1. Istituto di Fisica dello Spazio Interplanetario New concept for the BepiColombo Radio Science Experiment: rôle of ISA accelerometer XCIV Congresso Nazionale SIF Genova, 22-27 Settembre 2008 V. Iafolla, E. Fiorenza, C. Lefevre, S. Nozzoli, R. Peron, M. Persichini, A. Reale, F. Santoli Istituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Roma, Italy

2. Istituto di Fisica dello Spazio Interplanetario BepiColombo Radio Science Experiment (RSE) 2 The RSE uses the radiometric tracking of BepiColombo from ground-based antennas to precisely track the spacecraft and to obtain information on its gravitational dynamical environment Gravimetry Rotation General relativity Gravimetry Rotation General relativity Three main experiments:Involved instruments: Ka–band Transponder Star–Tracker High Resolution Camera Accelerometer Ka–band Transponder Star–Tracker High Resolution Camera Accelerometer

3. Istituto di Fisica dello Spazio Interplanetario RSE Objectives 3 global gravity fieldtemporal variationsThe global gravity field of Mercury and its temporal variations due to solar tides (in order to constrain the internal structure of the planet) local gravity anomaliesThe local gravity anomalies (in order to constrain the mantle structure of the planet and the interface between mantle and crust) rotation stateThe rotation state of Mercury (in order to constrain the size and the physical state of the core of the planet) orbit of the Mercury center–of–massThe orbit of the Mercury center–of–mass around the Sun (in order to improve the determination of the parametrized post–Newtonian (PPN) parameters of general relativity) Milani et al., Plan. Space Sci. 49, 1579 Milani et al., Phys. Rev. D 66, 082001 Milani et al., Plan. Space Sci. 49, 1579 Milani et al., Phys. Rev. D 66, 082001

4. Istituto di Fisica dello Spazio Interplanetario RSE measurements 4 Rangerange–rateRange and range–rate tracking of the MPO with respect to Earth–bound radar station(s) (and then of Mercury center–of–mass around the Sun) non–gravitational forcesDetermination of the non–gravitational forces acting on the MPO by means of an on–board accelerometer absolute attitudeDetermination of the MPO absolute attitude by means of a Star–Tracker angular displacements of reference points on the solid surfaceDetermination of angular displacements of reference points on the solid surface of the planet, by means of a fotocamera

5. Istituto di Fisica dello Spazio Interplanetario RSE science goals 5 Spherical harmonic coefficients of the gravity field of the planet up to degree and order 25 Degree 2 (C 20 and C 22 ) with 10 -9 accuracy (Signal/Noise Ratio 10 4 ) Degree 10 with SNR 300 Degree 20 with SNR 10 Love number k 2 with SNR 50 Obliquity of the planet to an accuracy of 4 arcsec (40 m on surface – needs also SYMBIO-SYS) Amplitude of physical librations in longitude to 4 arcsec (40 m on surface – needs SYMBIO-SYS). C m /C (ratio between mantle and planet moment of inertia) to 0.05 or better C/MR 2 to 0.003 or better

6. Istituto di Fisica dello Spazio Interplanetario RSE science goals 6 Spacecraft position in a Mercury-centric frame to 10 cm – 1m (depending on the tracking geometry) Planetary figure, including mean radius, polar radius and equatorial radius to 1 part in 10 7 (by combining MORE and BELA laser altimeter data ) Geoid surface to 10 cm over spatial scales of 300 km Position of Mercury in a solar system baricentric frame to better than 10 cm PN parameter (controlling the deflection of light and the time delay of ranging signals) to 2.510 -6 PN parameter (controlling the relativistic advance of Mercurys perihelion) to 510 -6 PN parameter (controlling the gravitational self-energy contribution to the gravitational mass) to 210 -5 Gravitational oblateness of the Sun (J 2 ) to 210 -9 Time variation of the gravitational constant (d(lnG)/dt) to 310 -13 years -1

7. Istituto di Fisica dello Spazio Interplanetario RSE total noise 7 Instrument bandwidth

8. Istituto di Fisica dello Spazio Interplanetario ISA accuracy requirement inside the frequency band 8

9. Istituto di Fisica dello Spazio Interplanetario Direct solar radiation pressure: solar irradiance 9 http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant From http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant

10. Istituto di Fisica dello Spazio Interplanetario Direct solar radiation pressure: the transversal acceleration and its spectrum 10

11. Istituto di Fisica dello Spazio Interplanetario ISA description 11 ISA sensing element ISA pick-up

12. Istituto di Fisica dello Spazio Interplanetario Differential Accelerometer 08/06/2006 Emiliano Fiorenza 12 Mechanical arrangement Seismic noise rejection

13. Istituto di Fisica dello Spazio Interplanetario Accelerations acting on ISA The acceleration on a point P (ISA proof–mass) close to the MPO COM is: X Y Z ISA proof–mass Inside the MPO frame = Planet gravity = MPO angular rate = MPO angular acceleration = MPO–proof–mass vector Acceleration due to the planet gravity field gradients Centrifugal acceleration Angular acceleration Coriolis acceleration Non–Gravitational accelerations 13

14. Istituto di Fisica dello Spazio Interplanetario ISA Positioning Best configuration of the accelerometer for MPO Best configuration of the accelerometer for MPO: the three sensitive masses aligned along the rotation axis of the MPO, and the com of the mass with sensitive axis along the rotation axis coincident with the com of the accelerometer as well as with the MPO one Z–sensitive axis Y–sensitive axis X–sensitive axis com ISA COM Rotation axis 14 Requirements

15. Istituto di Fisica dello Spazio Interplanetario Vibrational random noise on board the MPO inside the frequency band Micro-vibration random noise on board the MPO outside the frequency band 15 Vibrations

16. Istituto di Fisica dello Spazio Interplanetario ISA operative temperature: -20/+30 °C ISA non operative temperature: -40/+40 °C FEE electronic stability: 510 -8 m/s 2 / Hz ACC. mechanical stability: 510 -7 m/s 2 / Hz Active thermal control attenuation: 700 ISA operative temperature: -20/+30 °C ISA non operative temperature: -40/+40 °C FEE electronic stability: 510 -8 m/s 2 / Hz ACC. mechanical stability: 510 -7 m/s 2 / Hz Active thermal control attenuation: 700 ISA thermal system Over one orbital period (2.3 h) of the MPO Over one sideral period (44 days) of Mercury Random noise 4 °Cpp 25 °Cpp 4 °C/ Hz MPO Temperature Variations 16

17. Istituto di Fisica dello Spazio Interplanetario ISA Error Budget: Deterministic Contributions 17 ISA EID-B, BC-EST-RS-02520 It needs to be revised following the new RSE concept (see later) It needs to be revised following the new RSE concept (see later)

18. Istituto di Fisica dello Spazio Interplanetario ISA Error Budget: Random Contributions 18 ISA EID-B, BC-EST-RS-02520 It needs to be revised following the new RSE concept (see later) It needs to be revised following the new RSE concept (see later)

19. Istituto di Fisica dello Spazio Interplanetario 19 Performance and calibration For calibration we mean a characterization of the instrument and its response, in all the phases and operative conditions Transfer function Transducer factor Linearity of response Intrinsic noise Thermal stability Transfer function Transducer factor Linearity of response Intrinsic noise Thermal stability

20. Istituto di Fisica dello Spazio Interplanetario 20 Functional Tests Functional Tests Stability Tests Stability Tests Calibration by internal actuators Calibration by internal actuators Calibration by external accel. Calibration by external accel. On ground On ground After S/C integration After S/C integration Near Earth commissioning Near Earth commissioning In cruise In cruise Mercury commissioning Mercury commissioning Nominal operations Nominal operations PHASES TESTS (instrument): PRE-CALIBRATION AND PERFORMANCE TESTS (single sensor-instrument) Performance and calibration

21. Istituto di Fisica dello Spazio Interplanetario 21 New RSE concept In standard orbit determination and parameter estimation procedure, spacecraft equations of motion and observations are referred to the spacecraft Center of Mass (CoM). This requires precise knowledge of CoM position. ISA CoM HGA This could be a problem, due to CoM movements (fuel sloshing and consumption) This problem is related to the overall RSE concept, not to the single instruments This solution has been discussed by A. Milani (MORE PROGRESS MEETING, Roma, 13 March 2008), has been adopted as the new baseline and is currently under implementation (change of RSE Requirements …) direct referencing of MORE observables to ISA position To overcome this problem, it has been proposed by ASD a direct referencing of MORE observables to ISA position, thereby avoiding the need of a precise CoM position knowledge

22. Istituto di Fisica dello Spazio Interplanetario 22 Current status Instrument Science Requirement Review (ISRR)Instrument Science Requirement Review (ISRR) completed (scientific requirements frozen, apart from small changes due to the new RSE concept) Instrument Preliminary Design Review (IPDR)Instrument Preliminary Design Review (IPDR) foreseen before the end of the year Demonstration ModelDemonstration Model ongoing (developed technologies) ISA Team laboratoriesISA Team laboratories renewed for performance and calibration tests on the various models Instrument Science Requirement Review (ISRR)Instrument Science Requirement Review (ISRR) completed (scientific requirements frozen, apart from small changes due to the new RSE concept) Instrument Preliminary Design Review (IPDR)Instrument Preliminary Design Review (IPDR) foreseen before the end of the year Demonstration ModelDemonstration Model ongoing (developed technologies) ISA Team laboratoriesISA Team laboratories renewed for performance and calibration tests on the various models