1. 83230913-DOC-TAS-EN-002 5 th International GOCE User Workshop UNESCO, Paris, France 28 November 2014 From GOCE to the Next Generation Gravity Mission Stefano Cesare (1), Andrea Allasio (1), Alberto Anselmi (1), Sabrina Dionisio(1), Sergio Mottini (1), Manlio Parisch (1), Luca Massotti (2), Pierluigi Silvestrin (2) (1) Thales Alenia Space Italia (2) ESA-ESTEC

2. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space Contents GOCE A brief history of GOCE Mission profile Satellite Payload Mission performance Next Generation Gravity Mission Objectives and measurement technique Mission scenarios Payload outline Satellite outline Control challenges for NGGM From GOCE to NGGM 2

3. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 3 5th international GOCE user workshop A brief history of GOCE December 2000 GOCE Beginning of Phase B Stabilizer & full drag free (ion + cold-gas  -thrusters) 1998 GOCE Phase A study Electrostatic gradiometer selected for GOCE. Full drag-free control based on ion thrusters (along track) and cold-gas micro-thrusters (lateral) also used for attitude control. Operational altitude: 250 km. Lifetime: 20 months 1995 Gravity Explorer Mission Definition study Full-tensor (3D) gradiometer. Trade-off : electrostatic gradiometer (EG, operating at room temperature) vs. superconducting gradiometer (SG, cryogenic) Drag-free control based on ion thrusters + cold gas thruster for attitude control. Operational altitude: 270 km Lifetime: 12 (EG) or 6 months (SG) 1991 ARISTOTELES Phase A study Planar gradiometer (0.9 m side) Continuous calibration by oscillating masses. Drag variations control by flaps Magnetometer on 4.5m boom 6 months at 200 km altitude & 95  + 3 years at 480 km & 92  2300 kg launch mass, 960 kg hydrazine propellant

4. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 4 5th international GOCE user workshop GOCE evolution during Phase B/C/D February 2002 Preliminary Design Review Stabilizer replaced by Winglets. Cold gas micro-propulsion replaced by FEEP for later drag-free & attitude control. March 2008 Flight Acceptance Review 17 March 2009: Launch May 2005 Critical Design Review Electrical micro-propulsion discarded (in mid 2003) for maturity reasons. Single-axis drag-free control (along track) based on ion thrusters. Attitude control based on magnetic torquers. On/off cold-gas thrusters for gradiometer calibration

5. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 5th international GOCE user workshop GOCE planned mission profile Mission profile planned before launch based on solar activity prediction at that epoch. Lifetime = 20 months (nominal) + 10 months (extension) Two Mission Operational Phases during the eclipse- free/short eclipse seasons Hibernation Phase during the long-eclipse season Gradiometer and drag-free control were to be switched off and altitude increased Temperature-induced disturbances were considered incompatible with accurate measurements

6. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 5th international GOCE user workshop GOCE actual mission profile Solar activity during the GOCE mission Actual eclipse pattern during GOCE mission Actual mission profile lifetime = 55 months; initial operational altitude lower than planned and progressively decreased in the last 15 months; no measurement phase interruption and no orbit raise during long eclipses.

7. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 7 5th international GOCE user workshop A unique spacecraft Aerodynamic shape with small cross section in the flight direction: 1.1 m² Fixed solar panels (8.5 m² ), thanks to the dusk-dawn sun-synchronous orbit. Stable structure in carbon fibre composite. Drag-free control based on variable thrust engines (1 main + 1 redundant) with force range from ~1 to 20 mN, operated in closed loop with the payload accelerometers. Three magnetic torquers with fine (~36 Am²) and coarse (400 Am²) regulation mode as attitude control actuators. Mass at launch: 1060 kg. Propellant mass: 40.87 kg Xe (for the ion engines), 14 kg N2 (for the gradiometer calibration device) Ion thruster performance ~4 yr overall operation ~500,000 Ns total impulse ~3.9 mN average thrust ~1200 s average specific impulse

8. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 8 5th international GOCE user workshop With a unique payload Gradiometer Accelerometer (produced by ONERA): noise ~2  10 -12 m/s 2 /  Hz Accelerometer proof mass (Pt-Rh) GPS Receiver Structure in Carbon- Carbon (CTE: 10 -7 /K) Baseline: 0.5 m GPS receiver (produced by TAS-I): 12 channels; 2 frequencies per channel (L1, L2); measurement of C/A code, P code, carrier phase (performance < 1 mm on L1, < 1.5 mm on L2). Laser retro-reflector array Temperature stability ~100 µK over time periods of 200 s

9. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 9 5th international GOCE user workshop Spacecraft in flight performance Orbit and drag force control Drag acceleration reduced to: ~2  10 -7 m/s 2 (maximum) ~ 2  10 -9 m/s 2 /  Hz (SD) And without drag-free control. Spectral density of the GGT trace before and after the Gadiometer calibration with drag- free control.

10. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space NGGM objectives and measurement technique Objectives: provide the variations of Earth’s gravity field over long time span (possibly covering a full solar cycle) with the spatial and temporal resolution enabling the investigation and monitoring, with unprecedented detail of geophysical phenomena involving mass distribution and transport in the atmosphere, continental hydrosphere, oceans, cryosphere, and lithosphere. Measurement technique: “Satellite-to-satellite tracking” (SST) in low Earth orbit. “gravity sensor” consisting of a pair of satellites flying in loose formation; gravity field recovered from measuring (with a laser ranging system) the distance variation (  d ) induced between the satellites; non-gravitational effects produced by atmospheric drag (  d G ) separately measured by accelerometers and accounted for in the data processing. 10 28/11/2014 5th international GOCE user workshop

11. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 11 5th international GOCE user workshop NGGM satellite formations/constellations “pearl string” (or “in-line”) formation: the two satellites fly on the same orbit with different true anomalies (  ). Gravity field sampled in along-track direction only. “pendulum” formation: the two satellite fly on intersecting orbits, with different inclination or longitude of ascending node. Gravity signals captured alternately in along-track and cross-track directions. Two “pearl string” formations in “Bender-type” constellation: first pair in near polar orbit, second pair in medium inclination orbit (around 70  or 110  ). Faster, homogeneous sampling of the gravity field providing higher spatial resolution and reduced temporal aliasing. Satellite-satellite distance within each pair ~ 100 km.

12. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space GOCE mission period Presumable period of NGGM GRACE mission period 95% probability 50% probability 5% probability Solar Cycle 24Solar Cycle 25 28/11/2014 12 5th international GOCE user workshop Which altitude for NGGM? Minimum (constant) altitude limited by: mission duration (11 years), solar activity (cycle 25 predicted to have a “normal” behaviour, higher than 24), propulsion specific impulse, on-board propellant mass (~50 kg for a S/C total mass compatible with dual launch). Estimated lower limit for a constant flying altitude: 340 km for in-line formation; 420 km for 15  pendulum. Alternatively the altitude can be adapted to the encountered drag force.

13. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 13 5th international GOCE user workshop NGGM payload outline: laser ranging system Laser interferometry needed for measuring the inter-satellite distance variation with nm resolution. Up to a inter-satellite distance of 100 km it is still possible to use a simple retro-reflector installed on one satellite, transmitting the laser beam back towards the satellite which has emitted it. Above 100 km distance, the laser beam received by one satellite must “regenerated” by a local laser before re-transmission. Configuration and breadboard of the laser interferometer, based on back transmission by retro-reflector, designed and built by TAS-I in collaboration with the Italian Institute of Metrology. Breadboard of an auxiliary optical metrology supporting optical link acquisition and laser beam pointing Estimated limit to the spectral density of the relative error in the distance variation measurement.

14. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 14 5th international GOCE user workshop NGGM payload outline: accelerometers The non-gravitational acceleration measurement needs of NGGM can be fulfilled by GOCE-type accelerometers. 2 or 4 accelerometers (depending on the available volume and resources) for measuring the non-gravitational acceleration of the satellite COM can be symmetrically arranged around the COM, leaving a free volume for accommodating the laser retro-reflector. Further advantages of more than one accelerometer: redundancy; possibility of measuring some components of the gravity gradient too. Arrangement of 2 or 4 accelerometers around the spacecraft COM (occupied by the laser retro-reflector) providing the maximum sensitivity along the beam direction. retro-reflector Estimated limit to the measurement error spectral density of the relative non-gravitational acceleration of the two satellites along the line joining the satellite COMs

15. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 15 5th international GOCE user workshop NGGM spacecraft outline Spacecraft preliminary design (from TAS-I studies of the NGGM). Sun tracking strategy with deployable, fixed solar panels (the orbit is not sun- synchronous): the satellite performs a periodic roll manoeuvre (e.g. once per month) for sufficient panel area to be exposed to sunlight. Spacecraft configuration suitable for dual launch with a small launcher (Vega, Rockot, Dnepr). 8 electric thrusters arranged to provide forces and torque along/around all axes Duct for laser beam passage

16. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space NGGM control challenges Functions of the spacecraft dynamics & control system in NGGM: Orbit altitude control. Satellite formation control. Non-gravitational acceleration control (3-axis, in order to exploit the accelerometer full performance) Satellite pointing control along the satellite-to-satellite line (for laser beam pointing). Satellite attitude control around the satellite-to-satellite line (roll angle). Control of the inertial angular rates and accelerations around the 3 satellite axes. Provision of spacecraft shaking profiles needed for accelerometer calibration. Formation control must keep the relative position of the two satellites bounded: without interfering with the scientific measurements (the satellites must be “free” to move under the effect of the gravity field over time scales of 1000 s); without spoiling the drag-free environment (i.e. the formation control accelerations must fulfil the drag-free requirement too); minimizing thruster use in terms of dynamic range and propellant consumption. Drag-free control must reduce the non-gravitational accelerations of the two satellites without affecting the formation-control accelerations. 16 28/11/2014 5th international GOCE user workshop

17. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 17 5th international GOCE user workshop Predicted control performance Time history and spectral density of the non-gravitational linear accelerations of the satellite COM under the action of the drag-free control. Time history and spectral density of the pointing stability of the laser beam axis with respect to the satellite- satellite line (rotations around Y, Z).

18. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 18 5th international GOCE user workshop With the appropriate actuators Preliminary specifications for electric thrusters suitable to fulfil the NGGM needs: Thrust range: 50 μN to 2500 μN Thrust resolution: 1 μN Thrust noise Thruster specific impulse:

19. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 19 5th international GOCE user workshop GOCE vs NGGM GOCENGGM Mission objective High resolution mapping of Earth’s gravity field Monitoring of temporal variations of Earth’s gravity field LifetimeFew years~11 years (solar cycle) Measurement technique Gradiometry (measurement of Earth’s gravity gradient) Satellite-Satellite Tracking (measurement of inter-satellite distance variation due to gravity) Main payload instruments Three-axis gradiometer with 6 accelerometers Laser interferometer, accelerometer(s) SatellitesOne satellite One or more satellite pairs in loose formation. OrbitLEO (~250 km), SSOLEO (~350), polar, medium i Control needs Orbit, drag forces, angular accelerations, attitude (Earth pointing, loose control acceptable) Orbit, formation, drag forces, angular accelerations, attitude (tight, for laser beam pointing)

20. This document is not to be reproduced, modified, adapted, published, translated in any material form in whole or in part nor disclosed to any third party without the prior written permission of Thales Alenia Space -  2012, Thales Alenia Space 28/11/2014 20 5th international GOCE user workshop From GOCE to NGGM GOCE technologies and experience applicable to NGGM: Ultra-sensitive electrostatic accelerometers operating at room temperature Drag-free control techniques based on variable-thrust ion propulsion Ultra-stable structures and temperature stabilization techniques Design of a vibration-free spacecraft High performance GPS receiver Better knowledge of the LEO drag environment End-to-end system simulator Gradiometer in-flight calibration techniques To be specifically developed for NGGM: Laser ranging system Optical link acquisition devices and techniques Multi-function spacecraft control techniques Long-lifetime electric propulsion supporting all control functions Satellite COM position fine determination and adjustment techniques and devices