The GOCE Gravity Mission by yurtgc548

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THE GOCE GRAVITY MISSION: ESA’S FIRST
CORE EARTH EXPLORER

Mark R. Drinkwater, R. Haagmans, D. Muzi, A. Popescu, R. Floberghagen, M. Kern
and M. Fehringer




Published in

Proceedings of the 3rd International GOCE User Workshop, 6-8 November, 2006, Frascati, Italy,
ESA Special Publication, SP-627, ISBN 92-9092-938-3, pp.1-8, 2007.




Citation:

Mark R. Drinkwater, R. Haagmans, D. Muzi, A. Popescu, R. Floberghagen, M. Kern and M. Fehringer, The
GOCE Gravity Mission: ESA’s First Core Earth Explorer, Proceedings of 3rd International GOCE User
Workshop, 6-8 November, 2006, Frascati, Italy, ESA SP-627, ISBN 92-9092-938-3, pp.1-8, 2007.
2


         THE GOCE GRAVITY MISSION: ESA’S FIRST CORE EARTH EXPLORER

                 Mark R. Drinkwater(1), R. Haagmans(1), D. Muzi(2), A. Popescu(2), R. Floberghagen(2),
                                           M. Kern(1) and M. Fehringer(2)

         (1)
               Mission Science Division, European Space Agency, ESTEC, 2200 AG Noordwijk, The Netherlands
                                             Email: mark.drinkwater@esa.int
                 (2)
                    GOCE Project, European Space Agency, ESTEC, 2200 AG Noordwijk, The Netherlands



ABSTRACT

The Gravity field and steady-state Ocean Circulation          first of these two missions, with a launch scheduled in
Explorer Mission (GOCE) will be the first Core Earth          2007 [3].
Explorer mission in the context of ESA’s Living Planet
programme. Currently scheduled for launch in 2007,
GOCE will measure highly accurate, high spatial
resolution gravity gradients in three dimensions along a
well characterised orbit. The mission objectives are to
obtain gravity gradient data such that new global and
regional models of the static Earth’s gravity field and of
the geoid can be deduced with high spatial resolution
and accuracy. The goal is to achieve an accuracy of
1mGal for gravity anomalies and 2cm for the geoid at
length scales down to 100km. Such an advance in the
existing knowledge of the Earth’s gravity field will help
develop a more comprehensive understanding of the
physics of the Earth’s interior, the interaction between
continental plates and the ocean circulation. Further,
GOCE products will have broad application in the fields
of geodesy, oceanography, solid-earth physics and
glaciology.                                                        Figure 1. Artist’s Impression of the GOCE Satellite

1.   INTRODUCTION

The “Living Planet Programme” [1,2] defines the               2.     MISSION OBJECTIVES
European Space Agency’s (ESA’s) strategy and plans
for satellite Earth Observation (EO) in the 21st century.     The Earth’s gravity field is a fundamental physical force
Its establishment in the late 1990’s marked the               for every dynamic process on its surface and it’s
beginning of an era in which European EO missions are         interior. Since the start of the satellite era, the
smaller and more focussed than their predecessors (e.g.       determination of the global gravity field and the
ERS-1, -2, Envisat and MetOp). The programme is               associated geoid (i.e. the reference equipotential
user-driven in terms of addressing science and research       surface) has been considered a high priority goal. With
community measurement requirements with the Earth             GOCE we are aiming to achieve a significant step in
Explorer series of missions. The main objectives are to       characterisation of the high-resolution static component
further develop our knowledge of the complex Earth            of the Earth’s gravity field [4, 5, 6]. The new knowledge
system; to preserve the Earth and its environment and         which is accrued will help advance our present
resources; and to provide information with which to           understanding of how the Earth works and will have a
more efficiently and effectively manage life on Earth.        number of important practical applications.

Out of nine Earth Explorer core missions proposed in          Figure 2 shows the accuracy required by GOCE to
the first Call for Core Explorers (i.e. Announcement of       improve the geoid and gravity field to the point where
Opportunity) in 1996, two missions were ultimately            significant improvements can be expected in
selected for implementation in 1999. These were the           oceanography, solid-earth physics and geodesy
Gravity field and steady-state Ocean Circulation              applications of the data. It also shows the status of
Explorer (GOCE) and the Atmospheric Dynamics                  gravity field knowledge at the point in time when the
Mission (ADM-Aeolus), respectively. GOCE will be the          GOCE mission was proposed (see EGM96 curve)
                                                                                                                      3



                                                             important role in energy exchanges around the globe
 (a)                                                         (Figure 2a). Similarly, a higher-resolution gravity-field
                 EGM96                                       map of the anomalous density structure of the
                                                             lithosphere and upper mantle will provide new insights
                                                             into the physics and dynamics of processes in zones
                                                             impacted by natural hazards such as volcanoes and
                                                             earthquakes (Figure 2b). Such information will provide
                                                             better constraints for modelling the Earth’s interior,
                                                             particularly in plate margin locations. Together the new
                                                             GOCE data products will lead to the possibility for
                                                             global unification of height systems (Figure 2c), using
                                                             ‘pseudo levelled’ or orthometric heights referenced to a
                                                             common GOCE-derived geoid. Similar GPS levelling of
                                                             existing tide gauges will also facilitate a greater insight
                                                             into regional distribution sea-level change.
(b)
                                                             The aim of the GOCE mission is therefore to determine
               EGM96
                                                             the gravity anomalies and geoid heights. It shall do this,
                                                             by:

                                                                 •    measurement of the Earth’s gravity anomaly
                                                                      field with an accuracy of better than 1–2 mGal
                                                                      (1 mGal = 10−5ms-2) via combination of gravity
                                                                      gradients and satellite to satellite tracking.

                                                                 •    determining (from the measured gravity
                                                                      anomaly field) the geoid (i.e. the equipotential
                                                                      surface of a hypothetical ocean at rest) with a
                                                                      radial accuracy better than 1-2 cm.
 (c)                                                             •    achieving both these measurements at a length
                EGM96                                                 scale of 100 km or less (i.e. degree and order
                                                                      equal to or higher than 200 in a spherical
                                                                      harmonics expansion of the field).

                                                             A summary of the scientific applications of the GOCE
                                                             data are shown below in Figure 3.




Figure 2. Schematic diagram (adapted from [4])
showing geoid accuracy and scale, or horizontal
resolution, required to characterise (a) ocean circulation
features; (b) solid-earth processes; and for (c) geodesy
applications. Shaded areas indicate the expected
improvement over the existing EGM96 and more recent
GRACE geoid and gravity model.



through the present day (indicated by the GRACE
curves in Figure 2). For instance, since gravity is
directly linked to the distribution of mass within the       Figure 3. Summary of science applications areas using
Earth, an accurate global geoid model including high         GOCE data (blue) in conjunction with other satellite, in-
harmonics contributes to an improved understanding of        situ or other ancillary data (green).
key features of ocean circulation, which plays an
4



3.   MISSION CONCEPT                                           International Global Navigation Satellite Service (IGS).
                                                               Taking their orbits and the relative GPS distance
Satellite gradiometry is the measurement, ideally in all       measurement to the low-earth orbiting GOCE platform
three spatial directions, of differences in acceleration       into account, the exact orbit can be retrieved to cm-
between pairs of test-masses of an ensemble of 6               precision without interruption in three dimensions (see
accelerometers inside one satellite (Figure 4). The            Figure 4). Long wavelength distortions in the orbit due
measured signal is the difference in gravitational             to the effects of the gravity field will be detected by this
acceleration at the proof-mass locations inside the            technique.
spacecraft, where of course the gravitational signal
reflects the various attracting masses of the Earth.
Sources of uneven mass distribution include amongst
others the relative distribution of oceans, land and ice,      Table 1. Important technical parameters for the GOCE
ocean mass exchange by circulation, mountains and              System
valleys, and via ocean ridges, lithospheric subduction
zones and mantle inhomogeneities down to the core-               Electrostatic Gravity    EGG Instrument specifications:
mantle-boundary and beyond. The technique in                     Gradiometer (EGG)        -   3 pairs of 3-axis, servo-controlled, capacitive
                                                                                              accelerometers on an ultra-stable Carbon-
principle can resolve all these features as they appear in                                    Carbon structure
the observed gravity gradients, which are second                                          -   Pairs of accelerometers separated by a
derivatives of the gravitational potential.                                                   baseline of approx. 0.5 m
                                                                                          -   Accelerometer noise < 2 x 10-12 m s-2 Hz-1/2 in
Non-gravitational acceleration of the spacecraft (for                                         the defined measurement bandwidth (from
                                                                                              0.005 to 0.1 Hz)
instance due to air drag and radiation pressure) affects
                                                                 Satellite to Satellite   SSTI instrument specifications:
all accelerometers inside the satellite in the same              Tracking Instrument      -   12 channel, dual-frequency satellite-to-
manner. The non-gravitational accelerations ideally              (SSTI)                       satellite tracking receiver
drop out when taking differences between two                                              -   Geodetic-quality (~1cm) orbit determination
accelerometers along a gradiometer arm. Rotational               Spacecraft (S/C)         Rigid platform structure with fixed solar wings
motion of the satellite is addressed by correcting for the                                and no moving parts:
centrifugal accelerations. Due to the r-2 dependency of                                   -   octagonal space craft body, approximately 1
                                                                                              m diameter by 5 m long
gravitational forces a low orbit implies stronger signals
                                                                                          -   cross-section minimised in direction of flight
and greater accuracy.                                                                         to reduce drag
                                                                                          -   tail fins for passive stability
                                                                                          -   solar-illuminated side of spacecraft covered
                                                                                              with solar cells
                                                                 S/C Budgets              -   satellite mass < 1100 kg (including fuel)
                                                                                          -   electric power supply 1300 W
                                                                                          -   telemetry and telecommand (S-band) at 4
                                                                                              Kbit/s uplink; 850 Kbit/s downlink

                                                                 Attitude Control         Drag-free Attitude-Control System (DFACS)
                                                                                          comprising:
                                                                                          -   Actuators – Ion Thruster Assembly (Xenon
                                                                                              propellant) and magnetotorquers
                                                                                          -   Sensors – Star trackers, a 3-axis
                                                                                              magnetometer, a digital sun sensor, and a
                                                                                              coarse Earth and Sun sensor
                                                                                          Cold-gas thrusters for gradiometer calibration
                                                                                          only
Figure 4.        Measurement principle of the GOCE               Orbit                    -   sun-synchronous, dusk/dawn or dawn/dusk
Satellite, combining gravity gradiometry with satellite-                                      circular orbit
to-satellite tracking (SST).                                                              -   250 km mean altitude
                                                                                          -   96.5o inclination
The gradiometer measurements are supplemented by                 Mission Profile          Nominal mission duration of 20 months
exploiting the concept of satellite-to-satellite tracking in                              including:
the high-low mode (SST-hl). This means that the low                                       -   3-month commissioning and calibration
Earth orbiting GOCE is equipped with a Global                                             -   two nominal 6-month measurement phases
                                                                                              separated by long-eclipse hibernation period
Positioning System (GPS) dual-frequency receiver
                                                                                          Rockot: launch from Plesetsk Cosmodrome,
derived from the LagrangeTM receiver. Dual zenith-
                                                                 Launcher
                                                                                          Russia
pointing quadrifilar helix antennas (mounted on the              Flight Operations        -   command and data downlink ground station
solar wings for unobstructed visibility) ‘see’ up to                                          in Kiruna
twelve GPS satellites at any one time whose                                               -   mission control, at European Space
ephemeredes are determined very accurately by the                                             Operations Centre (ESOC), Darmstadt
large network of ground stations that participate in the
                                                                                                                   5



4.    GOCE SPACECRAFT ELEMENTS                             basic gradiometric quantity (differential measurement),
                                                           while half the sum is proportional to the externally
An advanced gravity mission such as GOCE requires          induced perturbing drag acceleration (or common mode
that the satellite and system of sensor and control        measurement). The three identical arms are mounted
elements function as one ‘gravity measuring device’.       orthogonal to one another (see Figure 5). The
Thus, in contrast to previous ESA satellite remote-        gradiometer axes so defined are nominally aligned in
sensing missions there is no division between the          the along-track, cross-track and a third direction
satellite platform and the instrument payload. GOCE        pointing approximately towards the Earth’s centre
has also benefited significantly from the CHAMP and        (forming a right-handed triad). The three resulting
GRACE mission experiences. As a consequence the            differential accelerations provide direct, independent
design ensures a stable thermal environment for the        measurements: not only of the diagonal gravity
gradiometer, and that the effects of thermoelastic         components, but also of the off-diagonal terms and the
deformation or any other potential contaminant of the      perturbing angular accelerations.
accelerometer data is minimised.
                                                           In-orbit calibration of EGG involves a carefully-
4.1    Electrostatic Gravity Gradiometer (EGG)             planned, coordinated series of random thruster impulses
                                                           using the cold-gas calibration thrusters together with the
The EGG instrument built at Alcatel Alenia Space,          reported digital force-feedback information from the
France (AAS-F) incorporates accelerometers designed        gradiometer. Such calibrations may be repeated to check
and developed at ONERA, and is based on an ambient         parameter stability with respect to thermal drifts and
temperature, closed loop, capacitive accelerometer         fluctuations. The objective of in-orbit calibration is to
concept. EGG is a three-axis gradiometer consisting of     determine relative scale factors and alignment angles
3 pairs of three-axis servo-controlled capacitive          between accelerometer readings.
accelerometers on an ultra-stable carbon-carbon
structure. The thermal control (passive with heaters)      4.2       Satellite to Satellite Tracking Instrument
provides approximately 10 mK stability during 200 s.                 (SSTI)
The resulting performance shall be better than 6 mE
Hz−1/2 across the measurement bandwidth. The EGG           The objective of the SSTI is to provide support to the
assembly has a mass of 180 kg and requires up to 100       gravity field recovery, by using the positioning provided
W of electric power.                                       by the simultaneous tracking of up to 12 GPS satellite
                                                           signals. As such this payload element is an integral part
                                                           of the system and not an independent instrument. In
                                                           addition, the SSTI provides data for precise orbit
                                                           determination and is used for real-time on-board
                                                           navigation and attitude-reference-frame determination.

                                                           The Lagrange SST instrument has a redundant 12-
                                                           channel dual-frequency receiver with a semi-codeless
                                                           tracking capability. It processes, demodulates and
                                                           decodes the signals from GPS, received through a pair
                                                           of hemispherical antennas pointing in the zenith
                                                           direction. The frequency bands L1 and L2 signals are
                                                           used to allow the compensation of ionospheric delays by
                                                           ground post-processing. Each channel of SSTI receives
                                                           GPS signals and provides the following measurements:
                                                           coarse acquisition pseudo range (L1; with provision for
Figure 5. Gradiometer structural thermal model in          L2), L1 and L2 carrier phase (with phase noise <1 mm),
testing at Alcatel Alenia Space, France.                   P1 and P2 code pseudo range (L1 and L2), L1-L2
                                                           differential carrier phase and P1-P2 differential pseudo
The principle of operation of the EGG is based on the
                                                           range. In addition, the Lagrange SSTI provides the
measurement of the forces needed to maintain a proof
                                                           following capabilities:
mass at the centre of a cage. A six degree of freedom
servo-controlled electrostatic suspension provides               •    Position,  velocity     and     time     (PVT)
control of the proof mass in terms of translation and                 measurements
rotation. Each pair of identical accelerometers, mounted
on the ultra-stable carbon-carbon structure about 0.5m           •    1 Hz output synchronized with GPS time
apart, form a “gradiometer arm”. The difference
between accelerations measured by each of the two                •    measurement time-tagging with respect to on-
accelerometers, in the direction joining them, is the                 board spacecraft time
6



      •    fully redundant      receiver    and    receiver    Magnetotorquers
           processing unit                                     Magnetotorquers aligned in the x, y, and z direction
      •    optimisation of the number of measurement           may be used to realign the spacecraft axes with respect
           channels for power saving.                          to the Earth’s magnetic field.

The total mass of the fully-redundant SSTI sub-system          Sensors
is approx. 12 kg, with a peak power demand of < 32 W.
                                                               The sensors responsible for providing information on
                                                               the satellite attitude are the Star trackers, a 3-axis
4.3       Laser Retro-reflector (LRR)                          magnetometer, a digital Sun sensor, and a coarse Earth
                                                               and Sun sensor. The star trackers (STR), shown in
The LRR allows acquisition of a supplementary data set         Figure 6, are used to provide data about the orientation
of satellite laser ranging (SLR) observations (by the          and angular rate of the spacecraft at a rate of 2Hz. Three
existing SLR ground network) as backup for precise             start tracker heads are employed together such as to
orbit determination post-processing. The LRR is a              provide redundancy in combating blinding from the
corner-cube array capable of reflecting laser pulses back      moon.
along the incident light path.

4.4       Satellite Attitude Control

The satellite is 3-axis stabilised, and is piloted in a yaw-
steering mode (i.e. allowing the yaw angle and roll
angle to vary slowly along the orbit). The Drag Free
Attitude Control System (DFACS) comprises actuators
and the various sensors (see Table 1):

Ion Propulsion Assembly
The Ion Propulsion Assembly (IPA) consists of an ion
thuster, a gas feed system and related power and control
electronics. The Ion Thruster Assembly is the primary
                                                               Figure 6. DTU star tracker sensor and processing unit.
actuation device on board GOCE and functions solely to
compensate drag in the along-track direction. At its
heart is a QinetiQ T5 MkV ion thruster which is
mounted on an adjustable alignment bracket to direct
the thrust vector through the spacecraft centre of mass.       5.   MISSION PROFILE
The Kaufman-type electron bombardment ion motor
runs on Xenon (Xe) which is fed into a 10 cm diameter          GOCE will be launched using a Rockot vehicle from the
cylindrical discharge chamber both via a hollow cathode        Eurockot Cosmodrome in Plesetsk, northern Russia in
and a normal feed pipe. The hollow cathode serves as an        late 2007. The satellite will be injected into orbit at an
electron source to ignite and sustain the Xe plasma            altitude of around 265 km, and will be allowed to decay
discharge proper inside the thruster chamber. An               down to the measurement altitude around 250 km (with
external magnetic field is applied to enhance the              inclination close to 96.5◦). Since the GOCE satellite is
ionisation efficiency of the electrons and to guide the        designed such that it’s fixed solar panels must face the
Xe ions towards the extraction grid system at the              sun, this constrains the launch to seasonal windows in
thruster exit. Two carbon grids, well aligned and              winter and summer, with a daily launch window of only
separated by about one mm, accomplish the acceleration         approximately 30 minutes. In its winter launch
of the Xe ions to 1170 eV and at the same time prevent         configuration, the GOCE orbit will have a 06:00 hrs
unwanted backstreaming of ambient plasma electrons             equatorial ascending node crossing (i.e. dawn-dusk
into the thruster. To prevent spacecraft charging, a           orbit), while in its summer launch configuration it will
second hollow cathode is used to emit an electron beam         have an ascending node equatorial crossing at 18:00 hrs
of equal magnitude but opposite sign compared the ion          (i.e. dusk-dawn orbit). In this orbit, global coverage
beam.                                                          outside the polar caps is reached after about 30–40 days,
                                                               while the orbit configuration also meets the requirement
The thruster can be throttled between 1 and 20 mN at           for a ground-track repeat period exceeding 60 days.
rates compatible with the targeted mission profile and         Figure 7 indicates the ground track coverage after two
expected drag changes over individual orbits. GOCE is          weeks, whilst the ground track density after two months
equipped with two fully redundant IPAs. The fuel tank          ensures that the maximum separation of tracks is less
is filled with 40 kg of Xe, this is sufficient for a 30        than 40 km.
months mission.
                                                                                                                                                         7



                                                              Flight Operations Segment (FOS) via ESA-ESOC,
                                                              Darmstadt. The FOS generates and uplinks commands
                                                              to programme the GOCE satellite operations, and
                                                              meanwhile processes the housekeeping and instrument
                                                              data to monitor the health status and performance of the
                                                              platform and the instruments.

                                                              The generation of the scientific Level 1b products of the
                                                              GOCE mission is done by the Payload Data Segment
                                                              (PDS), which also receives the GOCE data via Kiruna.
                                                              GOCE Level 2 products (Table 2) include gravity
                                                              gradients, precise orbit solutions, as well as the GOCE-
                                                              only gravity field models including supporting
                                                              information. These Level 2 data products will be
                                                              generated by the High-Level Processing Facility (HPF).
                                                              The HPF is a distributed processing chain being
Figure 7. GOCE ground track sampling pattern over
                                                              developed by a group of 10 European Institutes known
Europe after 14 days of a 60-day repeat pattern (in the
                                                              as the European GOCE Gravity Consortium (EGG-C).
reference orbit configuration). The red line indicates the
area within which the satellite is in line-of-sight contact
with the Kiruna ground receiving station (in northern
                                                              Table 2. GOCE Geophysical (i.e. Level 2) Products.
Sweden).
                                                               Product Name      Product Definition                                    Remarks

After the launch and early orbit phase (LEOP) of the           Gravity Gradients
                                                               EGG_NOM_2         Level 2 gravity gradients in GRF with                 Latency 2 weeks
mission, the nominal mission profile anticipates the                             corrections:
                                                                                 - Externally calibrated and corrected gravity
orbit altitude to be allowed to slowly decay. During this                        gradients
                                                                                 - Corrections to gravity gradients due to
controlled orbit decay the spacecraft will be                                    temporal gravity variations
commissioned and the gradiometer set-up and                                      - Flags for outliers, fill-in gravity gradients for
                                                                                 data gaps with flags
calibration will take place. Assuming nominal                                    - Statistical information
                                                               EGG_TRF_2         L2 gravity gradients in EFRF with corrections:        Latency 6 months.
instrument performance after this initial estimated                              - Externally calibrated gravity gradients in          Only on physical
commissioning interval (~3 months) the orbit decay will                          Earth fixed reference frame including error
                                                                                 estimates for transformed gradients
                                                                                                                                       media

be stopped at an operating altitude that matches the real                        - Transformation parameters to Earth fixed
                                                                                 reference frame
performance capabilities of DFACS to the real drag             GOCE Orbits
environment and its temporal variability.                      SST_PSO_2         Precise science orbits:                               Latency 2 weeks
                                                                                 - Reduced-dynamic and kinematic precise
                                                                                 science orbits
The mission profile foresees a minimum of two science                            - Rotation matrices between IRF and EFRF
                                                                                 - Quality report for precise orbits
measurement operations phases, each comprising up to           GOCE Gravity Fields
6 months of data acquisition. Potential science                EGM_GOC_2         Final GOCE gravity field model:                       Latency 9 months
                                                                                 - Spherical harmonic series including error
measurement phases may be interrupted by a long                                  estimates
                                                                                 - Grids of geoid heights, gravity anomalies and
eclipse season (of around 5 months duration) during                              deflections of the vertical
which time the satellite is in Earth shadow for over 25                          - Propagated error estimates in terms of geoid
                                                                                 heights
minutes of each orbit. During this long-eclipse interval                         - Quality report for gravity field model
                                                               EGM_GVC_2         Variance-covariance matrix for the final              Latency 9 months.
the power demands of the on-board systems (such as the                           gravity field in terms of spherical harmonic          Only on physical
ion-thruster) may exceed the power generated by the                              series                                                media

solar panels. This situation will require hibernation, and
thus the satellite will be raised using the Ion Thrusters
to a safe orbit altitude (~270 km) at which various           User access to Level 2 Data is facilitated by a proposal
subsystems may be safely switched off.                        response to the GOCE Data Announcement of
                                                              Opportunity released in Oct. 2006 via the
The present spacecraft design budgets for consumables         http://eopi.esa.int/goce web site. The following GOCE
(e.g. Xenon for ion thruster) that allow a nominal            products (see also Table 2) will be distributed to
mission duration of 20 months.                                approved and registered Category 1 GOCE AO data
                                                              users:
6.   GOCE DATA PRODUCTS                                             •      Externally calibrated and corrected gravity
                                                                           gradients (EGG_NOM_2/EGG_TRF_2)
The GOCE mission employs a single ground station in
Kiruna to exchange commands with the spacecraft and                 •      Precise science orbits (SST_PSO_2)
to downlink data to ground. During operations, the                  •      Global Earth gravity potential modelled as
satellite is continuously monitored and controlled by the                  spherical harmonic series up to deg/order 200 –
8



         corresponding to 100 km spatial resolution            4. European Space Agency: 1999, Gravity field and
         including coefficients and error estimates            Steady-State Ocean Circulation Explorer Mission, ESA
         (EGM_GOC_2)                                           SP 1233(1), 217pp.
     •   Global ground-referenced gridded values of:
                                                               5. Drinkwater, M.R., R. Floberghagen, R. Haagmans,
              o geoid heights (Earth geoid map)
                                                               D. Muzi, and A. Popescu, GOCE: ESA’s first Earth
              o gravity anomalies (Earth gravity map)          Explorer Core mission. In Beutler, G.B., M. R.
              o geoid slopes                                   Drinkwater, R. Rummel, and R. von Steiger, Earth
     •   Variance-covariance matrix of final GOCE              Gravity Field from Space - from Sensors to Earth
         Earth gravity field model (EGM_GVC_2)                 Sciences. In Space Science Reviews, Vol. 108, 1, 419-
                                                               432, 2003.

                                                               6. Johannessen, J.J., G. Balmino, C. Le Provost, R.
7.   SUMMARY AND CONCLUSIONS                                   Rummel, R. Sabadini, H. Sünkel, C.C. Tscherning, P.
                                                               Visser, P. Woodworth, C. W. Hughes, P. LeGrand, N.
The recent CHAMP and GRACE mission successes
                                                               Sneeuw , F. Perosanz, M. Aguirre-Martinez, H. Rebhan,
have already led to considerable improvement in our
                                                               and M. R. Drinkwater, The European Gravity Field and
knowledge of the geoid at long wavelengths, as well as
                                                               Steady-State Ocean Circulation Explorer Satellite
time variations in the Earth’s gravity field. GOCE,
                                                               Mission: Impact in Geophysics, Surveys in Geophysics,
however, will be the first satellite to deliver high spatial
                                                               24, 4, 339-386, 2003.
resolution gravity gradients from a very low-earth orbit
(~250km) using drag-free control. Further, GOCE is the
first gravity mission to employ the technique of satellite
gradiometry complemented by geodetic satellite to
satellite tracking. The GOCE instrument data will allow
recovery of a high resolution static gravity field with
homogeneous quality and of unprecedented accuracy
and very high resolution. The resulting products will
deliver a key step forward in improving ocean, solid
Earth and sea-level modelling. Furthermore, the data
will have a positive impact on resolving differences
between national height systems and in surveying
applications on land and sea. The launch of GOCE is
currently foreseen in late September 2007. For more
details about the GOCE mission please see:
www.esa.int/livingplanet/goce.

8.   ACKNOWLEDGEMENTS

The authors acknowledge the significant collective
contributions of the GOCE Mission Advisory Group,
the entire GOCE Project team, and the Industrial
consortium members; Alcatel Alenia Space, Italy,
EADS Astrium GmbH, Alcatel Alenia Space, France,
and ONERA.

9.   REFERENCES

1. European Space Agency, The Science and Research
Elements of ESA’s Living Planet Programme, ESA SP-
1227, 105pp, 1998.

2. European Space Agency, The Changing Earth: New
Scientific Challenges for ESA’s Living Planet
Programme, ESA SP 1304, 83pp, 2006.

3. European Space Agency, GOCE Mission Web Site,
http://www.esa.int/livingplanet/goce.

								
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