Fig.9.MonthlyverticaldisplacementtimeseriesfromGPS,GRACEandgeophysical modelsatBRAZsite.

Jinetal.,2005).Itneedstobefurtherinvestigatedusinglongerand moreGPSandGRACEmeasurements.

4.8. GeocenterandEarth’soblatenessvariations

The transfer and redistribution of the Earth’s atmosphere, oceanandland watermasses duetothetectonicactivitiesand global climate change can lead to variations of Earth’s oblateness(J2)andgeocentermotion(Jinetal.,2010a,2011b).Satellite laserranging(SLR)canwellestimatethegeocentermotionand degree-2zonalgravitationalcoefﬁcientvariations,butsubjecting tosparseandunevenlydistributedSLRstationsaswellashigher- altitudelasersatellites.Althoughthenewgenerationofsatellite gravityGRACE haslargely improvedthelower-order coefﬁcient estimateswithordersofmagnitude,butisnotsensitivetogeo- centerandJ₂.Nowadays,continuousGPSobservationscanprovide highprecision3-dimensionalcoordinate time series,whichcan

GRACE 10 mm GPS 10 mm MODEL 10 mm

1^o80W

12^o0W 60

oW

0^o 60o E

120_o E

180_o W 80^oS

40^oS 0^o

40^oN

80^oN

longitude

latitude

Fig.8. AnnualvariationsofverticaldisplacementsfromGPS,GRACEandgeophysicalmodels.

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2003 2004 2005 2006 2007 2008 2009 -8

-6 -4 -2

Time (year) ΔJ2 (X10

Fig.10. ThetimeseriesofEarth’soblatenessvariations.

estimatelow degree gravityﬁeld coefﬁcients (Wu et al., 2006;

JinandZhang,2012).Here,theGPScoordinatestimeseriesfrom ITRF2008solutions (Altamimietal., 2011)areusedtoestimate geocenter and J₂ together with OBP (ocean bottom pressure) andGRACE,respectively,whichshowgoodagreementwithSLR solutionsandgeophysicalmodelsestimatesinseasonal,intrasea- sonalandinterannualvariations.Forexample,Fig.10showsthe timeseriesofEarth’soblatenessvariationsfromGPS,GPS+OBP, GPS+OBP+GRACE,SLR,GRACEandModels(JinandZhang,2012).

ThevariationsofJ₂atseasonal,intraseasonalandinterannualscales aremainlydrivenbytheatmosphere,oceansandhydrology.

4.9. GeophysicalexcitationsofEarthrotation

TheEarth’s rotation variables are changing from secondsto decadesof years,including polarmotion(Xand Y)and change ofrotation rate(or length-of-day,LOD).Attimescales of a few yearsorless,theEarth’srotationalchangesaremainlydrivenby massredistributionintheatmosphere,oceansandhydrosphere fromgeophysicalfluidmodels(e.g.,Barnesetal.,1983;Grossetal., 2003).However,accuratelyassessingtheatmospheric,hydrologi- calandoceaniceffectsonpolarmotionandlength-of-dayvariations remainsunclearduemainlytothelackofglobalobservations(Jin etal.,2012).Heretheatmospheric,hydrologicalandoceanicmass excitationstopolarmotionareinvestigatedfromgeophysicalfluid models(NCEP+ECCO+GLDAS)andGravityRecoveryandClimate Experiment(GRACE)forAugust2002untilAugust2009.Results showthattheGRACEexplainsthegeodeticresidualpolarmotion excitationsatannual periodsbetter(see Fig.11).However,the geophysicalfluidmodelsdoabetterjobofcapturingtheintrasea- sonalgeodeticresidualsofpolarmotionexcitationinthePxandPy components.Inaddition,GRACEandthecombinedGRACEandSLR solutionsbetterexplainthegeodeticLODexcitationsatannualand semi-annualtimescales.Forlessthan1-yeartimescales,GRACE- derivedmassdoesworseatexplainingthegeodeticresiduals,while SLRagreesbetterwiththegeodeticresiduals.However,thecom- binedGRACE andSLR resultsaremuch improvedin explaining thegeodeticresidualexcitationsatintraseasonalscale(Jinetal., 2011b).

4.10. Atmospheric-solidEarthcoupling

GNSS/InSARandstrongmotionseismicmeasurementsprovide uniqueinsightsintothekinematicruptureandthesizeoftheearth- quake(Jinetal.,2010b).However,basedoncurrentmeasurements understandingandpredictingearthquakesisstillchallengingand difﬁcult.Possibly non-seismicmeasurements, e.g., atmospheric,

2003 2004 2005 2006 2007 2008 2009 2010

-40

Time (year)

Polar motion excitation (mas)

2003 2004 2005 2006 2007 2008 2009 2010

-60 -40 -20 0 20 40 60

Time (year)

Polar motion excitation (mas)

(b) Py

Geod.-Wind-Current Models GRACE

Fig. 11. Surface ﬂuid mass excitations of polar motion from geodetic observation residual (Geod.-Wind-Current) (blue line), geophysical models (NCEP+ECCO+GLDAS)(greenline)andGRACE(redline).(Forinterpretationofthe referencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionof thearticle.)

ionosphericorelectromagneticobservations,canhelpinthistask.A casestudyofthe2008Wenchuanearthquakewasperformedusing continuousGPS measurementsinChina.Significantionospheric disturbancesarefoundatcontinuousGPSsitesneartheepicenter withanintensiveN-shapeshock-acousticwavepropagatingsouth- eastward,almostconsistingwithseismometer,indicatingthatthe co-seismicionosphericTECdisturbancesweremainlyderivedfrom themainshock.Forexample,Fig.12showsthesignificantTECvari- ationsduringthemainshock,indicatingasignificantionospheric disturbance(Afraimovichetal.,2010;Jinetal.,2010b)thatoccurs simultaneouslywiththeco-seismicrupture.Furthermore,theco- seismic tropospheric anomalies during the mainshock are also found,mainlyinthezenithhydrostaticdelaycomponent(ZHD)(Jin etal.,2011c).Thisconclusionissupportedbythesamepatternof surfaceobservedatmosphericpressurechangesatco-locatedGNSS sitethataredrivenbytheground-coupledairwavesfromground verticalmotionofseismicwavespropagation.Therefore,theco- seismicatmosphericdisturbancesappeartoindicateanacoustic couplingbetweentheatmosphereandsolid-Earthwithairwave propagationfromthegroundtothetopatmosphere.

5. Challengesandfuturedevelopments

Although high precision space geodesy plays a key role in observing and understanding the Earth system, a number of questions still remain, e.g., the precision and temporal–spatial resolutionofthetechniques.Forexample,currentlyVLBIcannot continuouslyobserveandSLRhasafewglobaltrackingstations, particularlyinthesouthernhemisphere.Furthermore,laserran- gingsatellitesarenotsensitivetothehigh-frequencyvariations oflow-degree gravityﬁeldbecauseofthehighaltitudeof their orbits,e.g.,LEGEOS1/2withaltitudeof6000km(ChengandTapley, 2004).Satellitegravimetry,inparticularGRACE,isalsorestricted inprecisionandresolutionduetoitsorbitalaltitude,orbitalincli- nation,hardwarenoiseandﬁlteringmethods(SwensonandWahr, 2006).Inaddition,theaccuracyofgeophysicalmodels,post-glacial rebound(Zhangand Jin,2013)and tidemodelsalsoaffectmass

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Fig.12.TheﬁlteredTECseriesdI(t)forLUZH-PRN22(a)withblack,red,andgray lineson11,12and13May,respectively.(b)DemonstratestheevolutionofN-shaped responsewithdistancefromepicenterincrease;theﬁrstmaximumsofN-response forLUZH-PRN14andLUZH-PRN22aresuperposed.Onpanels(c)and(d)theblack linesshowtheaccumulatedTECseriesdIP(t),obtainedontheexperimentalstage ofprocessingfortheplaneandsphericalwavefront,respectively(QOAmethod).

Theredandgraylinesonpanels(c)and(d)markthemodeleddIP(t)forwhich themaximumoffunctionCwasreached.Themomentofthemainshockofthe Wenchuanearthquakeismarkedbyblueshadedtriangles.(Forinterpretationofthe referencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionof thearticle.)

transportresults.Withthelaunchofthenextgenerationofgravity satellites,improvementofmeasurementprecision,dataprocessing methods,geophysicalmodels,and extensionof theobservation time,wewillbeabletodetectwithhigherprecisionglobalterres- trialwaterstorageandoceanmassvariationstogetmoredetailed informationofEarthsystem.

GNSShasgreatlyadvancedgeoscienceand Earthsystemsci- encesince1994,but lotsof errorsources arestill not resolved (e.g.,Jinetal.,2005;Chenetal.,2013).Inaddition,activitiesin theEarth’sinterior,suchasprocessesandstresstransferduring pre-, co- and post-earthquake, cannot be monitored. With the developmentof moreground-basedGNSS networksand multi- frequency/systemGNSSsatelliteconstellations,higherresolution and accurateparameters can, in the future, be retrieved from GNSS,including,coordinate,Earthorientationparameters,orbit, tropsphericandionosphericproducts.Withmoreandmorespace- borneGPSreflectometryandrefractometrymissionsinthenear future (e.g., follow-onFORMOSAT-7/COSMIC-2 mission, CICERO andTechDemoSat-1),therefracted,reflectedandscatteredGNSS signalswillremotelysensemoredetailedatmosphere,ocean,land, hydrologyandcryosphere(JinandKomjathy,2010).AlsotheGNSS reflectometrytogetherwithgroundobservationnetworksofseis- mologyandgeodesyareexpectedtobeappliedintoageohazard warningsystem.Moreover,GNSSreflectedsignalsmaybeusedto monitorcrustaldeformationinthesamewayasSyntheticAperture Radar(SAR)andincombinationwithothersensors,isexpectedto observeglobal-scalegeodynamicprocesses.

6. Summary

Inthispaper,themainspacegeodetictechniqueshavebeen brieﬂy introduced, including GNSS, VLBI, SLR, DORIS, InSAR, satellite gravimetry and altimetry. These techniques play key

rolesinmonitoringand understandingtheprocessesand inter- actions between the components of the Earth system with a high resolution and accuracy. Recent results of observing and understanding the Earth system variations are presented, e.g., atmospheric–ionosphericvariations,sealevelchange,hydrologic cycle,glaciermelting,crustaldeformationandloadings,geocenter motion and J₂ variations, excitation of Earth rotation and atmospheric-solidEarthcouplingetc.Furthermore,somequestions andchallengesonobservingandunderstandingtheEarthsystem variationshavebeendiscussed,aswellasfuturedevelopmentsin spacegeodesy.

Acknowledgement

This research is supported by the Main Direction Project of Chinese Academy of Sciences (Grant No. KJCX2-EW-T03), ShanghaiScienceandTechnologyCommissionProject(GrantNo.

12DZ2273300),ShanghaiPujiangTalentProgramProject(GrantNo.

11PJ1411500)andNationalNaturalScienceFoundationofChina (NSFC)Project(GrantNos.11173050and11373059).

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