1. Laccelerometro 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 IX CONGRESSO NAZIONALE DI PLANETOLOGIA AMALFI IX CONGRESSO NAZIONALE DI PLANETOLOGIA AMALFI

2. BepiColombo Radio Science Experiments (RSE) 28/09/2009 Gruppo di Gravitazione Sperimentale 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 Three main experiments:Involved instruments: Ka–band Transponder Star–Tracker High Resolution Camera Accelerometer

3. RSE Objectives 28/09/2009 Gruppo di Gravitazione Sperimentale 3 global gravity fieldtemporal variations The 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 anomalies The local gravity anomalies (in order to constrain the mantle structure of the planet and the interface between mantle and crust) rotation state The 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–mass 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 Milani et al., Plan. Space Sci. 49, 1579 Milani et al., Phys. Rev. D 66, 082001

4. RSE measurements 28/09/2009 Gruppo di Gravitazione Sperimentale 4 Rangerange–rate 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) non–gravitational forces Determination of the non–gravitational forces acting on the MPO by means of an on–board accelerometer absolute attitude Determination of the MPO absolute attitude by means of a Star–Tracker angular displacements of reference points on the solid surface Determination of angular displacements of reference points on the solid surface of the planet, by means of a fotocamera

5. RSE science goals 28/09/2009 Gruppo di Gravitazione Sperimentale 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. RSE science goals 28/09/2009 Gruppo di Gravitazione Sperimentale 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. Role of the accelerometer 28/09/2009 Gruppo di Gravitazione Sperimentale 7 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

8. Role of the accelerometer 28/09/2009 Gruppo di Gravitazione Sperimentale 8 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) non- gravitational perturbations 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

9. Role of the accelerometer 28/09/2009 Gruppo di Gravitazione Sperimentale 9 RP, Master and PhD Theses work Lucchesi et al., Plan. Space Sci. 52, 699 (2004) Rough measure of uncertainty size Estimation of direct solar radiation pressure from tracking data: the case of LAGEOS satellites estimation bias A correlation with other phenomena can lead to an estimation bias Probably false signalTrue signal? RP, Master and PhD Theses work

10. New RSE concept 28/09/2009 Gruppo di Gravitazione Sperimentale 10 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 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 …) 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

11. ISA measurements 28/09/2009 Gruppo di Gravitazione Sperimentale 11 ISA Measurements definition: Finalin an inertial RFISA vertex acting on the MPO, and to MPO motion 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. 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.

12. RSE total noise 28/09/2009 Gruppo di Gravitazione Sperimentale 12 Instrument bandwidth

13. ISA accuracy requirement 28/09/2009 Gruppo di Gravitazione Sperimentale 13

14. Direct solar radiation pressure 28/09/2009 Gruppo di Gravitazione Sperimentale 14 http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant

15. Direct solar radiation pressure 28/09/2009 Gruppo di Gravitazione Sperimentale 15

16. ISA measurements 28/09/2009 Gruppo di Gravitazione Sperimentale 16 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, …)

17. ISA description 28/09/2009 Gruppo di Gravitazione Sperimentale 17 ISA sensing element ISA pick-up

18. Differential Accelerometer 28/09/2009 Gruppo di Gravitazione Sperimentale 18 Mechanical arrangement Seismic noise rejection

19. Accelerations acting on ISA 28/09/2009 Gruppo di Gravitazione Sperimentale 19 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

20. Dropped requirements 28/09/2009 Gruppo di Gravitazione Sperimentale 20 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 Requirements

21. Vibrations 28/09/2009 Gruppo di Gravitazione Sperimentale 21 Vibrational random noise on board the MPO inside the frequency band Micro-vibration random noise on board the MPO outside the frequency band

22. ISA thermal issues 28/09/2009 Gruppo di Gravitazione Sperimentale 22 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 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

23. ISA thermal issues 28/09/2009 Gruppo di Gravitazione Sperimentale 23 ISA thermal control system performance

24. ISA Error Budget 28/09/2009 Gruppo di Gravitazione Sperimentale 24

25. Data reduction procedure dd/mm/yyyy Gruppo di Gravitazione Sperimentale 25 28/09/2009 25 Gruppo di Gravitazione Sperimentale

26. Performance and calibration 28/09/2009 Gruppo di Gravitazione Sperimentale 26 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

27. Calibration on ground 28/09/2009 Gruppo di Gravitazione Sperimentale 27 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

28. ISA operations in cruise 28/09/2009 Gruppo di Gravitazione Sperimentale 28 RSE support In-cruise calibration Periodical checkouts Long-term stability tests ISA cruise science RSE support In-cruise calibration Periodical checkouts Long-term stability tests ISA cruise science

29. ISA operations in cruise 28/09/2009 Gruppo di Gravitazione Sperimentale 29 Superior Conjunction Experiment (SCE) Quiet dynamical environment (no thrust) High-precision tracking ISA on ISA support of POD

30. ISA operations in cruise 28/09/2009 Gruppo di Gravitazione Sperimentale 30 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

31. ISA operations in cruise 28/09/2009 Gruppo di Gravitazione Sperimentale 31 Required information for in-cruise calibration tracking accuracy during SCEEstimation of tracking accuracy during SCE, and therefore of the accuracy in recovering the MCS acceleration expected acceleration signal acting on the MCSEstimation of the expected acceleration signal acting on the MCS, to be confronted with the tracking accuracy and accelerometer sensitivity expected signals acting on the sensing elementsRe-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) tracking accuracy during SCEEstimation of tracking accuracy during SCE, and therefore of the accuracy in recovering the MCS acceleration expected acceleration signal acting on the MCSEstimation of the expected acceleration signal acting on the MCS, to be confronted with the tracking accuracy and accelerometer sensitivity expected signals acting on the sensing elementsRe-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

32. ISA operations in cruise 28/09/2009 Gruppo di Gravitazione Sperimentale 32 Periodical checkouts Long-term stability tests Zero position of the sensing massesZero position of the sensing masses: potential drifts in the working positions of the sensing masses will be detectable by a continuous read- out Noise levelNoise 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

33. ISA operations in cruise 28/09/2009 Gruppo di Gravitazione Sperimentale 33 Cruise science Direct measurement of solar radiation pressure Flybys (anomalies, gradiometric measurements) 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

34. Calibration in orbit 28/09/2009 Gruppo di Gravitazione Sperimentale 34 Nominal procedure Nominal procedure: calibration using the internal actuators before every measurement arc Backup procedure 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 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 To be taken into account

35. Current status 28/09/2009 Gruppo di Gravitazione Sperimentale 35 Feasibility study and proposal to ESA Feasibility study and proposal to ESA (May 2004) ISA selected for MPO payload ISA selected for MPO payload (November 2004) Phase A/B1 Kick Off Phase A/B1 Kick Off (January 2007) Instrument Science Requirement Review 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 Instrument Preliminary Design Review (January 2009) Review completed Demonstration Model Demonstration Model ongoing (developed technologies) ISA Team laboratories ISA Team laboratories renewed for performance and calibration tests on the various models Feasibility study and proposal to ESA Feasibility study and proposal to ESA (May 2004) ISA selected for MPO payload ISA selected for MPO payload (November 2004) Phase A/B1 Kick Off Phase A/B1 Kick Off (January 2007) Instrument Science Requirement Review 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 Instrument Preliminary Design Review (January 2009) Review completed Demonstration Model Demonstration Model ongoing (developed technologies) ISA Team laboratories ISA Team laboratories renewed for performance and calibration tests on the various models