1. GRUPPO DI GRAVITAZIONE SPERIMENTALE – IAPS - INAF V. Iafolla, E. Fiorenza, C. Lefevre, D.M. Lucchesi, M. Lucente, C. Magnafico, S. Nozzoli, R. Peron, F. Santoli Istituto di Astrofisica e Planetologia Spaziali – Istituto Nazionale di Astrofisica BEPICOLOMBO ISA ACCELEROMETER TUTORIAL Luogo, dataSWT12

2. INTRO

3. 3 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ITALIAN SPRING ACCELEROMETER BASICS

4. 4 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ITALIAN SPRING ACCELEROMETER BASICS Gravity act on the sensing mass and the satellite structure with the same acceleration All other Surface Forces (S) doesn’t ISA measures only what is not caused by the gravity

5. 5 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 Control capacitors Pick–up capacitors Proof–mass ITALIAN SPRING ACCELEROMETER BASICS Each ISA sensing element is a flexural harmonic oscillator presently obtained by working a single piece of aluminium 5056. The pick-up system of the accelerometer is based on capacitors read–out flexural foil proof–mass Pick-up Plates acceleration

6. 6 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE INTRODUCTION The BepiColombo RSE represents a complex mix of measurements and scientific objectives and, very interesting, it is not possible to separate them neatly in independent experiments. However, we can distinguish: 1) a gravimetry experiment 2) a rotation experiment 3) a relativity experiment Basically, on–board the MPO, the instruments used for these experiments are: Ka–band Transponder Star–Tracker High Resolution Camera Accelerometer 1. Tracking: L. Iess (Univ. Roma) PI – MORE 2. Accelerometer:V. Iafolla (IAPS-INAF Roma) PI – ISA 3. Hi-Res Camera:E. Flamini (ASI Roma) PI – SYMBIOSYS 4. Orbit determination:A. Milani (Univ. Pisa) - MORE

7. 7 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE BepiColombo aims to perform: 1. a detailed study of the planet Mercury and its environment 2. and to test Einstein’s General Relativity to an unprecedented level of accuracy The MPO will be a 3–axis stabilized spacecraft, Nadir pointing, and equipped with a complete set of instruments in order to perform Radio Science Experiments (RSE)

8. 8 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE SCIENTIFIC GOALS global gravity fieldtemporal variations solar tides I.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 II.the local gravity anomalies (in order to constrain the mantle structure of the planet and the interface between mantle and crust) rotation state III.the rotation state of Mercury (in order to constrain the size and the physical state of the core of the planet) orbitcenter–of–mass propagation of electromagnetic waves IV.the orbit of Mercury’s center–of–mass around the Sun and the propagation of electromagnetic waves (in order to improve the determination of the parameterized post–Newtonian γ and β parameters of General Relativity) Milani et al., Plan. Space Sci. 49, 1579 Milani et al., Phys. Rev. D 66, 082001 Gravity Experiment Rotation Experiment Relativity Experiment

9. 9 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE RSE - SCIENTIFIC GOALS Milani et al., Phys. Rev. D 66, 082001 (2002)

10. WHAT’S THE ISA ROLE IN RSE

11. 11 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE a. Range and range–rate measurements of the MPO position and velocity with respect to Earth–bound radar station(s) (and then of Mercury’s center–of–mass around the Sun) b. the determination of the MPO absolute attitude by means of a Star–Tracker c. the determination of angular displacements of reference points on the solid surface of the planet by means of an High Resolution Camera d. the determination of the non–gravitational signals on the MPO by means of an on–board Accelerometer NEEDED MEASUREMENTS The requirements for the tracking system are:  Range:20 ÷ 30 cm (2-way)  Range rate:3  10  4 cm/s (2-way) Solar Radiation Pressure Anisotropic MPO IR emissions Gas Leaks

12. 12 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE OBSERVABLES AND DATA NEEDED BepiColombo RSE, will use the following data: a. Range and range–rate measurements of the MPO position and velocity with respect to Earth–bound radar station(s) b. the determination of the non–gravitational signals on the MPO by means of an on–board Accelerometer c. the determination of the MPO absolute attitude by means of a Star–Tracker d. the determination of angular displacements of reference points on the solid surface of the planet by means of an High Resolution Camera The requirements for the tracking system are:  Range: 20 – 30 cm (2-way)  Range rate: 3  10  4 cm/s (2-way) RSE measurements are based on the difference between the real observed orbit of the spacecraft and a computed modelled orbit. Observed obs – Computed Obs = residuals By adjusting a set of parameters it can be possible to minimize residuals and determine a best solutions for the desired parameters (ie. Gravity field coefficients) Mercury MPO

13. 13 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 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 our case

14. 14 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 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) 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

15. RSE METHODOLOGY

16. 16 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 BEPICOLOMBO RSE ISA MEASUREMENTS Accelerations acting on the spacecraft can be divided in: Gravitational: Mercury gravity field Sun gravity field Other planets and moons Relativistic effects of the sun space-time distortions Non Gravitational Solar radiation pressure MPO gas leaks MPO unbalanced forces of RW desaturation thrusters Mercury albedo Anisotropic thermal emissions Non Gravitational accelerations are very hard to model, so the best is to directly measure it: ISA is insensible to gravity accelerations, it reacts only to non-gravitational accelerations. RSE will use ISA data to remove non-gravitational acceleration from the observables

17. 17 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 WHAT ISA MEASURES http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant

18. 18 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 In order to reach the very ambitious goals of the RSE, ISA main role is to measure the non- gravitational accelerations (NGA) in the strong radiation environment around Mercury with an accuracy of 10  8 m/s 2 over one orbital period of the MPO (about 8355 s) This acceleration corresponds to an along-track accuracy of about 1 m on the same time span The NGA are complex and subtle and the strong radiation environment of Mercury is characterized by strong day/night asymmetries and a huge variation of the solar irradiance  during the sidereal year (14448 W/m 2 – 6272 W/m 2 ) The NGA are proportional to the area-to-mass ratio of the body, and are very difficult to be properly modelled for a complex in shape (S/A) and active (HGA) satellite like the MPO WHAT ISA MEASURES

19. 19 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 WHAT ISA MEASURES

20. 20 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA MEASUREMENTS 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 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) and has been adopted as the new baseline 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

21. 21 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA MEASUREMENTS ON BOARD 1.Non–gravitational accelerations (NGA), our main target for the RSE 2.Accelerations due to gravity gradients 3.Apparent accelerations 4.MPO center-of-mass (CM) accelerations 5.Accelerations due to thruster maneuvers In formula, this translates in: which would impose requirement and knowledge in the position of the test masses and in the attitude of the spacecraft, as well as in the spacecraft CM drifts and accelerations.

22. 22 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 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 follows: “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”.

23. 23 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA MEASUREMENTS  Measurement of non-gravitational perturbations acting on MPO spacecraft  Support to RSE during Superior Conjunction Experiment (SCE)  Measurement of  V during MPO maneuvers  Gradiometry (currently, ISA is not on MPO Center of Mass…)  In general, disentangling between gravitational and non- gravitational effects (anomalies, …)

24. 24 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA MEASUREMENTS ISA TO MORE DATA FORMAT AND DEFINITIONS ISA Measurements definition: Final output of ISA measures are the components, in an body 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 follows: “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”. Fg OBT Ax Ay Az dAx dAy dAz Ox Oy Oz dOx dOy dOz On board time Status flag X-Y-Z accelerations wrt body-fixed MPO rf X-Y-Z accelerations uncertainties recentering acc. offset recentering offset uncertainties 1Hz data table in the following format:

25. ISA HARDWARE

26. 26 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA DM SENSING ELEMENTS The industrial contractor in charge of design, building and test of the space version of ISA accelerometer

27. 27 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 INSTRUMENT DEVELOPMENT ENGINEERING QUALIFICATION MODEL ISA (IDA) EQM

28. 28 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 INSTRUMENT DEVELOPMENT

29. 29 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 RSE TOTAL NOISE Instrument bandwidth

30. 30 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 RSE TOTAL NOISE Instrument bandwidth Orbital Period

31. 31 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA ACCURACY REQUIREMENT Frequency Hz Acceleration m/s2/√Hz 3 × 10 -5 3 × 10 -8 10 -4 - 10 -3 10 -8 10 -1 10 -7 Electronic noise level: 10 -10 g/√Hz This level has been reached via rejection between two ISA-like accelerometers

32. 32 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 INSTRUMENT DEVELOPMENT IPDRJan 2009 RSE-ReqDoc review of draft 3Jan 2009 EID-B first signed draftFeb 2009 ISA STM PSRJun 2010 EID-B second signed draftFeb 2011 ISA EM deliveryMar 2011 ICDR Nov 2011 – March 2012  ISA Demonstration Model (not del.) testingOngoing  ISA EQM (not del.) Jul 2012  ISA FM deliveryDec 2012 WARNING NOT UP TO DATE

33. 33 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 INSTRUMENT DEVELOPMENT ACTIVITIES ON FM Ongoing activities FM sensors finalization: tuning and characterization (to be completed by February). Calibration facilities/procedures verification (to be completed by February). Definition of in flight alignments calibration approach (to be completed by ???) Future activities FM performance tests (February 10-26) –Thermal sensitivity –Instrumental noise –Transfer function –Cross-talk –Polynomial corrector FM sensors calibration of internal calibration source for in flight calibrations (March 3-24) Internal alignments calibration (April 21 – May 12) Update of accelerometer error budget (February 27 – May 12) EQM refurbishment to FS (May 14 – July 23) Update of the ISA model for RSE simulations (May 14 – July 23) FS test and calibration (July 23 – October 8) WARNING NOT UP TO DATE

34. ISA PERFORMANCE

35. 35 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 PERFORMANCE AND ACCURACY TABLE ISA oscillator parameters: Mass200 g Resonance frequency3.9 Hz Mechanical quality factor (Q)10 ISA performance: Measurement bandwidth 3 × 10  5 ÷ 1 × 10  1 Hz Intrinsic noise 1 × 10  9 m/s 2 /  Hz Measurement accuracy 1 × 10  8 m/s 2 Dynamics 300 × 10  8 m/s 2 A/D converter saturation 3000 × 10  8 m/s 2 ISA thermal stability: Sensor thermal sensitivity 5 × 10  7 m/s 2 /°C Electronic thermal sensitivity 1 × 10  8 m/s 2 /°C Active thermal control attenuation700 Temperature variations: Mercury half sidereal period (44 days) 25°C peak-to- peak MPO orbital period (2.325 h) 4°C peak-to- peak Random noise 10°C /  Hz

36. 36 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 UPDATED ISA ERROR BUDGET Error termError Share MPO mechanical noise + Data reduction residuals Errors related to the reconstruction of acceleration vector at ISA vertex. Due to ancillary data knowledge accuracy. (MPO motion, attitude, ISA geometry,…) Periodic 55%A 0 Random 90%S 0 Instrument Calibration errors Periodic 10%A 0 Random Negl. Thermal effects Periodic 15%A 0 Random 30%S 0 ISA intrinsic noise (FEE noise, Brownian Noise, dissipation in damping resistors,…) + ISA response to in-band accelerations (FRF flatness, Non linearity, Crosstalk) Periodic 10%A 0 Random 10%S 0 Out of band accelerations effects Periodic 10%A 0 Random 30%S 0

37. 37 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 UPDATED ISA ERROR BUDGET Error term Error Share Main requirements Derived requirements / parameters to be verified Requirement verification Method MPO generated mechanical noise + Data reduction residuals Errors related to the reconstruction of acceleration vector at ISA vertex. Due to ancillary data knowledge accuracy. (MPO motion, attitude, ISA geometry,…) Periodic 55%A 0 TBC Random 90%S 0 “Acceleration disturbances” spectral density (RSE-385) Final verification by ASD MPO structural vibrations (No req.)Analysis (ASD) MPO AOCS ancillary data (No req.)Analysis (ASD) MPO contribution to AME (No req.)Analysis (ASD) Sensing axes directions (ISA-SR-306)DM test (TAS) ISA contr. to AME (ISA-SR-308) Axes directions stability (depends on delta T at IDA feet) Analysis + DM test (TAS) Axes direction calibration accuracy ISA calibration facility test (PI) Sensing masses position knowledge (ISA-SR-304)Analysis (TAS) Instrument Calibration errors Periodic 10%A 0 TBC Random Negl. Overall calibration accuracy Final verification by PI ISA on-ground calibration performance ISA calibration facility test (PI) ISA in-flight calibration performanceAnalysis (PI) Operability in inverted pendulum (ISA-SR-413)Design (TAS) Internal calib. signal perf. (ISA-SR-418)DM test (TAS) Maximum signal (RSE-38) Analysis (ASD) Thermal effects Periodic 15%A 0 Random 30%S 0 Effects of external temperature variations Final ver. by TAS Periodic and ‘random’ temperature variations (RSE-47) Analysis (ASD) ISA overall thermal sensitivity (ISA-SR-106) FEE th. sens. SH th. sens. TCS performance DM test (TAS) ISA intrinsic noise (FEE noise, Brownian Noise, dissipation in damping resistors,…) + ISA response to in-band accelerations (FRF flatness, Non linearity, Crosstalk) Periodic 10%A 0 Random 10%S 0 ISA intrinsic noise (ISA-SR- 101) Internal determ. error (ISA- SR-103) Rigid structure (ISA-SR-109) Final ver. by TAS FEE noiseDM test (TAS) Crosstalk, FEE linearityDM test (TAS) Mechanical Transfer FunctionDM test (TAS) Maximum signal (RSE-38)Analysis (ASD) Out of band accelerations effects Periodic 10%A 0 Random 30%S 0 Effects of out-of-band accelerations (ISA-SR-108). Final ver. by TAS ISA sensitivity to out-of-band accelerationsDM test (TAS) Out of band periodic vibr. (RSE-39)Analysis (ASD)

38. 38 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 PERFORMANCE ACCURACY

39. 39 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 PERFORMANCE RANDOM NOISE

40. ISA CALIBRATION STATEGIES

41. 41 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION DIFFERENT MEANINGS A distinction must be made between: Intrinsic instrument calibration It corresponds to the usual meaning of the term, i.e. it denotes the procedures to be performed in order to relate the instrument output to its input. This implies adequate knowledge of all the relevant parameters. RSE “calibration” This is related to the fact that ISA measurements are used in the RSE context. In particular, these are relative measurements (due to the limited bandwidth of the instrument). This requires a different type of “calibration”, performed during the parameter estimation procedure, in order to obtain absolute measurements.

42. 42 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION DIFFERENT MEANINGS A distinction must be made between: Actuator Transduction factor (-> on-ground ) Parameter that links given actuator Voltage to an acceleration acting on the mass. Pick-up Transduction factor (-> in-flight ) Parameter to convert Voltage to m/s 2 Internal Axes Misalignment (-> both on-ground and in-flight) The angles between ISA URF and the real ISA sensing axes

43. 43 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION INTRINSIC INSTRUMENT CALIBRATION Procedures to be performed in order to relate the instrument output to its input. This implies adequate knowledge of all the relevant parameters. Sensing Mass Zero Positioning Find the center of sensing mass with respect to the pick-up plates (electric-zero) Actuator Calibration Find the relationship between a voltage applied to the actuation plates and the relative acceleration acting to the sensing mass. On-Ground Sensing axes alignments Find orientation of three ISA sensing axes with respect to ISA optical cube Pick-up Transduction factor What is the real input to output calibration: Relate a Voltage output to a real displacement of the sensing mass. In-Flight Sensing axes alignments to position ISA ILS with respect to MPO-RF On-ground In-flight

44. 44 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION OPPORTUNITIES Functional checks and instrument calibration on-ground (before integration on the MPO) In the first step all the functionality tests of the accelerometer and the measurements of all the quantities involved in the evaluation, from the raw output data of the accelerometer, of the inertial acceleration of one point of the accelerometer with respect to the inertial reference frame, are performed. Functional checks on-ground (after integration on the MPO) The functional checks have the goal of verifying the general instrument functionality once it has been installed on MPO (no performance and calibration tests are envisaged after its integration on MPO). Instrument calibration (in-flight calibration) Concerning the instrument calibration in-flight, three different procedures are envisaged: a nominal one using internal actuators (both in cruise and in orbit), a backup one using MPO manoeuvres (in orbit only and without high-precision knowledge of MPO COM) and a further possibility using linear acceleration measured by tracking (cruise only).

45. 45 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION ON GROUND 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

46. 46 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION ON GROUND AXES DIRECTIONS Goal: measure the direction of each sensing axis with respect to IDA UOAF Measurement principle and setup architecture already addressed New “simplified” procedure for ISA on- ground calibration: Performed at “sea-level” air pressure Clean room Only one linear stage and two rotation stages Angles measured by laser range sensors No stringent requirements on rotations accuracy Mechanical I/Fs needed to put the unit in six different positions

47. 47 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION ON GROUND AXES DIRECTIONS Goal: measure the direction of each sensing axis with respect to IDA UOAF Procedure for ISA on-ground calibration: Performed at “sea-level” and at air pressure Clean room (controlled temperature and humidity) Only one linear stage and two rotation stages Angles measured by laser range sensors Mechanical Facility (now complete) to put the unit in six different positions

48. 48 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION ON GROUND AXES DIRECTIONS

49. 49 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION ON GROUND ACTUATOR Purpose of this calibration: finding the relationship between the voltage applied to the actuators plates and the equivalent acceleration Acceleration induced with known tilt Displacement induced with voltage

50. 50 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION ON GROUND ACTUATOR 25 arcsec (about 121 µrad) peak-to-peak → acceleration equal to ± 6 ∙10 -4 m/s 2 Autocollimator autocollimator ISA Piezo actuator

51. 51 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION IN FLIGHT CRUISE ESA, BC-ESC-RP-05500, Issue 3.2 Trajectory with launch on 15 August 2015

52. 52 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION IN FLIGHT CRUISE 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) 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) Transduction factor

53. 53 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION IN FLIGHT 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

54. 54 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 Y X zz X isa Z Z isa X isa YY xx The advantages of an in-cruise calibrations for sensing axis misalignments are: 1.Weak SRP 2.No gravity gradients perturbation on ISA 3.No planetary infrared radiation perturbation CALIBRATION IN FLIGHT AXES DIRECTIONS, IN CRUISE

55. 55 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION IN FLIGHT 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 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

56. 56 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 IN CRUISE Maneuver to be performed during cruise Weak SRP No gravity gradients perturbation on ISA No planetary infrared radiation perturbation Long integration time available. The latest simulation indicate large errors due to thermoelastic distortions between in-cruise and operative conditions) CALIBRATION IN FLIGHT IN FLIGHT CALIBRATION STRATEGIES Y X zz X isa Z Z isa Y isa YY xx Y X zz X isa Z Z isa Y isa YY xx IN ORBIT Maneuver to be performed during Mercury orbit and in eclipse. Operative conditions Planet IR perturbation IR emissions due to MPO radiator cooling Gravity Gradients perturbation on ISA Real challenge is to remove CoM movements due to Fuel Sloshing and appendix, during calibration rotations

57. 57 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION IN FLIGHT IN FLIGHT CALIBRATION SIMPLIFIED PROCEDURE  xy x y z X Y Z yaya xaxa zaza r CoM  xz  zy  yz  zx  yx Misalignment Matrix Isa measured acceleration ISA occurring Acceleration Acceleration MCS Rotating Acceleration MCS Standing OBSERVED OBSERVABLES RESIDUALS C O M PositionMisalignments r x r y r z  xy  xz  yx  yz  zx  zy PARAMETERS

58. 58 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 Y X zz X isa Z Z isa X isa YY xx Calibration of axes direction will be done by performing three rotation maneuvers around the expected ISA axes. ISA team will collect apparent accelerations and will compare them with AOCS data to reconstruct the real alignments of the axes. CALIBRATION IN FLIGHT AXES DIRECTIONS, IN ORBIT During the maneuver calibration ISA needs a very quite spacecraft environment. This could be performed by blocking all moving mechanism and executing the calibration during eclipse.

59. 59 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CALIBRATION IN FLIGHT AXES DIRECTIONS, IN ORBIT – POINTING NEEDS Goal: to perform alignments calibration (ISA vs AOCS ). 2 types of S/C maneuvers under study. Preliminary outcomes: –To be performed in Mercury orbit. –at least two rotations axes needed. –to be performed during eclipses.

60. OPERATIONS

61. 61 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 Typical scenario with two Earth tracking stations: During the nominal phase, ISA will be always ON and in Nominal Observations mode; exception to this could be for security reasons only Indeed, ISA measurements will be very useful for the RSE also during the un- observed (from Earth) arcs ISA turning OFF would have a negative impact on RSE, due to data loss of the order of a few days for each turning OFF (IDA thermal stabilization taks about two days) Therefore, the strategy to be developed for ISA operations will be very important in order to obtain the best performance and provide very long time series to MORE for the RSE objectives X–band linkX–band + Ka–band links 0h3h11h19h24h 8h20h DM OPERATIONAL STRATEGY

62. 62 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 ISA operations fall into these Operative Modes: 1.Nominal Observations (RSE); 2.High Rate Observations; 3.Calibration; 4.DeltaV measurements; From our point of view, all these phases are to be considered in the Science Modes of the accelerometer, see EID-B. In particular, if the saturation problem of the accelerometer is avoided during the DeltaVs measurements (no change in the nominal dynamics …), item 4 may be included in item 1, from the point of view of the Operative Modes, but not from the point of view of the Operations, because we need information about the events of the offloading maneuvers. OPERATIONAL STRATEGY

63. 63 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY ISA OPERATIVE MODES

64. 64 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY

65. 65 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY THERMAL CONTROL About 2h off-duty due to thermalization time. Baseline of TAS-I is to change SetPoint manually, via TC. Heater power sufficient to overlap the set- point of 5°C. 8 set-point changes per sidereal year (baseline). Heater power capabilities are now freezed; they cannot be changed. Boundary temperature sidereal variation ±12.5°C Boundary temperature orbital variation ±2°C 5 set-point in order to save power consumption Together with TAS-I we are studying different SP change approach: Temperature forecast OBCP Other information about thermal perturbations given by random switches of instruments in the proxymity of ISA are needed from ASD.

66. 66 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY NEAR EARTH COMMISSIONING PHASE 1.SW Functional Verification: Will be almost the same as used in TBTV SW test 2.General instrument functionality: To check instrument functionality after launch shock and masses off-centering and to check the on-board noise 3.Thermalization: To check thermal control functionality and to thermalize the instrument in order to fulfil following activities 4.Self-Calibration 5.Chain Calibration Health and functionality CheckPerformance Test 1.Thermal Test (FEE set point changes transfer function, Thermalization times check): To check BELA and Kat switch-on effects. 2.High-Rate Observation campaign: Inspect out of band noise for the cruise phase. 3.Nominal Observation campaign Two kind of test are foreseen and right now under discussion with ESA 1 2

67. 67 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY PERFORMANCE CHECKS @ MERCURY During commissioning phase: Instrument functionality checks Thermal control system verification and tuning Self Calibrations (full version)* S/C assisted calibration (axes orientation)* High-Rate Observation campaign* Nominal Observation campaign * To be repeated periodically to track performance.

68. 68 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 During the RSE with both X-band and Ka-band tracking, ISA must work fully exploiting its best performance. When out of the tracking session, if ISA measurements are not necessary for the RSE, ISA may measure with reduced performance when the thermal and mechanical environmental conditions (see Perihelion phase) do not allow to exploit the nominal performance of the instrument. - Therefore, the thermal environmental conditions and the on board noise must both fulfill ISA requirements, as explained in the RSE-RD. - Also the down-link of the accelerometer data shall be ensured permanently. OPERATIONAL STRATEGY NOMINAL OBSERVATIONS - RSE

69. 69 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 This mode is useful to estimate the impact of the out of band noise on the accelerometer measurements and, especially, to calculate the needed damping factor. In this mode the accelerations signal of one sensor will be sampled at 500 Hz, with regard to the other two sensors the accelerations signals will be sampled at 10 Hz. OPERATIONAL STRATEGY HIGH RATE OBSERVATIONS - HRO

70. 70 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY CALIBRATION 1. Self Calibration. An internal calibration to be performed periodically using ISA's actuators. - This calibration needs to include first the measurement and adjustment of the working- point of the capacitive bridge and then the measurement of the transducer factor for each of the three sensing elements of the accelerometer. - To be done during the unobserved arc and before the next tracking arc. - During this calibration the nominal environmental conditions are necessary. 2. Chain Calibration. Instrument total transfer function determination. - To be done when necessary and during the unobserved arc and before the next tracking arc. 3. S/C Assisted Calibration. ISA calibration using MPO maneuvers. - To be done when necessary (also during the calibration of other instruments and/or the Flip Over maneuvers).

71. 71 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY DELTA V MEASUREMENTS These measurements will help in the MPO orbit reconstruction after the offloading maneuvers of the S/C Reaction Wheels, especially in the along- track direction, the most important for the RSE objectives. We received (recently) from ASD new information on the thrust profiles with some changes with respect to previous information. Hence, we need to carefully analyze such information, which is not anyway definitive, in order to check if we are able to measure the DeltaVs avoiding the accelerometer saturation. Obviously, the accuracy of the measurements is a function of the effective dynamics of the accelerometer.

72. 72 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 DELTA V MEASUREMENTS Time domain result Frequency domain result Results from previous simulations: 0.6% discrepancy with respect to  a z 

73. 73 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 OPERATIONAL STRATEGY CALIBRATIONS 1. Self Calibration. An internal calibration to be performed periodically using ISA's actuators. - This calibration needs to include first the measurement and adjustment of the working- point of the capacitive bridge and then the measurement of the transducer factor for each of the three sensing elements of the accelerometer. - To be done during the unobserved arc and before the next tracking arc. - During this calibration the nominal environmental conditions are necessary. 2. Chain Calibration. Instrument total transfer function determination. - To be done when necessary and during the unobserved arc and before the next tracking arc. 3. S/C Assisted Calibration. ISA calibration using MPO maneuvers. - To be done when necessary (also during the calibration of other instruments and/or the Flip Over maneuvers).

74. 74 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 VEGASO Possible measurements / calibrations Measurement of non gravitational perturbations acting on the composite during the flybys Measurement of gravitational gradients induced by the primary (i.e., Venus) gravitational field Measurement conditions ISA can work during flyby (no need to access to outside the spacecraft) Subject to power / thermal conditions

75. 75 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 VEGASO SAP

76. 76 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 VEGASO SAP

77. DATA HANDLING & ARCHIVING

78. 78 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 DATA FLOW

79. 79 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 NOMINAL DATA REDUCTION

80. 80 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 DATA HANDLING AND ARCHIVING High-level processing scheme Input/output data for each processing step Computation of expected data volumes Proposal of different definitions for PDS4 processing levels Preparation of PDS4 data models

81. 81 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 SCIENCE GROUND SEGMENT DEVELOPMENT DATA HANDLING Purpose of the ongoing activity is to design and implement the instrument data processing pipeline which performs the data selection, reduction and calibration and formats data to be passed to MORE Team (and to the community).

82. 82 INSTRUMENT DATA PROCESSING DEVELOPMENT STATUS ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM

83. 83 INSTRUMENT DATA PROCESSING DEVELOPMENT SCHEDULE ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM

84. 84 INSTRUMENT DATA PROCESSING DEVELOPMENT SCHEDULE ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM MILESTONE

85. 85 INSTRUMENT DATA PROCESSING DEVELOPMENT SCHEDULE ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM MILESTONE Final programming language

86. 86 INSTRUMENT DATA PROCESSING DEVELOPMENT SCHEDULE ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM MILESTONE

87. 87 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 SCIENCE GROUND SEGMENT DEVELOPMENT DATA ARCHIVING Purpose of the ongoing activity is to select the instrument TM to be formatted and grouped in order to make a proper input to the data processing pipeline and to prepare the PDS4 data models of raw data.

88. 88 PDS PRODUCTS DEFINITION STATUS ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM BC-SGS-ICD-015 ISA Experiment to Archive ICD TMRaw Field NameByte / bit offset PTC/PFCTM2RAW field FormatConversion or extraction needed TimeStamp0N/A (48 bit) OBTASCII_Date ASCII_Time Y (TBC) Acc 0 Max64-12MAX 0ASCII_Integer Acc 1 Max84-12MAX 1ASCII_Integer Acc 2 Max104-12MAX 2ASCII_Integer GROUP REPETION X 10 Acc 0 Data 04-14POS 0ASCII_Integer Acc 1Data 44-14POS 1ASCII_Integer Acc 2Data 84-14POS 2ASCII_Integer Temp 0 Data 104-12TEMP 0ASCII_Integer Temp 1 Data 124-12TEMP 1ASCII_Integer Temp 2 Data 144-12TEMP 2ASCII_Integer Example taken from technical Note ISA Data Archiving, BC-ISA-TN- 10018 (currently Draft C) and related to scientific observations TM Defined RAW science products HK data for quality control BC-ISA-TN-10018 draft C BC-ISA-TN-10018 draft D Ongoing (ready until end 02/2015) HK data for the quick look Auxiliary MPO data Calibration products MPO geometry

89. 89 NEED TO PROCESS AND / OR ARCHIVE DATA ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM Before Launch e.g data resulting from the on-ground calibration efforts on ground calibration data Alignment matrix Transduction factors During the Near-Earth Commissioning Phase (NECP) In-flight calibration data Transduction factors Transfer function

90. 90 QUICK LOOK ANALYSIS IDEAS ESA - ESTEC 27 Jan 2015INAF – IAPS ISA TEAM

91. 91 SCIENCE WORKING TEAM MEETINGLondon, 10 September 2015 CONCLUSIONS ISA accelerometer is an important part of the BepiColombo RSE aimed at establishing important results on Mercury geophysics and fundamental physics A new concept for the RSE has been suggested for this mission, referring the measurements to the accelerometer reference point and thereby avoiding the problems connected with MPO center of mass knowledge ISA to MORE Data exchange format is now available Data handling pipelines is under construction A data reduction procedure will enable full exploiting of data information content Calibration procedures in all the phases of the mission are essential to fully employ the instrument characteristics