1 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: email@example.com (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 . 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 ) 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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