Goce Newsletter Issue2

Issue 2 | March 2011

GOCE satellitelaunched on 17 March 2009

In this issue:

ForewordGOCE Mission Accomplished!GOCE Mission Status March 2011Release of the second generation GOCE gravityfield solutionsThe GOCE gravity field solutions: DIR, TIM and SPW• Improvements on the geoid model from thenew solutions

PubblicationsHow to obtain GOCE Data• Available datasets• Data Access

Processing GOCE Data• L1b and Level 2 product readers• The GUT Software suite

Staying up-to-date: the GOCE PortalGetting Help

This is the second Issue of the GOCENewsletter, following up on the first issue ofMay 2010. It is being released prior to the4th International GOCE User Workshop,

which takes place in Munich on 31 Marchand 1 April 2011, and the 2011 EuropeanGeosciences Union General Assembly in Vienna,3-6 April 2011.

ForewordAll GOCE newsletters can be obtained from theGOCE web portal: http://earth.esa.int/GOCE

GOCE Mission accomplished!On 2 March 2011, ESA’s Director of EarthObservation Programmes, Prof. Volker Liebig,announced to all GOCE users and to MemberStates delegations that GOCE has completedits nominal mission, after having completedtwelve full months of gravity field mapping.This achievement matches the nominal

GOCE satellite and its payload are in goodhealth and continue to deliver top sciencedata. On 24 November 2011, and based onthe excellent achievements of the mission sofar, ESA’s Earth Observation ProgrammeBoard approved the extension of GOCEoperations through 2012.

mission design which foresaw two so-calledMeasurement Operations Phases, each of 6months duration. The accompanying release of the second-generation gravity field products marks thismilestone.

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→ ESa’S Gravity MiSSiOn

GOCE Mission Status March 2011At the completion of its second year in orbit,on 17 March 2011, the GOCE satellite and itspayload are in excellent conditions andoperating nominally. The configuration ofboth payloads and all satellite subsystemhas remained unchanged since the issue ofthe first GOCE newsletter in May 2010. Inparticular, the orbital altitude of 254.9 kmand the respective 61 days repeat cycle

which foresaw hibernating the satelliteduring eclipses.

As already stated, the nominal mission’s lifetimeof 12 months continuous operations, hasbeen accomplished in early March. Currently,measurement cycle #7 is being executed. Theindividual measurement cycles are shiftedwith respect to each other along the equator

have remained constant since launch. Thepersistent low solar activity throughout2010, the good health of the powersubsystem and the excellent thermo-elasticstability of satellite and gradiometer haveallowed to continue science measurementsduring eclipse phases. Operating all yearround significantly increases the overall datareturn compared to pre-launch planning,

in order to decrease the distance betweenascending node crossings. An averagedistance of ascending node crossings of 10km has been achieved by now. The advantageof this strategy, compared to longer repeatcycles which would yield a denser groundcoverage pattern, lies in a reduced sensitivityto measurement interruptions due to satelliteanomalies.

Solar activity has increased in recent weeksand its evolution is being carefully monitored.The average drag levels have gone up byroughly a factor two since launch and are

and relaying data towards the antenna,completely stopped the capability to transferboth science and housekeeping data toground. Due to that anomaly no science dataare available between 8 July and 27September 2010.A second anomaly related to a softwareproblem with the GPS payload SSTI causedan interruption of the scientific measurementsbetween 2 and 19 January 2011. Both anomalies have been resolved and donot impact the successful continuation of themission even beyond the current end ofmission date in December 2012.

expected to double further until themaximum of the current solar cycle will bereached in 2013. Peaks due to solar flares willstill reach higher levels. Current predictionsindicate that it will be possible for the majorpart of 2011 to keep the orbital altitudeconstant at 254.9 km.

Two satellite anomalies leading to significantinterruptions of science measurements wereencountered since the publication of the lastnewsletter. First, a problem in the datacommunication between the main computerand the telemetry module, which is formatting


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release of the second generation GOCE gravity field solutions

The second-generation gravity field solutionshave been released on March 11, 2011.These new solutions are based on 8 months ofdata, November 2009 to July 2010 (effectively6 months after reduction of data gaps andcalibration phases). So-called direct and time-wise numericalsolutions are available, both representingsatellite-only gravity field solutions. Theproducts associated with these solutions, are: • Direct solution :

GO_CONS_EGM_GOC_2__20091101T000000_20100630T235959_0001.TGZ

• time-Wise solution:GO_CONS_EGM_GOC_2__20091101T000000_20100705T235500_0001.TGZ

A third solution, so-called space-wise solution,will be released later on.

http://earth.esa.int/object/index.cfm?fobjectid=7484 (Time-wise solution)as well as from ICGEM.

A first generation of GOCE gravity fieldmodels, based on 2 months of data (1 Nov2009 through 31 Dec 2009), had already beenreleased at ESA's Living Planet symposium inBergen, Norway, July 2010.

The accuracy improvement of the new-generation solutions over these first-generationones is relevant and, essentially, as expectedfrom the augmented statistics. in accordancewith the increase of the data amount by afactor three, the improvement on degree-errormedian is of the order of √3, over a widespectral range.

These new solutions are available free ofcharge, through the usual data services,EOLI and the GOCE Virtual Online Archive(see below). Full error variance-covariance matrices for thesemodels are available and may be downloadedfrom the GOCE Virtual Online Archive, wherea more complete description of the modelscharacteristics may also be found.

A subset of these data, the models’ gravityfield spherical harmonics coefficients, hadalready been released by ESA in February2011. They may be directly downloaded inplain ASCII format from the GOCE webportal, see: http://earth.esa.int/object/index.cfm?fobjectid=7490 (Direct solution)

the GOCE gravity field solutions: Dir, tiM and SPW

Three different solutions for the gravity fieldare being released by ESA, representing theTime-wise solution (TIM), the Direct solution(DIR) and the Space-wise solution (SPW).

The time-wise solution is inferred from GOCEdata exclusively, i.e. it does not containgravity field information through a backgroundreference model. Therefore, it is representativeof the GOCE mission performance and constitutesan independent means of comparison to othermodels. This is illustrated in Fig. 1.

The direct numerical solution, in contrast withthe time-wise one, has been constructedtaking prior gravity field information througha background reference mode. As such, itincorporates data from other satellitemissions, such as GRACE. The low degrees ofthe direct solution are consequently moreaccurate than those of the time-wise solution.

to deliver gridded data in a local geographicalreference frame. Grids may either be referredat mean satellite altitude, returning thegravitational potential and its second orderderivatives thus representing the originalinformation from Mission data, or, throughdownward continuation, at ground level,returning the diagonal component of thegravity gradient tensor. External information,such as other independent global modelsbased on satellite and/or ground data, may beeasily integrated in gridded representations.Easier integration in geophysical applicationswill also result. High resolutions may beachieved by gridding.

None of these two new solutions incorporatesurface data or airborne data in any way.

The Space-wise (SPW) model makes use ofboth satellite tracking data, derived from theon-board GPS receiver, and gravity gradientsobserved by the on-board electrostaticgradiometer. Reduced dynamic orbits are usedfor geo-locating gravity gradients. EGM2008 isused for degree variance modelling and forerror calibration of the estimated gravitationalpotential along track, thus affecting the lowdegrees of the solution. As mentioned above,the second generation SPW model has notbeen released together with the DIR andTIM models. It will be released as soon asit has been produced and validated by therelevant science teams.

During the extended mission, the processingteam behind the Space-wise solution plans

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The blue curve in Figure 1 represents thedegree error median of the current 6-monthsolution and shows the significant improvementachieved compared to the previous solution (redcurve), which was based on 2-months of data.The improvement is of the order of √3 overa wide spectral range, in accordance with the

increase of the data amount by a factor ofabout 3.It can be shown that this factor of √3 is notonly present in the formal errors, but is a realimprovement of the gravity field accuracy. Forthis purpose, gravity anomaly differencesbetween the 2-months solution (Fig. 2, left),

the new solution (Fig. 2, right),and EGM2008 up to degree/order200 have been computed. Signifi-cant noise reduction is clearlyachieved over the open oceans andregions with high-quality terrestrialgravity field data (Europe, NorthAmerica, Australia) incorporatedin EGM2008. The large differencesover the continents, which aredue to inaccurate surface dataand a non-optimum combinationof satellite and surface data inEGM2008, are presently particularlywell visible (Fig. 2, right).

The propagated geoid heighterrors to degree/order 200 are

shown in Fig. 3, the three mainfeatures of which are:• Almost no striping due tohomogeneous orbit coverage

• Asymetry due to lower meanorbit altitude in Northernhemisphere

• Increased error south of Australiadue to data interpolation

As mentioned above, the directnumerical solution is inferredfrom the GOCE data, but, contraryto the time-wise model, it hasbeen constructed taking prior

gravity field information through a backgroundreference model into account. Therefore, it is

improvements on the geoid model from the new solutions

not representative of the GOCE missionperformance only, but constitutes a modelthat is based on both GRACE and GOCE. Thelow degrees of the direct solution are conse-quently more accurate than those of the time-wise solution. A second difference is due tothe regularization method used when solvingfor the Stokes coefficients. Regularization ofthe gravity field solutions is necessary dueto the polar gaps in the data coverage, andthe direct solution applies spherical capregularization (Metzler and Pail [1]). Thisregularization approach consists in employingan analytical continuous geopotential functionusing a gravity field model.Figure 4 presents the geoid height differencesof several models with EIGEN-51C [2], whichis a recent GRACE and surface data combinedmodel. The new GOCE model by means ofthe direct approach, labelled E-GOCE-69-6 inFig. 4, is a satellite-only model that has beendetermined with 6 months of GOCE data,whereas the background model ITG-Grace2010s[3] constitutes the GRACE contribution. Thelatter model has been used in the spherical

cap regularization. The first direct approachgravity field model, labelled DIR in Fig. 4, isvery similar to EIGEN-51C because it wasused in the spherical cap regularization.Because of the strong regularization appliedand the higher resolution of the latter model,the first direct model is not a pure satellite-onlymodel and as a result compares better tosurface data than the new model. The modelslabelled TIM and SPW in Fig. 4 are the 2-monthtime-wise and space-wise gravity fieldmodels, respectively. Differences for ITG-Grace2010s are alsoshown in Fig. 4 in order to show the impactof the GOCE data especially above degree140.The importance and the effect of regularization

Fig. 3Propagated geoid height errors (degree/order200) of the new 6-month time-wise solution.

Fig. 1The degree (error) medians of the 2-monthand 6-month time-wise gravity field solutions.

Fig. 2Gravity anomaly differences between EGM2008 and the 2-month (left) and 6-month (right) time-wise gravity field models.


are demonstrated in Fig. 5 by comparing to thereference model EGM2008. The left framepresents very large differences betweenEGM2008 and the free solution, and thisill-conditioned problem due to the polar gapclearly requires regularization. The rightframe of Fig. 5 shows the differences whenregularization, in this case spherical capregularization, has been applied.

An example of external validation is shown inFig. 6, namely by comparing the models to theGerman GPS/levelling data set as a function ofmodel resolution (degree). A significantlybetter agreement of the GOCE models fordegrees higher than 150 is observed comparedto the GRACE-only model ITG-Grace2010. Thecomparison also improves for the time-wise

models when more data are assimilated, i.e.,EGM-TIM1 versus EGM-TIM2. The agreementfor the direct models is worse presently (EGM-DIR2 versus EGM-DIR1) at high degree dueto the different regularization, based onsatellite data only, that was applied. Finally,based on the increasing accuracy of the GOCEmodels with time, the agreement withGPS/levelling data set will probably be nearthe level of EGM2008 at degree/order 200at the end of the first extended mission inDecember 2012.

[1] Metzler B, Pail R (2005) GOCE dataprocessing: the Spherical Cap RegularizationApproach. Stud Geophys Geod, 49:441-462.doi:10.1007/s11200-005-0021-5[2] Bruinsma SL, Marty JC, Balmino G, BiancaleR, Förste C, Abrikosov O, Neumayer H (2010)GOCE Gravity Field Recovery by Means of theDirect Numerical Method. Paper presented atthe ESA Living Planet Symposium, 28 June –2 July 2010, Bergen, Norway[3] Mayer-Gürr T, Kurtenbach E, Eicker A(2010) The Satellite-only Gravity Field ModelITG-Grace2010s. http://www.igg.uni-bonn.de/apmg/index.php?id=itg-grace2010

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Fig. 6Validation of the 2-months(‘1’) and 6-months (‘2’)time-wise (‘TIM’) and direct(‘DIR’) solutions based onthe German GPS/levellingdata set. Results for theGRACE-only ITG-Grace2010sand reference combinationmodel EGM2008 are alsogiven for comparisonpurposes.

Fig. 4Geoid height differencesof ITG-Grace2010s, the2-month direct (DIR), the2-month time-wise (TIM),the 2-month space-wise(SPW), and the new directmodel (E-GOCE-69-6) withEIGEN-51C.

Fig. 5Geoid differences between EGM2008 andthe free (left) and spherical cap regularized

(right) solutions.


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For all data released, reports on instruments,spacecraft or processing issues affecting thequality of data may be found on the GOCEQuality Control (QC) web page:

http://earth.esa.int/GOCE/ → “Level 1b QC

The GOCE QC page provides information on the performance of GOCE's EGG and SSTIinstruments, through daily and monthlyreports.

instruments Performance and Data quality


Information are provided in several formatsand views, such as:• EGG summary tableSynthetic view for EGG data quality

• SSti summary tableSynthetic view for SST data quality

• Monthly reportsReference QC reports for all released data.

• reportsOther Reports/Announcements of generalinterest.

Fig. 7Summary table for EGG instrument. See earth.esa.int/GOCE/ → “Level 1b QC” → EGG

In-flight calibration operations for InverseCalibration Matrix determination is alwaysconnected with EGG data unavailability (24hours, usually). Kalman filter reinitializationsalso occur after calibration, affecting thefollowing half a day. Relevant data loss or corruption haveresutlted only by the major spacecraftanomalies, so far: 12/02 to 1/03/2010: CDMU-A failure andswitchover to the redundant CDMU-B. Duringthe anomaly no EGG data are available.Some of the SSTI data are missing (fordetails see the February 2010 monthly QCreport).20/03/2010: EGG control SW anomaly. A gapof 197 seconds in EGG datasets results, withconsequent, Kalman filter reinitialization.

Fig. 10Beam Out event, as seen

in common modeaccelerations data.

Fig. 8Effect on trace spetral density varying the

STR used for the attitude control

Fig. 9STR1 flagged in darkgreen, STR2 flagged

in light green

30/05/2010: Communication link betweenIPCU and CDMU occurred. This forced a fallbackto FPM.Other special events are usually related toeither a change of the STR used in the atti-tude control loop or to beam-outs from theIon Propulsion system.

November 2009 through November 2010reports are available, to date. Summary tables present a view of the overallstatus of instrument data, see Figure 7.Events are colour coded, indicating satellite outages, calibrations or special events (anynon-routine event). Special events cells areusually clickable, for access to detailed analysis.

Data gaps on EGG are most often a result ofprocessing issues on-ground.

Use of STR1 instead of STR2 in the attitudecontrol loop is a known cause of performanceworsening, limited to the lower part of themeasured bandwidth, see Figure 8.An overview the STR in use during Operationsmay be read out from Figure 9. April 2010has been the most affected period, with 23


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days of STR1 being used in the attitudecontrol loop. Beam Out events alter common modeaccelerations data, see Figure 10.Beam-out induced oscillations enter the

Fig. 11Gravity gradients PowerSpectral Density over one dayLeft panel includes effect ofbeam-out.

accurate GOCE orbits from the SSt_PSO_2 product

Ranging (SLR) measurements, which areprovided by the International Laser RangingService (ILRS). This important contributionof the ILRS to the mission is gratefullyacknowledged. The SLR residuals in Figure12 (Mean: 0.81 cm, RMS: 2.16 cm) aresorted in ascending (dusk) and descending(dawn) passes. Due to the sun-synchronousorbit, passes are always occurring duringdusk or dawn.

The SSTI delivers high-quality and continuous1 Hz GPS measurements on 12 channels.Since the very beginning of the mission, theexcellent SSTI data quality has allowed togenerate precise continuous GOCE orbits(SST_PSO) with the required accuracy. TheSST_PSO product consists of two differentorbit types, a reduced-dynamic (SST_PRD)and a kinematic (SST_PKI) orbit. The latterone is accompanied by covariance information

(SST_PCV) over a few subsequent epochs.The purpose of the reduced-dynamic orbit isto geolocate the satellite and its instrumentswith highest accuracy. The kinematic orbitdoes not depend on GOCE dynamic orbitmodels, and is thus best suitable for therecovery of the long wavelength part of theEarth’s gravity field.The validation of the GPS-derived orbits isdone with independent Satellite Laser

Fig. 12SLR residuals of the reduced-dynamic orbits (SST_PRD)

Pubblications

A special issue of the Journal of Geodesy dedi-cated to the GOCE Mission is in preparation.It will be a reference resource for all scientific

and technical output from the GOCE Missionto date. Publication of the Journal special issuewill be announced on GOCE’s website.


gradients time series, notably the Uxxcomponent, see Figure 11. Impact on data is limited to the event itself,not propagating forward. Ten Beam Outevents have occurred, so far.


How to obtain GOCE Data

Table 1AvailableGOCEproducts

Fig. 14The ESA Multi Mission

Archive EOLI-SA


Level Product Single product span

Level 1b EGG_NOM_1b 1 orbit

Description

Nominal Gradiometer Instrument data

SST_NOM_1b 1 orbit Nominal SSTI Instrument (GPS) data

SST_RIN_1b 1 orbit Nominal SSTI Instrument (GPS) data inRINEX format

EGG_NOM_2 1 day Calibrated and corrected gravity gradientsin the gradiometer reference frame

Level 2

EGG_TRF_2 1 day to 1 month Calibrated and corrected gravity gradients inthe terrestrial reference frame

SST_PSO_2 1 day Precise Science Orbits

EGM_GOC_2 > 2 months Gravity solution. First and Second Generationsolutions are available with three differentprocessing techniques: direct numericalsolution, Time-wise and Space-wise solutions.

Level 2 gravitysolutions

EGM_GVC_2 > 2 months Calibrated and corrected gravity gradients inthe terrestrial reference frame

Fig. 13The GOCE Virtual Online Archive at http://eo-virtual- archive1.esa.int/Index.html

available datasetsAvailable GOCE Level-1b and Level-2 datasets are listed in Table 1.

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parsing tool for extracting the datasets ofinterest, ESA provides two basic tools forprocessing L1b and L2 data: • XML Parser for Level 1b and Level 2 data • Basic Matlab routines for specific L1b data.

The XML parser runs on Mac, Windows andLinux platforms.

Processing GOCE Data

Both XML parser and the Matlab routines maybe downloaded from the GOCE website, underthe right-menu bar entry: “Key Resources”(see Figure 13): • http://earth.esa.int/object/index.cfm?fobjectid=7042

• http://earth.esa.int/object/index.cfm?fobjectid=7530

L1b and Level 2 product readersGOCE data conform to the Earth Explorers FileFormat, which is based on XML. Product handbooks for both Level-1b andLevel-2 products are available from the GOCEportal, containing most of the informationneeded for understanding GOCE’s dataformats and contents. While each user is free to develop its own

provide an account with ordering privileges. GOCE data will then be accessible eitherthrough GOCE’s Virtual on-line Archive(http://eo-virtual-archive1.esa.int/index.html)or through ESA’s Multi-Mission archive, EOLI-SA(see http://earth.esa.int/EOLi/EOLi.html).

From EOLI-SA, products are made availableeither as single products or as Global products,collecting one week of data each. No quotalimitation applies but orders may be limitedto a maximum of 20 products. Global (weekly)products should be used for larger orders, inplace of single products.

EOLI users receive, for each order, a link fordownloading the data via ftp, while VirtualArchive users can directly download datathrough http, without the need of submittingorders. EOLI interface offers, however, moreflexibility and tools for data selection. Variance/Covariance matrices (EGM_GVC_2)are only available through the GOCE VirtualArchive. All details and up-to-day news on GOCE dataavailability can be found on GOCE’s portal(http://earth.esa.int/GOCE).

Data accessGOCE data access is open and free of charge,in line with the new ESA Earth Observationpolicies on free datasets. More detailedinformation on ESA Earth Observation dataaccess may be found at: http://earth.esa.int/dataproducts/accessingeodata. In order to access GOCE data, users need toregister via the WWW site: http://eopi.esa.int/registration. Users may also contact the ESA’s Help andOrder Desk ([email protected]), for guidanceon the registration process. Once users are registered, ESA’s helpdesk will


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Conventions and ancillary datasets,including a priori surfaces (DEM, MSS, MDT).

• Compute geoid heights at a chosenmaximum degree and order over a grid ortransect.

• Compute gravity anomalies, height anomaliesand vertical deflections on the surface of theterrain for a range of maximum degree andorder expansions over a grid or transect.

• Compute a gridded field from its sphericalharmonic expansion.

• Compute the spherical harmonic expansionof a gridded field.

• Compute the ocean’s mean dynamictopography and the geostrophic velocities,with the option of isotropic filtering in thespatial or spectral domains.

• Transform data between different referenceellipsoid and tide-systems.

• Support configurable high-level processing.• Produce final output products in netCDF-CFformat, GRAVSOFT, KML and TIFF.

The version 2.0 will be released and distributedto users during the 4th GOCE User Workshop,featuring: • Support for IGCEM Format • Support to solid Earth community

• Support for multi-data netCDF• Anisotropic spatial filtering • Bicubic spline interpolation• Removal of scalar bias from a grid/transect• Adjustment of geoid heights in spectraldomain by height correction terms

• Handling of time-system attribute fortime-varying surfaces

• Enhancement of the statistic tools

Toolboxes will be demonstrated on the firstday of the GOCE User Workshop (31 March)and also presented on an oral presentation.GUT software can be freely downloaded fromGUT’s website: http://earth.esa.int/gut , or from the relatedlink on GOCE webpage (under “Key Resources”).

the GUt Software suiteThe GUT (GOCE User Toolbox) is the resultof the work of a team of European researchinstitutes and industries led by DanishTechnical University Space (DTU Space)under ESA Contract. The objectives of theproject are to develop a toolbox that facilitatesthe use, viewing and post-processing ofGOCE Level 2 mission data and to pursuethe research in the fields of geodesy,oceanography and solid Earth physics, whileexploiting the newly released GOCE dataset.

The GUT software is a command-lineprocessor that has been designed for users atall levels of expertise. For the novice user,pre-built workflows allow the rapidcomputation of geophysical parameters,including geoid heights, gravity anomaliesand ocean mean dynamic topographies in asingle step, with very little user inputrequired. More experienced users can generatetheir own workflows for enhanced or morespecialised processing.

The current GUT release 1.1 features:

• Reading the GOCE XML Level 2 Products(EGM_GOC_2), GRAVSOFT, netCDF in CF-

gutGOCE USER TOOLBOX

Staying up-to-date: the GOCE Portal


The GOCE portal is the single and comprehensiveaccess point for all GOCE-related information,resources and data access. Please, visit itregularly for announcements and updates.

http://earth.esa.int/GOCE

Fig. 16The GOCE main

portal athttp://earth.esa.int/

GOCE


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ESA’s Earth Observation User Services inESRIN, Italy, is the entity ensuring a user-friendly interface between the satellite systemand the data users. Services provided to usersinclude:

• Help services from the EO Help Desk team• Order Handling by the Order Desk• Mission planning and production planning • Maintenance of catalogues and ordering tools• On-line information services: (Earthnetonline, EO Portal and Disasters Charter)

All these interfaces are further described onGOCE’s web site.


Getting Help

Contact:European Space agency - goce newsletterESRIN | Frascati (Rome) | Italyhttp://earth.esa.int/goce

Copyright © 2011 European Space Agency

On behalf of the GOCE team rune Floberghagen

[email protected]

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