The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 1 GOCE Technical Presentation Mission Design Geoid Gravity Anomalies

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 2 Mission Rationale Science & Application Mission Design Performance Conclusions - System Concept - Instruments - Attitude and Drag Control - Conventional Mission Elements - Programmatics System Concept Presentation Outline

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 3 From Scientific to Satellite Requirements The scientific requirements are 1 mgal (gravity) and 1 cm (geoid) at 100 km resolution To carry a gradiometer and a GPS/GLONASS receiver Using the mission simulation tools, these scientific requirements, have been transformed in mission and satellite requirements: –gravity gradient (mE), –satellite position (cm), –orbit altitude –and mission duration System Concept

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 4 Derived Satellite Requirements Orbit altitude: 250 km Orbit inclination: 96.5º (Sun-synchronous) Orbit local time of ascending node: 6:00 Mission duration: 20 months Gradiometric performance target: 4 mE/  Hz SST-hl performance target: 2 cm System Concept

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 5 Satellite Configuration Symmetric Slender (0.8 m 2 ) Large Solar Array Without mechanisms 770 kg 1100 W 4 m long Velocity Zenit Sun Mission Design

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 6 Mission Timeline System Concept

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 7 Observables Frequency Ranges The gravity and the perturbing forces change in space and time The satellite observe them as time series Signal and noise are studied in the frequency domain F x (along velocity) F y (transversal) 77 km 770 km 7700 km System Concept Hz Acceleration PSD (m/(s2sqrtHz)) Drag force (mN) s

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 8 Instruments Synergy The gradiometer has good performance at high frequency and the SST-hl receiver at low frequency Overlapping frequency is Hz The gradiometer provides the external accelerations to the SST-hl receiver that provides long term stability to the gradiometer System Concept Resolution (km) Frequency (Hz) Gradiometer range ( Hz) SST-hl receiver range

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 9 Satellite Error Budgets System Concept SST-hl total error : 2 cm (1 cm, receiver, 1 cm GPS ephemeris, 1 cm satellite accelerations) Gradient error:

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 10 The Gradiometer It provides the high resolution terms of the gravity field Three pairs of accelerometers perpendicular to each other. Baseline 0.5 m –The difference of read-out of a pair of accelerometers provides one component of the gravity gradient –The addition of the read-out provides the external linear acceleration –Angular accelerations are also obtained Measurement bandwidth (mbw): Hz Resources: 125 kg, 75 w, 1 Kbps, 0.8  0.8  1.2 m Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 11 The Gradiometer: Accelerometer Principle C1 C2 S S Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 12 The Gradiometer Accelerometer Design and Status Benefits from many years of development Pt-Rh proof mass (4  4  1 cm, 320 g) grounded by a gold wire. Control electrodes in gold coated ULE glass. External body in Invar. Benefits from many years of development GOCE drag control allows better accuracy Proof Mass Electrodes Exploded View Integrated View Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 13 The Gradiometer: Instrument Resolution 3 mE/  Hz specified The predicted performance curve has been derived from a combination of analysis and test Predicted results in line with requirements Specified noise value Predicted noise values Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 14 The Gradiometer: Instrument Satellite Coupling Errors External linear and angular accelerations couple with instrument missalignments to produce errors 1 mE/  Hz allocated to this error source. The resulting gradiometer alignment accuracy is rad. It has been verified by test. dd dd cc cc Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 15 The Gradiometer: Pendulum Test Bench A servo controlled pendulum test bench has been developed for the testing of the gradiometer Tilting angles can be controlled down to rad By tilting the platform, alignments and scale factors can be measured to rad Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 16 The Gradiometer: Configuration (Exploded View) External thermal protection Internal thermal protection Gradiometer core Thermally regulated platform Structural support Platform Mechanical decoupling device Mechanical decoupling device Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 17 The Gradiometer: Configuration (Integrated View) Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 18 The Gradiometer: Thermal Stability Lack of dimensional stability will produce errors 0.2 mE/  Hz allocated Two thermal domains configuration Ultra stable Carbon&Carbon structure The performances (0.8 mK over 200 s and 9 µK over 10 s) fulfill these requirements Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 19 Calibration Principle dd dd cc cc dd dd The satellite will be shaken in orbit with specified forces and torques by the micro-thrusters and the accelerometers alignment errors will be measured This will also be done on ground using the pendulum bench as shaker Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 20 SST-hl GPS/GLONASS Receiver It provides the low resolution part of the gravity field 12 channel dual-frequency GPS and GLONASS receiver Less than 1 cm of measurement noise. Two off-the shelf receivers fulfilling GOCE needs will be available in Europe soon: GRAS and Lagrange Reference interface data are: –Planar hemispherical zenith looking antenna –System is: 10 kg, 40 w, 2 Kbps. Electronic box is 250  164  203 mm, Antenna is 300  300  50 mm, Instruments

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 21 System Requirements Linear acceleration: m/s 2 (total), 2.5·10 -8 m/s 2  Hz (mbw) Angular acceleration: rad/s 2 (total), 2.5·10 -8 rad/s 2  Hz (mbw) Pointing: 0.35 mrad (total), 8.6 ·10 -6 rad/  Hz (mbw) The analysis including close-loop simulation, has demonstrated that the requirements are fulfilled Low flying altitude drives: redundant system for the ‘nominal’ modes plus fully independent emergency mode sensors and actuators Attitude and Drag Control

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 22 Architecture Attitude and Drag Control and drag Normal mode Safe mode

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 23 Performance Pointing requirement: 8.6·10 -6 rad/  Hz fulfilled  (2 mE  Hz) Drag control requirements: 2.5·10 -8 m/s 2  Hz fulfilled  (0.9 mE  Hz) Drag requirement Pointing requirement Velocity requirement Acceleration requirement Attitude and Drag Control m/s 2  Hz unit/  Hz Frequency (Hz)

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 24 Ion-Thrusters Principles and Requirements It is used to compensate the atmospheric drag Xe gas is first ionised, then accelerated by high voltage and expelled. This produces thrust The main requirements are: –Normal thrust range: 1-12 mN. Orbit change thrust: 20 mN –Minimum thrust step: 18  N –Thrust modulation speed: 10 mN in 1000 s and 25  N in 0.1 s –Bandwidth 10 Hz Two thrusters (full redundancy) located at the bottom of the satellite Attitude and Drag Control

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 25 Ion-Thrusters Development Status Most requirements have been verified by analysis or by test The verification of the long term thrust direction stability is pending 25  N step in 1 ms Ion-thruster test set-up Thruster step test result Attitude and Drag Control

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 26 Thermal and Structural Elements Structure allows easy assembly and disassembly Conventional thermal control Upper equipment bay Lower equipment bay Instrument bay Equipment radiator Instrument radiators Equipment radiator Conventional Mission Elements

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 27 Avionics Ion Propulsion SA V power bus Power control electronics Gradiometer Radiation Monitor Attitude and Drag Control Communications Thermal Control Data bus 1 Mbps S band Solar Array W SST receiver Data Handling Battery 265 Wh Data bus MIL-1553 Conventional Mission Elements 4 Kbps 1 Gbit memory

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 28 Satellite Mass and Power Budget Conventional Mission Elements

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 29 GOCE Operations 5 Kbps, no real time data, instruments always on Once a month recalibration Change of altitude several times during the mission Robust strategy to avoid mission loss in case of failure of the drag or attitude control –Sophisticated attitude and drag control modes –Autonomous and resilient satellite S band 1 Mbps down-link rate Two passes per day are enough for data downlink Conventional Mission Elements

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 30 Ground Segment Architecture IGS data for POD Geoid and gravity fields are produced during the mission and consolidated once the mission is finished Real time checking of the data quality is done using the trace- less property Conventional Mission Elements

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 31 Development and Mission Risk The gradiometer benefits from the accelerometers development The SST-hl receiver is available The already performed pre-development on the ion-thrusters provide a very high degree of confidence on the approach The proportional micro-thrusters have not yet gone through all its key development stages but the last developments are encouraging Launch window is one month. If it is not met it would imply one year launch delay. Low flying altitude is necessary. Specific features have been implemented to minimize this risk: redundancy, safe mode, aerodynamic stability, autonomy. Up to 20 days without ion- thrusters can be recovered Programmatics

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 32 Satellite at ESTEC in 2004 Programmatics

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 33 GOCE Testing Programmatics

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 34 Schedule Phase A Phases C/D Phase B ESA GOCE User Milestones AO’s National Entities ESAG ? Airborne Gravity Survey 2005 ESTEC Workshop ISSI EGS EGG-C: Level 1 - Level 2 Data Processing Architecture Gravity User Workshops ESRIN Workshop 2006 Cal/Val AOData AO HPF/CMF&RPF Development Validation Campaign? AO Workshop Phase E Launch IAG/IAPSO Data Processing

The Four Candidate Earth Explorer Core Missions Consultative Workshop October 1999, Granada, Spain, Revised by CCT GOCE T 35 Development and Mission Risk The gradiometer benefits from the accelerometers development The SST-hl receiver is available The already performed pre-development on the ion-thrusters provide a very high degree of confidence on the approach The proportional micro-thrusters have not yet gone through all its key development stages but the last developments are encouraging Launch window is one month. If it is not met it would imply one year launch delay. Low flying altitude is necessary. Specific features have been implemented to minimize this risk: redundancy, safe mode, aerodynamic stability, autonomy. Up to 20 days without ion- thrusters can be recovered Programmatics