L’accelerometro ISA per la missione BepiColombo

1. IX Congresso Nazionale di Planetologia Amalfi L’accelerometro ISA per la missione BepiColombo 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. BepiColombo Radio Science Experiments (RSE) 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 Three main experiments: Involved instruments: • Gravimetry • Rotation • General relativity • Ka–band Transponder • Star–Tracker • High Resolution Camera • Accelerometer Gruppo di Gravitazione Sperimentale

3. RSE Objectives The global gravity field of Mercury and its temporal variations due to solar tides (in order to constrain the internal structure of the planet) The local gravity anomalies (in order to constrain the mantle structure of the planet and the interface between mantle and crust) The rotation state of Mercury (in order to constrain the size and the physical state of the core of the planet) The 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 Gruppo di Gravitazione Sperimentale

4. RSE measurements Range 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) Determination of the non–gravitational forces acting on the MPO by means of an on–board accelerometer Determination of the MPO absolute attitude by means of a Star–Tracker Determination of angular displacements of reference points on the solid surface of the planet, by means of a fotocamera Gruppo di Gravitazione Sperimentale

5. RSE science goals Spherical harmonic coefficients of the gravity field of the planet up to degree and order 25 Degree 2 (C20 and C22) with 10-9 accuracy (Signal/Noise Ratio ~ 104) Degree 10 with SNR ~ 300 Degree 20 with SNR ~ 10 Love number k2 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). Cm/C (ratio between mantle and planet moment of inertia) to 0.05 or better C/MR2 to 0.003 or better Gruppo di Gravitazione Sperimentale

6. RSE science goals 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 107 (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 2.5∙10-6 PN parameter  (controlling the relativistic advance of Mercury’s perihelion) to 5∙10-6 PN parameter  (controlling the gravitational self-energy contribution to the gravitational mass) to 2∙10-5 Gravitational oblateness of the Sun (J2) to 2∙10-9 Time variation of the gravitational constant (d(lnG)/dt) to 3∙10-13 years-1 Gruppo di Gravitazione Sperimentale

7. Role of the accelerometer The analysis of experimental data to obtain the properties of a physical system requires models System dynamics Measurement procedure (Reference frame) The availability of good experimental data implies taking out a lot of “noise” in order to reach the phenomenology of interest – many orders of magnitude, in case of relativistic effects Gruppo di Gravitazione Sperimentale

8. Role of the accelerometer When available models for a particular effect are not accurate enough (or not present at all) the relevant information in experimental data is not correctly assessed (e.g., worst fit) A typical case is that of non-gravitational perturbations (direct solar radiation pressure, albedo radiation pressure, thermal effects, …) An on-board accelerometer can measure directly these effects and provide important information to improve the fit Gruppo di Gravitazione Sperimentale

9. Role of the accelerometer Estimation of direct solar radiation pressure from tracking data: the case of LAGEOS satellites Rough measure of uncertainty size A correlation with other phenomena can lead to an estimation bias Probably false signal True signal? • RP, Master and PhD Theses work • Lucchesi et al., Plan. Space Sci.52, 699 (2004) • RP, Master and PhD Theses work Gruppo di Gravitazione Sperimentale

10. New RSE concept HGA 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. 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 ISA This could be a problem, due to CoM movements (fuel sloshing and consumption) CoM This problem is related to the overall RSE concept, not to the single instruments This solution has been discussed by MORE Team (MORE PROGRESS MEETING, Roma, 13 March 2008), has been adopted as the new baseline and is currently under implementation (change of RSE Requirements …) Gruppo di Gravitazione Sperimentale

11. ISA measurements ISA Measurements definition: Final output of ISA measures are the components, in an inertial RF, of ISA vertex acceleration due to external (non gravitational) perturbations acting on the MPO, and to MPO motion; this is recovered (a-posteriori during the data analysis phase) using the “raw” acceleration data measured by ISA and ancillary data, produced by other MPO systems. • ISA measurements error definition: • The “Total measurement error” (i.e. the difference between measured value and true value) is considered to be composed of two parts: “total random noise” and “total deterministic error” that are defined as follow: • “Measurement deterministic error”: is the part of the “Total measurement error” formed by the harmonic components of the MPO orbital period, that are in the ISA measurement frequency band. • “Measurement random noise”: is defined as the difference between the “Total measurement error” and the “Measurement deterministic error”. Gruppo di Gravitazione Sperimentale

12. RSE total noise Instrument bandwidth Gruppo di Gravitazione Sperimentale

13. ISA accuracy requirement Gruppo di Gravitazione Sperimentale

14. Direct solar radiation pressure http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant Gruppo di Gravitazione Sperimentale

15. Direct solar radiation pressure Gruppo di Gravitazione Sperimentale

16. ISA measurements Measurement of non-gravitational perturbations acting on MPO spacecraft Support to RSE during Superior Conjunction Experiment (SCE) Measurement of V during MPO manoeuvres Gradiometry (currently, ISA is not on MPO Center of Mass…) In general, disentangling between gravitational and non-gravitational effects (anomalies, …) Gruppo di Gravitazione Sperimentale

17. ISA description ISA sensing element ISA pick-up Gruppo di Gravitazione Sperimentale

18. Differential Accelerometer Mechanical arrangement Seismic noise rejection Gruppo di Gravitazione Sperimentale

19. Acceleration due to the planet gravity field gradients Centrifugal acceleration Non–Gravitational accelerations Angular acceleration Coriolis acceleration Z ISA proof–mass Inside the MPO frame Y X Accelerations acting on ISA The acceleration on a point P (ISA proof–mass) close to the MPO COM is: = Planet gravity = MPO angular rate = MPO angular acceleration = MPO–proof–massvector Gruppo di Gravitazione Sperimentale

20. comISA COM Z–sensitive axis X–sensitive axis Rotation axis Y–sensitive axis Dropped requirements 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 Requirements Gruppo di Gravitazione Sperimentale

21. Vibrations Vibrational random noise on board the MPO inside the frequency band Micro-vibration random noise on board the MPO outside the frequency band Gruppo di Gravitazione Sperimentale

22.  4 °Cpp  25 °Cpp  4 °C/Hz ISA thermal issues MPO Temperature Variations • Over one orbital period (2.3 h) of the MPO • Over one sideral period (44 days) of Mercury • Random noise ISA operative temperature: -20/+30 °C ISA non operative temperature: -40/+40 °C FEE electronic stability: 5∙10-8 m/s2/  Hz ACC. mechanical stability: 5∙10-7 m/s2/  Hz Active thermal control attenuation: 700 Gruppo di Gravitazione Sperimentale

23. ISA thermal issues ISA thermal control system performance Gruppo di Gravitazione Sperimentale

24. ISA Error Budget Gruppo di Gravitazione Sperimentale

25. Data reduction procedure 25 dd/mm/yyyy Gruppo di Gravitazione Sperimentale Gruppo di Gravitazione Sperimentale

26. 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 Gruppo di Gravitazione Sperimentale

27. Calibration on ground Measurement of rotation matrix between ISA axes and optical cube Measurement of transduction factors for ISA sensing elements Check of alignment constancy after vibrational tests Measurement of sensing masses position at zero gravity Check of alignment constancy in time and possible measurement of aging Check of alignment constancy after thermal stress Gruppo di Gravitazione Sperimentale

28. ISA operations in cruise • RSE support • In-cruise calibration • Periodical checkouts • Long-term stability tests • ISA “cruise science” Gruppo di Gravitazione Sperimentale

29. ISA operations in cruise • Superior Conjunction Experiment (SCE) • Quiet dynamical environment (no thrust) • High-precision tracking • ISA on ISA support of POD Gruppo di Gravitazione Sperimentale

30. ISA operations in cruise The direct solar radiation pressure signal is above ISA sensitivity (possibly inside ISA band if MCS is rotating) Possibility of instrument calibration by comparison with tracking Gruppo di Gravitazione Sperimentale

31. ISA operations in cruise Required information for in-cruise calibration • Estimation of tracking accuracy during SCE, and therefore of the accuracy in recovering the MCS acceleration • Estimation of the expected acceleration signal acting on the MCS, to be confronted with the tracking accuracy and accelerometer sensitivity • Re-assessment of expected signals acting on the sensing elements, taking into account the different positioning of ISA with respect to MCS COM (instead of MPO COM) Transducer factor Gruppo di Gravitazione Sperimentale

32. ISA operations in cruise • Periodical checkouts • Long-term stability tests • Zero position of the sensing masses: potential drifts in the working positions of the sensing masses will be detectable by a continuous read-out • Noise level: the solar radiation pressure and the residual vibrations on MCS being the only source of vibration noise, it will be possible to test the instrument intrinsic noise with high accuracy Gruppo di Gravitazione Sperimentale

33. ISA operations in cruise “Cruise science” • Direct measurement of solar radiation pressure • Flybys (“anomalies”, gradiometric measurements) An accelerometer disentangles gravitational and non-gravitational effects Out of the spacecraft center of mass, an accelerometer measures also gravity gradients Gruppo di Gravitazione Sperimentale

34. Calibration in orbit • Nominal procedure: calibration using the internal actuators before every measurement arc • Backup procedure: calibration using the external acceleration produced by dedicated MPO manoeuvres every TBD days and every time the calibration with internal actuators shows an anomalous change of ISA parameters To be taken into account • The allowed manoeuvres, both in type and temporal allocation (this will require close co-operation with ESOC) • The MPO COM knowledge, still an important factor for this type of calibration (TBC) • ISA measurement band and sensitivity: the calibration signal must be inside ISA band and should be inside its dynamics Gruppo di Gravitazione Sperimentale

35. Current status • Feasibility study and proposal to ESA (May 2004) • ISA selected for MPO payload (November 2004) • Phase A/B1 Kick Off (January 2007) • Instrument Science Requirement Review (October 2007) • Review completed (scientific requirements frozen, apart from small changes due to the new RSE concept) • Instrument Preliminary Design Review (January 2009) • Review completed • Demonstration Model ongoing (developed technologies) • ISA Team laboratories renewed for performance and calibration tests on the various models Gruppo di Gravitazione Sperimentale