E S A ’ s G R A V I T Y M I S S I O N

BR-209October 2003

METEOSAT - In 1977 the first of seven Meteosat meteorological satellites was

launched to monitor the weather over Europe and Africa from a vantage

point high over the Equator. The success of the first satellites led to the

formation of the European Organisation for the Exploitation of Meteorological

Satellites (EUMETSAT) in 1986. Meteosat-7, the last in the series, was

launched in 1997.

ERS-1 and 2 - ERS-1, launched in 1991, was ESA’s first remote-sensing

satellite; it carried a comprehensive payload including an imaging Synthetic

Aperture Radar (SAR), a radar altimeter and other powerful instruments to

measure ocean surface temperature and winds at sea. ERS-2, which

overlapped with ERS-1, was launched in 1995 and added the Global Ozone

Monitoring Experiment (GOME) for atmospheric ozone level research.

ENVISAT - Launched in 2002, is the largest Earth-observation satellite ever

built. It carries 10 sophisticated optical and radar instruments to provide

continuous observation and measurement of the Earth’s oceans, land, ice

caps and atmosphere for the study of various natural and man-made

contributors to climate change. It also provides a wealth of information

needed for the management of natural resources.

MSG (Meteosat Second Generation) - MSG is a joint project between ESA

and EUMETSAT and follows up the success of the first-generation Meteosat

with satellites of higher performance. Four satellites are planned at present,

the first of which was launched in 2002.

CRYOSAT - An Earth Explorer Opportunity mission, due for launch in 2004,

will determine variations in the thickness of the Earth’s continental ice

sheets and marine ice cover. Its primary objective is to test and quantify the

prediction of thinning polar ice due to global warming.

METOP (Meteorological Operational) - Europe’s first polar orbiting satellite

system dedicated to operational meteorology. MetOp is a series of three

satellites to be launched sequentially over 14 years, starting in 2005, and

forms the space segment of EUMETSAT’s Polar System (EPS).

GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) - An Earth

Explorer Core mission, due for launch in 2006, will provide the data set

required to accurately determine global and regional models of the Earth’s

gravity field and the geoid. It will also advance research in the fields of

steady-state ocean circulation, physics of the Earth’s interior, geodesy and

surveying and sea-level change.

ADM-Aeolus (Atmospheric Dynamics Mission) - An Earth Explorer Core

Mission, due for launch in 2007, will make novel advances in global wind

profile observation. It is intended to provide much-needed information to

improve weather forecasting and climate research.

SMOS (Soil Moisture and Ocean Salinity) - An Earth Explorer Opportunity

Mission, due for launch in 2007, will provide global observations of soil

moisture and ocean salinity to improve modelling of the weather and climate.

It will also monitor vegetation water content, snow cover and ice structure.

Further information can be obtained via the ESA Observing the Earth and Living Planet web sites at:

E S A ’ s E A R T H O B S E R V A T I O N M I S S I O N S

Prepared by:

Roger Haagmans, Rune Floberghagen

& Bernd Pieper

Published by:

ESA Publications Division

ESTEC, PO Box 299

2200 AG Noordwijk

The Netherlands

Editors: Honora Rider, Michael Rast &

Bruce Battrick

Design & Layout:

AOES Medialab & Carel Haakman

Illustrations: AOES Medialab

Price: 10 Euros

ISSN No.: 0250-1589

ISBN No.: 92-9092-663-5

http://www.esa.int/earthobservation

http://www.esa.int/livingplanet

G O C E :G r a v i t y F i e l d a n d S t e a d y - S t a t e O c e a n C i r c u l a t i o n E x p l o r e r

Contents

GOCE: Gravity Field and Steady-State Ocean Circulation Explorer

The Earth’s gravity field

Why we need to map the Earth’sgravity field

GOCE gets the low-down on gravity

The instruments

The satellite

From Newton to GOCE

GOCE overview

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GOCE:Gravity Field and Steady-StateOcean Circulation Explorer

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Gravity is a fundamental force of nature that influences manydynamic processes within the Earth’s interior, and on and aboveits surface. It is sometimes assumed that the force of gravity onthe surface of the Earth has a constant value, but this is actuallynot the case and would only be true if the Earth were a perfectsphere, and if each layer of material within the interior wereevenly distributed. In reality, the shape of the Earth is slightlyflattened because as our planet rotates, centrifugal force tendsto pull material outwards around the Equator where thevelocity of rotation is at its highest. This results in the Earth’sradius being 21 km greater at the Equator compared to thepoles, which is the main reason why the force of gravity isweaker at the Equator than it is at the poles. Rather than beingsmooth, the surface of the Earth is relatively ‘lumpy’ iftopographical features such as mountain ranges are taken into

consideration. In fact there is about a 20 km difference in heightbetween the highest mountain and the deepest part of theocean floor. Also, the different materials that make up the layersof the Earth’s crust and mantle are far from homogeneouslydistributed, and even the depths of the layers vary from place toplace. For instance, the crust beneath the oceans is a lot thinnerand denser than the continental crust. It is factors such as thesethat cause the gravitational force on the surface of the planet tovary significantly from place to place.

Although terrestrial data on the subject of gravity have beencollected for many years now, the advent of the space age hasprovided the opportunity to acquire a detailed map of theglobal gravity field within a short period of time. GOCE(Gravity Field and Steady-State Ocean Circulation Explorer) isan ESA mission dedicated to measuring the Earth’s gravity fieldand modelling the geoid, with extremely high accuracy andresolution. An improved knowledge of gravity anomalies willcontribute to a better understanding of the Earth’s interior,such as the physics and dynamics associated with volcanismand earthquakes.

The geoid, which is defined by the Earth’s gravity field, is asurface of equal gravitational potential. It follows a hypotheticalocean surface at rest, in the absence of tides and currents. Aprecise model of the Earth’s geoid is crucial in deriving accuratemeasurements of ocean circulation, sea-level change and

terrestrial ice dynamics, all of which are affected byclimate change. The geoid is also used as a referencesurface from which to map all topographicalfeatures on the planet, whether they belong to land,ice or the ocean - a reference that can be used, forexample, to accurately compare height measurementsof features such as mountain ranges on differentcontinents.

GOCE is the second Earth Explorer satellite to bedeveloped as part of ESA’s Living Planet Programmeand is scheduled for launch from Russia in 2006.The gravitational signal is stronger closer to theEarth, so GOCE has been designed to fly in a verylow orbit. However, the ‘atmosphere’ at low altitudescreates a very demanding environment for thesatellite in terms of resources such as propellant andelectrical power, and therefore data will be collectedfor just 20 months. This is sufficient time to gatherthe essential data to advance our knowledge of theEarth’s gravity field and geoid, which will be vital forthe next generation of geophysical research and will contribute significantly to furthering ourunderstanding of the Earth’s climate.

Papendrecht, The Netherlands 1953 flood

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GOCE will contribute significantly to a better understanding of general ocean circulation patterns

The geoid (EGM96 model):The geoid represents deviations

from the Earth’s ellipsoid.Although it has ‘highs’

and ‘lows’ it is a surfaceof equal gravitationalpotential. Therefore, if an object were placedon this surface it would

not move. The surface of the geoid defines the

horizontal.We can now measurehow g varies to morethan 8 decimal places, butwhat causes these small butsignificant changes?

The most significant deviation from the standard value of g is aresult of the Earth’s rotation. As the Earth spins, its shape isslightly flattened into an ellipsoid, so that there is a greater

distance between the centre of the Earthand the surface at the Equator, than thecentre of the Earth and the surface at thepoles. This greater distance results in the

force of gravity being weaker at the Equator than at the poles.Secondly, the surface of the Earth is very uneven; high mountainsand deep ocean trenches cause the value of gravity to vary.Thirdly, the materials within the Earth’s interior are not uniformlydistributed. Not only are the layers within the crust and mantleirregular, but also the mass distribution within the layers isinhomogeneous. Petroleum and mineral deposits or ground-water reservoirs can also subtly affect the gravity field, as can a risein sea level or changes in topography such as ice-sheet movementsor volcanic eruptions. Even large buildings can have a minoreffect. Of course, depending on location, many of these factors aresuperimposed upon each other, and can also change with time.

When thinking of gravity, most of us conjure up an image of SirIsaac Newton observing an apple falling from a tree more than300 years ago. The mass of the Earth is greater than that of theapple; therefore the apple falls towards the Earth and not viceversa. It was Newton who established the basic principles ofgravitation, and the concept more commonly known as the ‘g’force. This force is the result of the gravitational attractionbetween any two objects as a consequence of their mass andtheir separation.

At school we generally learn that g = 9.8 m/s2. Indeed this valuefor gravitational acceleration was for a long time assumed to beconstant for the entire planet. However, as more sophisticated andsensitive tools have been developed to measure g, it has becomeapparent that the force of gravity actually varies from place toplace on the surface of the planet. The standard value of 9.8 m/s2

refers to the Earth as a homogeneous sphere, but in reality thereare many reasons for this value to range from a minimum of9.78 m/s2 at the Equator to a maximum of 9.83 m/s2 at the poles.

The irregular gravity field shapes a virtual surface at mean sealevel called the ‘geoid’. This is the surface of equal gravitationalpotential of a hypothetical ocean surface at rest and is oftenemployed as a reference for our traditional height systems,used for levelling and construction. The surface of the geoid can deviate by as much as 100 m from an ellipsoidal modelrepresenting the Earth.

“Every body attracts every other body with a force directly proportional tothe product of their masses and inversely proportional to the square of thedistance between them".

Sir Isaac Newton (1642-1727).

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The Earth’s gravity fieldDid you know that:

- The word ‘gravity’ is derived from the Latin word ‘gravitas’,meaning ‘weight’.

- ‘Mass’ is a measure of how much matter an object has, and‘weight’ is a measure of how strongly gravity pulls on that matter.

- You weigh less at the Equator than you do at the North Pole.

- Dutchman Christiaan Huygens invented the pendulum clock in1656, which was the first practical means of measuring ‘therate of falling of a heavy body’, or gravity. French astronomersnoted afterwards that pendulum clocks lost time near theEquator when compared with observations made in Paris,implying that the intensity of gravity was less at low latitudes- this observation initiated a long debate on the shape of theEarth.

- In a vacuum, all objects have the same acceleration due togravity because there is no air drag. Therefore, if a rock and afeather were dropped off the top of a building at exactly thesame time, they would hit the ground simultaneously.

- Although invisible to the naked eye, the sea surface hasundulations that echo the topography of the ocean floor - on areduced scale. For example, the extra mass of a 2 km-highseamount attracts water over it, causing a ‘bulge’ in the seasurface about 2 m high and about 40 km across. Similarly, thereduced gravity over trenches in the sea floor means that lesswater is held by gravitational attraction over these regions, sothat locally the sea surface is depressed.

- The word ‘tide’ is a generic term used to define the alternatingrise and fall in sea level with respect to the land, as a result ofthe gravitational attraction of the Moon and the Sun. To amuch lesser extent, tides also occur in large lakes, theatmosphere, and within the solid crust of the Earth. Forexample, in some places the Earth's crust can rise and fall upto a total of 30 cm every day.

- Birds would choke and die in a zero-gravity environmentbecause they need gravity to help them swallow!

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As a result of the ice, the water level ‘at rest’ can be used to transfer heights

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Why we need to mapthe Earth’s gravity field

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Most of us take the Earth’s continuous pull of gravity for granted.Although invisible, it provides us with the sense of the horizontaland the vertical. However, if gravity is studied in detail weappreciate how complex this force actually is. We are currentlyaiming to achieve a better understanding of the Earth’s gravityfield and its associated geoid. This will significantly advance ourknowledge of how the Earth works and also have a number ofimportant practical applications. For instance, because gravity isdirectly linked to the distribution of mass within the Earth, animproved gravity-field map will provide an insight into thephysics and dynamics of the Earth’s interior. An accurate globalgeoid model will contribute to an improved understanding ofocean circulation, which plays an important role in energyexchanges around the globe. It will also lead to a greater insightinto sea-level change, and to a global unification of height systems,so that for example, mountain ranges in America can be measuredagainst those in Europe or Africa.

Ocean CirculationOcean circulation plays a crucial role in climate regulation bytransporting heat from low to high latitudes in surface waters,while currents cooled at high latitudes flow in deeper waters backtowards the Equator. The Gulf Stream, which carries warm surfacewaters northwards from the Gulf of Mexico, is a good example ofhow important ocean currents are in redistributing heat. Thanksto this current, the coastal waters of Europe are actually 4°Cwarmer than waters at equivalent latitudes in the North Pacific.However, knowledge of the role that the oceans have inmoderating the climate is currently insufficient for the accurateprediction of climate change. In order to study ocean circulationmore effectively it is necessary to have an accurate map of theEarth’s geoid. The GOCE derived model of the geoid will serve asa reference from which to study circulation patterns of not onlysurface waters but also deeper currents. Radar altimetry providesus with the actual shape of the ocean surface and then bysubtracting the shape of the geoid this provides the oceantopography. Surface ocean circulation can then be derived directlyfrom these ‘1-2 metre mountains’.

Solid EarthHow are mountain roots formed? Where are oil fields and oredeposits hidden? Why do plates in the Earth’s crust move andcause earthquakes? Where does lava rise through the crust to thesurface of the Earth? Detailed mapping of density variations in the lithosphere andupper mantle, down to a depth of 200 km, derived from a

combination of the gravity field map and seismic data will providean improved understanding of processes such as these. GOCE willalso further our knowledge of land uplift due to post-glacialrebound. This process describes how the Earth’s crust is rising inplaces as it has been relieved of the weight of thick ice sheets sincethe last Ice Age, when the heavy load caused the crust to depress.Currently, Scandinavia is rising at rates of up to 1 cm per year, andCanada is rising at rates of up to 2 cm a year.

GeodesyGeodesy is concerned with mapping the shape of the Earth, to thebenefit of all branches of Earth sciences as well as havingnumerous practical applications. For example, an improved geoidwill be used for levelling and construction, ensuring for instancethat water flows in the direction intended. It will replace expensiveand time-consuming procedures that are currently undertaken. Itwill also be used for a high-accuracy global height reference systemfor studying the topography of the Earth. This will also facilitateone global system for tide gauge records, so that sea levels can becompared all over the world.

Sea-level ChangeData derived from the GOCE mission will contribute to observingand understanding sea-level change as a result of meltingcontinental ice-sheets associated with a changing climate andpostglacial rebound.

Mean dynamic ocean topography model

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1 - Dams typically built as protection from the sea, theoperation of which depends upon the prevailing sealevel

2 - Expensive and time-consuming levelling, which GOCEcan replace by the combination of GPS and anadvanced map of the geoid

3 - The ancient use of an aqueduct relied upon gravity totransport water

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GOCE gets the low-downon gravityThe traditional method of measuring the Earth’s gravity field isbased on comparing the differences in how a ‘test mass’, such asa pendulum or gravimeter, responds to the gravitationalattraction at different locations on the surface of the Earth.Although data on gravity have been collected from many partsof the world by traditional means, they vary considerably inquality and are also incomplete. The process of acquiring dataon land and at sea is very time-consuming and expensive -global coverage with data of a consistent quality would takedecades. With the need for a global map of the Earth’s gravityfield, coupled with recent advances in the accuracy of thenecessary instrumentation, the obvious solution is to make theappropriate observations from space.

One of the main problems with observing gravity from space isthe fact that the strength of the Earth’s gravitational attractiondiminishes with altitude. The orbit of the satellite musttherefore be as low as possible to observe the strongest gravityfield signal. However, the lower the orbit the more air-drag thesatellite experiences. This causes disturbances in the motion ofthe satellite that therefore cannot be attributed to gravity alone.The effects of non-gravitational accelerations caused by air-drag have to be kept to a minimum and accounted for in orderto derive the best possible gravity-field model. As a compromisebetween gravity attenuation and the influence of theatmosphere, the optimum orbital altitude for GOCE is about250 km, which is considerably lower that that of other remote-sensing satellites. However, the details of the large fluctuations

in the gravity signal such as in the Indonesian Archipelagowould not easily be read in the diminished signal retrieved atthis altitude. In order to counteract the attenuation effect andto amplify the gravity signal GOCE is equipped with agradiometer instrument. The gradiometer contains six proofmasses capable of observing detailed local changes ingravitational acceleration in three spatial dimensions withextremely high precision. By capturing changes in localvariations of the gravity field at satellite height, the ‘blurringand flattening’ of the signal is largely counteracted. In effect, thegradiometer ‘zooms-in’ on details of the gravity field fromsatellite height.

Although the gradiometer is highly accurate, it is not possibleto map the complete gravity field at all spatial scales with thesame quality. To overcome this limitation the position of theGOCE satellite is tracked by GPS relative to GPS satellites at analtitude of 20 000 km - this procedure is known as satellite-to-satellite tracking. The gradiometer is used to measure high-resolution features of the gravity field whilst GPS is used toobtain low-resolution data.

The combination of these two principles provides theopportunity to derive a global geoid model with a 100 kmspatial resolution and 1 cm accuracy. A model for gravity can bedetermined with a precision of 0.000001 g (the standard valueof g = 9.8 m/s2) at a spatial resolution of 100 km or better.

The gradiometer contains three pairs of proof masses positioned at the outer ends ofthree, 50 cm long orthogonal axes. All experience the gravitational acceleration of theEarth slightly differently because of their different positions in the gravitational field. Thecommon acceleration of each pair of accelerometers is proportional to non-gravitationalforces such as air-drag acting on the satellite. The difference in acceleration for eachpair, scaled with the length of the connecting arm, called the gravity gradient, is used forgravity field analysis. An angular acceleration correction is needed however, since theinstrument rotates around one axis during each orbital revolution around the Earth.

The influence of gravity on the instrument is the compound effect of all constituents of g- for example, the Himalayas have a small influence on the value of g sensed in Europe.It takes the art of mathematical gravity inversion to disentangle the compound effect andidentify each individual contribution.

The 3 axes of the GOCE gradiometer allow the simultaneous measurement of 6 independent but complementary components of the gravity field. Shown here are global maps of residual gradientsreferring to an ellipsoidal model of the Earth.1 E (Eötvös) = 10-9 s-2

Accelerometer pair1

Ultra-stable carbon-carbon structure

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Panel regulated by heaters4

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The instruments

An advanced gravity mission such as GOCE requires that thesatellite and the system of sensor and control elements formone ‘gravity-measuring device’; this is because the satellite itselfalso acts as a prime sensor. In other words, in contrast to mostremote-sensing missions, there is virtually no division betweenthe satellite and the instruments.

The GOCE concept is unique in meeting four fundamentalcriteria for a high-resolution and high-accuracy gravity-fieldmission, namely:

- uninterrupted tracking in three spatial dimensions- continuous compensation for the effect of non-gravitational

forces such as air-drag and also radiation pressure- selection of a low orbital attitude for a strong gravity signal,

and- counteraction of the gravity-field attenuation at altitude by

employing satellite gravity gradiometry.

From a scientific standpoint, these are the building blocks ofthe GOCE mission. In general, the lower the orbit and thehigher the gravity signal, the more demanding are therequirements on all subsystems, as well as on the structure ofthe spacecraft and on the choice of materials. In turn, thesebuilding blocks dictate the choice of a related set of technicalsolutions for the instruments, sensors and actuators, namely:

- an onboard GPS receiver used as a Satellite-to-SatelliteTracking Instrument (SSTI)

- a compensation system for all non-gravitational forces actingon the spacecraft, including a very sophisticated propulsionsystem, and

- an Electrostatic Gravity Gradiometer (EGG) as the maininstrument.

The Satellite-to-Satellite TrackingInstrument (SSTI)The Satellite-to-Satellite Tracking Instrument (SSTI) consists ofan advanced dual-frequency, 12-channel GPS receiver and anL-band antenna. The SSTI receiver is capable of simultaneouslyacquiring signals broadcast from up to 12 spacecraft in the GPSconstellation. The SSTI instrument delivers, at 1Hz, so-calledpseudo-range and carrier-phase measurements on both GPSfrequencies, as well as a real-time orbit navigation solution.

The drag-free and attitude-control systemThe advanced drag compensation and attitude-control systemis a key feature required to keep the sensor heads in near ‘freefall motion’ and to maintain the average orbital altitude atabout 250 km. The system is based on ion-propulsiontechnology. A particular feature of the GOCE system design isthat the drag-free and attitude-control system uses the scientificpayload as a sensor. In addition to the fusion between ‘satellite’and ‘measuring device’ in terms of scientific data quality, thereis therefore also some ‘fusion’ between the satellite system andthe measuring device in terms of how the GOCE satellite willactually operate.

The Electrostatic Gravity Gradiometer(EGG)The principle of operation of the gradiometer relies onmeasuring the forces that maintain a ‘proof mass’ at the centreof a specially engineered ‘cage’. Servo-controlled electrostaticsuspension provides control of the ‘proof mass’ in terms oflinear and rotational motion. Three pairs of identicalaccelerometers, which form three ‘gradiometer arms’, aremounted on the ultra-stable structure. The difference betweenaccelerations measured by each pair of accelerometers is thebasic gradiometric datum, where half the sum is proportionalto the externally induced drag acceleration (common modemeasurement). The three arms are mounted orthogonally toone another: one aligned with the satellite’s trajectory, oneperpendicular to the trajectory, and one pointingapproximately towards centre of the Earth. By combining thedifferential accelerations, it is possible to derive the gravitygradient components as well as the perturbing angularaccelerations.

GOCE Facts

- The accelerations measured by each accelerometer can be as small as 1 part in10,000,000,000,000 of thegravity experienced on Earth.

- The GOCE accelerometers areabout a factor of 100 moresensitive than previously flownaccelerometers.

- In order to guide the satellite ina smooth trajectory around theEarth, free from all effectsexcept the gravity field itself, ithas to be equipped with a dragcompensation and angular-control system. All non-gravitational forces need to berejected to 1 part in 100,000.The angular control requires theorientation of the spacecraft tobe known relative to the stars(star trackers). The drag controlrequires the position of thespacecraft to be known bySSTI/GPS.

- The satellite is aligned to thevelocity vector, which isdetermined in real-time by theSSTI.

- The satellite is very rigid andhas no moving parts in order toavoid any internal microscopicdisturbances that wouldotherwise cause ‘false gravity’readings by the highly sensitiveaccelerometers.

Gradiometer instrument excluding harness

Gradiometer instrument including a radiator for thermal control

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Orbit Altitude Eclipse Duration

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Spacecraft Commissioning 1.5 month

Gradiometer Setup and Calibration 1.5 month

First Measurement Phase 6 months

Measurement Interruption 4.5 months

Gradiometer Calibration 1.5 month

Second Measurement Phase 6 months

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The satellite

ConfigurationUnlike other missions, where various independent instrumentsare carried onboard a satellite, GOCE is unique in that theinstrumentation actually forms part of the structure of thesatellite. The satellite has no mechanical moving parts becauseit has to be completely stable and rigid to ensure the acquisitionof true gravity readings. In order to receive the optimumgravity signal, GOCE has been designed to fly in a particularlylow orbit - just 250 km above the Earth. The satellite isconfigured to keep aerodynamic drag and torque to an absoluteminimum and allow for the flexibility of either a dusk/dawn ordawn/dusk orbit, depending upon the actual launch date. Themass and volume of the satellite have been limited by thecapacity of the launch vehicle.

The result is a slim, octagonal, 1200 kg satellite about 5 metresin length with a cross-sectional area of 1m2. The satellite issymmetrical about its flight direction and two winglets provideadditional aerodynamic stability. The side of the satellite thatfaces the Sun is equipped with four body-mounted, and twowing-mounted solar panels, using the maximum availablevolume under the fairing. The gradiometer is mounted close tothe centre of mass.

StructureThe satellite consists of a central tube with seven internal floorsthat support all the equipment and electronic units. Two of thefloors support the gradiometer. To ensure that the satellite is aslight as possible and that the structure is stable under varyingtemperatures, it is built largely of carbon-fibre-reinforcedplastic sandwich panels.

Thermal ControlThe thermal design has to cope during nominal measurementmodes with eclipses lasting up to 10 minutes, and duringsurvival modes with eclipses lasting up to 30 minutes. It alsohas to be compatible with either a dawn/dusk or a dusk/dawnorbit. Thermal control is achieved mainly by passive meanssuch as coatings and blankets and active control by heaterswhere necessary. The internal equipment is protected againstthe hot temperatures of the solar panels by multi-layerinsulation blankets, which are positioned between the solarpanels and the main body of the satellite. The cold side thatfaces away from the Sun is used extensively as a radiator area todissipate heat into space. All the external coatings, in particularthose situated in the direction of flight, are protected against

high atomic oxygen flux, which would otherwise erodeunprotected materials very quickly in this low orbit. Due to itsstringent temperature stability requirements (for thegradiometer core, in the range of milli-degrees Kelvin withinthe measurement bandwidth) the gradiometer is thermallydecoupled from the satellite and has its own dedicated thermalcontrol system. An outer active thermal domain is kept at a verystable temperature by heaters and is separated by blankets froman inner passive domain, thus providing an extremelyhomogenous environment for the accelerometers.

Power SubsystemThe solar-array panels, which use highly efficient gallium-arsenide solar cells, provide the necessary power for thesatellite. At times when the solar array is not illuminated by theSun, i.e. during the launch and early orbit phase and during theeclipse periods, a lithium-ion battery delivers the powerrequired. A power control and distribution unit maximises thepower output from the solar array and controls the charge anddischarge processes of the battery.

Due to the satellite configuration and orbit, the solar panels willexperience extreme temperature variations. It has thereforebeen necessary to use materials that will tolerate temperaturesas high as 160°C and as low as -170°C.

ESA’s quiet mission

It is vital to this mission that the measurements taken are of truegravity and not influenced by any relative movement of thesatellite. Therefore, strict requirements apply to the dimensionalstability of the structure. In addition, any micro disturbancescaused by moving parts, sudden stress releases due todifferential thermal expansions, inductive electromagneticforces, gas and liquid flows, etc. are eliminated or minimised tothe greatest extent possible. A comprehensive test programme,covering structural items, electronic units and the multi-layerinsulation blankets, will verify potential noise sources, whichcould affect the gradiometric measurements.

Launch vehicle

GOCE will be launched by one of the modified RussianIntercontinental Ballistic Missile (ICBM) SS-19 launchers,which are being decommissioned in the process of theStrategic Arms Reduction Treaty (START). The adaptationof the SS-19, called ‘Rockot’, uses the original two lowerliquid propellant stages of the ICBM, which has anexcellent record of successful flights, in conjunction witha new third stage for commercial payloads. Rockot ismarketed and operated by EUROCKOT, a German-Russianjoint venture. Launch is from the Plesetsk Cosmodrome innorthern Russia.

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From Newton to GOCE

Our present-day understanding of gravity is based upon IsaacNewton’s Philosophiae Naturalis Principia Mathematica (1687).Over the last 300 years many instruments have been developedand many experiments conducted to observe local, regional andglobal characteristics of the Earth’s gravity field. Until a fewdecades ago, terrestrial gravity variations were mainly observedwith ‘relative’ gravimeters, but more recently high-precision‘absolute’ gravimeters have been developed. An absolutegravimeter uses the principle of accurately measuring the timeit takes for a prism to free-fall inside a tube. Nowadays aprecision of 0.000000005 g (g = 9.8 m/s2) can be obtained. Thecombination of relative and absolute measurements allows thepossibility of accurately connecting gravity surveys in differentlocations. The areas of the world that have been extensivelysurveyed may contribute to the validation of the results derivedfrom the GOCE satellite.

From Newton’s theory we know that any satellite in the Earth’sorbit is actually free falling in the Earth’s gravity field. Thetraditional technique of gravity-field determination fromsatellite trajectories led to the development of global gravity-field models, albeit only representing large-scale features. In theearly 1960’s the concept of satellite-to-satellite tracking wasdeveloped. This technique is based upon tracking the orbitdifferences between satellites that experience differentgravitational accelerations at different locations. The concept ofgradiometry followed soon after and is based upon analysingthe difference in gravitational acceleration of proof masseswithin one satellite. In fact, the GOCE gradiometer is based

Gravity data in the polar region have beencollected as part of the Arctic GravityProject over the last few years. During theEuropean Survey of Arctic Gravity (ESAG)airborne campaign, which was supportedby ESA, gravity data were collected in May2002 from the north of Greenland andCanada. This data is used for calibrationand validation of the GOCE mission

upon principles similar to those used by Loránd Eötvös (1848-1918), who developed the ‘torsion balance’ that could observelocal horizontal gravity-gradients.

CHAMP, a German satellite launched in 2000, exploits thesatellite-to-satellite tracking concept between a low orbitingsatellite at an altitude of approximately 400 km and high GPSsatellites at an altitude of around 20,000 km. This has led to animpressive improvement in global gravity models for featuresof up to a few thousand kilometres. For example, there has beena significant improvement in the data from polar regions,which are traditionally difficult to access.

GRACE, a US-German satellite, launched in 2002, makes use ofsatellite-to-satellite tracking between two low-flying satellites

about 200 km apart, at an altitude of 400 km. In addition, bothsatellites track signals transmitted by the GPS constellation. Aswith CHAMP non-gravitational forces are measured by microaccelerometers. The GRACE mission aims to map monthlychanges in the gravity field for features down to 600 -1000 km,with high accuracy.

GOCE exploits a combination of the principle of gradiometryand satellite-to-satellite tracking relative to GPS satellites. Thedata derived from GOCE will provide an unprecedented modelof the Earth’s gravity field. The GOCE mission iscomplementary to the aforementioned missions in that it is ahigh-resolution gravity-field mission and will address acompletely new range of spatial scales, in the order of 100 km.

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E S A ’ s L I V I N G P L A N E T P R O G R A M M E

GOCE MissionThe Gravity Field and Steady-State Ocean Circulation Explorer(GOCE) mission will measure high-accuracy gravity gradientsand provide global models of the Earth’s gravity field and of the geoid. The geoid (the surface of equal gravitationalpotential of a hypothetical ocean at rest) serves as the classicalreference for all topographical features. The accuracy of itsdetermination is important for surveying and geodesy, and instudies of Earth interior processes, ocean circulation, icemotion and sea-level change.

GOCE was selected in 1999 as an Earth Explorer Core missionas part of ESA’s Living Planet Programme.

Mission Objectives- To determine the gravity-field anomalies with an accuracy

of 1 mGal (where 1mGal = 10-5 m/s2).- To determine the geoid with an accuracy of 1 cm.- To achieve the above at a spatial resolution better than 100 km.

Mission DetailsLaunch: 2006Duration: Nominally 20 months, including a nominal 3-month commissioning and calibration phase and twoscience measurement phases (each lasting 6 months),separated by a long-eclipse hibernation period.

Mission OrbitSun-synchronous, near circular, dawn/dusk or dusk/dawn lowEarth orbit.Inclination: 96.5°Measurement altitudes: 250 km and 240 kmHibernation altitude: 270 km

Configuration Rigid structure with fixed solar wings and no moving parts.Octagonal spacecraft body approximately 1 m diameter ˘ 5 mlength. Cross-section minimised in direction of flight toreduce drag. Tail fins act as passive stabilisers.

Payload- Gradiometer; 3 pairs of 3-axis, servo-controlled, capacitive

accelerometers (each pair separated by a distance of 0.5 m).- 12-channel GPS receiver with geodetic quality.- Laser retroreflector enabling tracking by ground-based lasers.

GOCE overviewSatellite Attitude Control - 3-axis stabilised- Drag-Free and Attitude-Control System (DFACS) comprising:

- Actuators - an ion thruster assembly (xenon propellant)and magnetotorquers.

- Sensors - star trackers, a 3-axis magnetometer, a digitalSun sensor and a coarse Earth and Sun sensor.

BudgetsMass: <1250 kg (including 40 kg xenon fuel).Power: 1300 W (solar-array output power) including a 78 Ah li-ion battery for energy storage.Telemetry and Telecommand: S band (2kbit/s up-link;850 kbit/s down-link).

Launch VehicleRockot (converted SS-19), from Plesetsk, Russia.

Flight OperationsMission control from European Space Operations Centre(ESOC) via Kiruna ground station.

Satellite ContractorsIndustrial Core Team:- Alenia Spazio (Satellite Prime Contractor)- EADS Astrium GmbH (Platform Contractor)- Alcatel Space Industries (Gradiometer)- ONERA (Accelerometers & System Support)

The GOCE mission uses a single ground station in Kiruna, Sweden, to exchangecommands and data with the ground.The satellite is monitored and controlled by the Flight Operations Segment (FOS). The FOSgenerates and uplinks the commands to programme GOCE operations via Kiruna. It alsoprocesses the housekeeping data received from the satellite to monitor the status of theplatform and the instruments.The generation of the scientific products of the GOCE mission is done by the Payload DataSegment (PDS), which also receives the GOCE science data via Kiruna. The final productsare distributed to the GOCE user community to be used for different applications.

Flight Operations Segment (FOS)

Payload Data Segment (PDS)

GOCE User Community

Contact: ESA Publications Divisionc/o ESTEC, PO Box 299, 2200 AG Noordwijk, The NetherlandsTel. (31) 71 565 3400 - Fax (31) 71 565 5433