New concept for the BepiColombo Radio Science Experiment: rôle of ISA accelerometer

1. XCIV Congresso Nazionale SIF Genova, 22-27 Settembre 2008 New concept for the BepiColombo Radio Science Experiment: rôleof ISA accelerometer 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 Experiment (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

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

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

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

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

7. RSE total noise Instrument bandwidth

8. ISA accuracy requirement inside the frequency band

9. Direct solar radiation pressure: solar irradiance From http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant

10. Direct solar radiation pressure: the transversal acceleration and its spectrum

11. ISA description ISA sensing element ISA pick-up

12. Differential Accelerometer Mechanical arrangement Seismic noise rejection 08/06/2006 Emiliano Fiorenza

13. 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

14. comISA COM Z–sensitive axis X–sensitive axis Rotation axis Y–sensitive axis ISA Positioning 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

15. Vibrations Vibrational random noise on board the MPO inside the frequency band Micro-vibration random noise on board the MPO outside the frequency band

16.  4 °Cpp  25 °Cpp  4 °C/Hz ISA thermal system 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

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

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

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

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

21. 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 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 …)

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