Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario V. Iafolla, E. Fiorenza, C. Lefevre, S. Nozzoli, R. Peron, M. Persichini, A. Reale, F. Santoli The BepiColombo ISA accelerometer: new error budget and calibration strategies 37th COSPAR Scientific Assembly Montreal, Canada, July 2008 Istituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Roma, Italy

Abstract Some references The ESA BepiColombo mission, for the exploration of planet Mercury, has been rescheduled for launch in 2014 with a redesigned two–satellite configuration. Among the main scientific objectives of this mission are the gravimetry and rotation of Mercury and tests of Einsteins theory of general relativity, to which the ISA (Italian Spring Accelerometer) instrument on– board the MPO (Mercury Planetary Orbiter) spacecraft will give a fundamental contribution by providing a continuous measurement of the non–gravitational accelerations acting on the spacecraft itself. Following a recent redefinition of the Radio Science procedure, in which the equations of motion are formulated with respect to the accelerometer reference point, the error budget for the instrument has been updated. This will be discussed, together with the calibration strategies which are being developed for the various mission phases. Iafolla, V. and Nozzoli, S., Plan. Space Sci. 49, 1609 (2001) Iafolla, V. et al., Adv. Space Res. 38, 639 (2006) Iafolla, V. et al., Celest. Mech. Dyn. Astron. 97, 165 (2007) Iafolla, V. et al., Plan. Space Sci., accepted for publication (2009)

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario 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 Gravimetry Rotation General relativity Gravimetry Rotation General relativity Involved instruments Ka-band Transponder Star-Tracker High Resolution Camera Accelerometer Ka-band Transponder Star-Tracker High Resolution Camera Accelerometer

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, Milani et al., Plan. Space Sci. 49, 1579 Milani et al., Phys. Rev. D 66, RSE Objectives ISA accelerometer non-gravitational forces purely gravitational orbit The ISA accelerometer is a fundamental part in BepiColombo RSE. It will directly measure the non-gravitational forces acting on BepiColombo MPO spacecraft (particularly strong around Mercury). These measurements will be used in the orbit determination and parameter estimation procedure to recover a purely gravitational orbit (a posteriori drag-free). In this way, the gravitational part of the tracking signal will be clearly distinguished with respect to the non- gravitational one, thereby enabling a precise estimation of the quantities of interest (Mercury geophysics and fundamental physics).

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario 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, and could be a problem, due to CoM movements (fuel sloshing and consumption); it is related to the overall RSE concept, not to the single instruments. ISA CoM HGA direct referencing of MORE observables to ISA position To overcome this problem, it has been proposed by the BepiColombo prime contractor (ASD) a direct referencing of MORE observables to ISA position, thereby avoiding the need of a precise CoM position knowledge. 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 …) ISA HGA

The new RSE concept changes the way the accelerometer is related to the overall scheme of the measurement. While in the previous concept the reference point for the measurement was the spacecraft COM, in the new concept the reference point is a given point of the accelerometer. So the effective free falling masses of the experiments are now the three ISA test masses. This effects directly the way the accelerometer signal is processed. While in the previous concept the contributions due to apparent forces and gravity gradients were spurious signals which had to be removed (modelled), in the new concept only the internal part of these effects (from each test mass to ISA reference point) is spurious. This has direct impact on the accelerometer error budget, since the error terms associated to spurios signals removal are significantly reduced.

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA error budget Deterministic contributions Signal, except for the internal part Same meaning Becomes part of the first contribution TypeDue toSpectral contentRequirement onError Gravity gradientsnOrbital period P and ½ P 85 % A 0 Apparent forcesOrbital period P and ½ P Thermal effects2 °COrbital period P and ½ P15 % A 0 Components couplingMisalignment angleOrbital period P and ½ PNegligible Total100 % A 0 A 0 = m/s 2

Random contributions Same meaning TypeDue toSpectral contentRequirement onError Apparent forcesRandom60 % S 0 Thermal effectsRandom30 % S 0 Noise on the MPOMovements due to the HGA, fuel sloshing, etc. Random10 % S 0 MPO COM displacement Movements due to the HGA, fuel sloshing, etc. Random70 % S 0 Components couplingMisalignment angle RandomNegligible ISA intrinsic noiseRandomNegligible Total (not correlated noise)< 100 % S 0 Signal in the new RSE concept S 0 = m/s 2 /Hz 1/2

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA calibration For calibration we mean a characterization of the instrument and its response, in all the phases and operative conditions Transfer function Transducer factor Transducer factor Linearity of response Intrinsic noise Thermal stability Pre-calibration Functional checks In-flight calibration by internal actuators In-flight calibration by internal actuators In-flight calibration by MPO spinning Calibration during the cruise Calibration procedures Baseline

Goals: Raw calibration to be used for performance tests Final calibration of internal actuators (not affected by earth gravity) Quantities to be measured: Internal actuators chain transduction factor (the transduction factor will be derived from the measure of parameters of mechanical oscillators, taking into account the effect of Earth gravity) Sensing axes directions with respect to UOAF Calibration signal: Fraction of earth gravity and inertial accelerations by means of external shakers (for axes directions measurement only) Performed on assembled ISA (along with performance tests) Instrument pre-calibration Goal: to check the general instrument functionality once it has been installed on MPO (no performance and calibration tests after integration on MPO) Known signals will be applied to sensing heads by means of internal actuators The instrument shall feature a reduced sensitivity to withstand seismic noise and the residual Earth gravity due to misalignment with respect to the local vertical MPO GSE shall allow to rotate the MPO in order to put ISA sensing axes in the horizontal plane Functional checks

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA calibration Performed in flight Three different (possible) procedures: 1. Using internal actuators (both in cruise and in orbit) 2. Using MPO spinning (in orbit only, without hi-precision knowledge of MPO COM) 3. Using linear accelerations measured by tracking (in cruise only) Instrument calibration Calibration signal: internally generated by means of electrostatic actuators Calibration of actuators performed on ground is not affected by earth gravity It will be possible to measure the transfer function of ISA sensing heads (this will require high data rate for short time) Performed periodically in orbit (before or after each measurement arc) Performed in cruise too (if allowed by MCS environment) Instrument calibration by internal actuators

To be considered as a back-up in case of internal actuators factor change Calibration signal: apparent forces due to MPO spinning (external signal) To be performed in orbit The feasibility of this procedure depends on: Allowed MPO manoeuvres (applicable angular rate and acceleration) Actual value of rotation axis to ISA sensing masses distance MPO COM stability during manoeuvres Instrument calibration by MPO spinning Calibration signal: MPO acceleration applied by means of thrusters or given by solar radiation pressure and measured by tracking (external signal) To be performed in cruise The feasibility of this procedure depends on: In-cruise environmental conditions (thermal and mechanical) Tracking performance Acceleration that can be applied to MPO Pointing knowledge Instrument calibration during cruise

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA in-flight calibration The difficulty is to guarantee that the gap between the actuators capacitors does not change more than The standard calibration procedure makes use of internal actuators to provide a reference acceleration in order to determine the transducer factor Voltage relative uncertainty

Accelerometer off-set changing due to repeated shocks Some shocks have determined an off-set changing, anyway the total off-set changing has caused an equivalent moving of m over a distance from the two plates equal to m

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA calibration during cruise Superior Conjunction ExperimentSCE An interesting calibration possibility raises from the fact that during the first part of the cruise the Mercury Composite Spacecraft (MCS) will travel in a relatively stable dynamical environment and there will be at least a possibility of precise tracking, during the Superior Conjunction Experiment (SCE). Comparing the accelerometer readings with tracking data, a calibration of the instrument could in principle be achieved. ESOC 2013 launch scenario

Regarding the physical source of the "calibration acceleration", the possibilities in principle are two: thrust by Mercury Transfer Module (MTM) or solar radiation pressure. In fact, only the second appears feasible, due to its more clean spectral content and especially to the fact that SCE will be performed in a no-thrust arc. 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 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 MCS COM (instead of MPO COM). The cruise phase will also offer a unique opportunity to perform a long-term stability test, due to the low noise on MCS during cruise. 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 (nearly constant) solar radiation pressure and the residual vibrations on MCS being the only source of vibrational noise, it will be possible to test the instrument intrinsic noise with high accuracy. This will be a further validation of the instrument capabilities.