Laccelerometro ISA per la missione BepiColombo V. Iafolla, E. Fiorenza, C. Lefevre, S. Nozzoli, R....

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, ItalyIstituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Roma, Italy

IX CONGRESSO NAZIONALE DI PLANETOLOGIA

AMALFI

BepiColombo Radio Science Experiments (RSE)

28/09/2009Gruppo di Gravitazione Sperimentale

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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

RSE Objectives


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The global gravity fieldglobal gravity field of Mercury and its temporal variationstemporal variations due to solar tides (in order to constrain the internal structure of the planet)The local gravity anomalieslocal gravity anomalies (in order to constrain the mantle structure of the planet and the interface between mantle and crust)The rotation staterotation 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–massorbit 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, 1579Milani et al., Phys. Rev. D 66, 082001

RSE measurements


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RangeRange and range–raterange–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 forcesnon–gravitational forces acting on the MPO by means of an on–board accelerometerDetermination of the MPO absolute attitudeabsolute attitude by means of a Star–TrackerDetermination of angular displacements of reference points on the solid angular displacements of reference points on the solid surfacesurface of the planet, by means of a fotocamera

RSE science goals


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Spherical harmonic coefficients of the gravity field of the planet up to degree and order 25Degree 2 (C20 and C22) with 10-9 accuracy (Signal/Noise Ratio 104)Degree 10 with SNR 300Degree 20 with SNR 10Love number k2 with SNR 50Obliquity 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

RSE science goals


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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 kmPosition 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

Role of the accelerometer


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The analysis of experimental data to obtain the properties of a physical system requires models

System dynamicsMeasurement 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



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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-non-gravitational perturbationsgravitational 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



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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

A correlation with other phenomena can lead to an estimation biasestimation bias

Probably false signalTrue signal?

RP, Master and PhD Theses work

Scm

ACa SunRSun

ˆ

New RSE concept


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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 …)

To overcome this problem, it has been proposed by ASD a direct referencing of direct referencing of MORE observables to ISA positionMORE observables to ISA position, thereby avoiding the need of a precise CoM position knowledge

ISA measurements


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ISA Measurements definition:FinalFinal output of ISA measures are the components, in an inertial RFin an inertial RF, of ISA vertexISA vertex acceleration due to external (non gravitational) perturbations acting on the MPO, and acting on the MPO, and to MPO motionto 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 definition:FinalFinal output of ISA measures are the components, in an inertial RFin an inertial RF, of ISA vertexISA vertex acceleration due to external (non gravitational) perturbations acting on the MPO, and acting on the MPO, and to MPO motionto 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”.

RSE total noise


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Instrument bandwidth

ISA accuracy requirement


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Frequency Hz 5103 34 1010 110

Acceleration values Hzsm // 2 8103 810 710

Direct solar radiation pressure


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http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant



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ISA measurements


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Measurement of non-gravitational perturbations acting on MPO spacecraftSupport to RSE during Superior Conjunction Experiment (SCE)Measurement of V during MPO manoeuvresGradiometry (currently, ISA is not on MPO Center of Mass…)In general, disentangling between gravitational and non-gravitational effects (anomalies, …)

ISA description


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ISA sensing element

ISA pick-up

Differential Accelerometer


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Mechanical arrangement

Seismic noise rejection

Accelerations acting on ISA


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The acceleration on a point P (ISA proof–mass) close to the MPO COM is:

NGPARRRRgR 2

X

Y

Z

R

ISA proof–mass

Inside the MPO frame

= Planet gravity

= MPO angular rate

= MPO angular acceleration

= MPO–proof–mass vector

g

Acceleration due to the planet gravity field gradients

Centrifugal acceleration

Angular acceleration

Coriolis acceleration

Non–Gravitational accelerations

R

Dropped requirements


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Best configuration of the accelerometer for MPOBest 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

comISA COM

Rotation axis

000

10500

105002

2

000

000

000

0

0

0

ZYX

ZYX

ZYX

Z

Y

X

ZZZ

YYY

XXX

R

R

R

Position ranges Position accuracies

0X 0 0X ±4 mm

tX ±30 mm ±30 mm

tX ±1 mm ±5 mm

0Y 0 0Y ±5.5 mm

tY ±20 mm ±20 mm

tY ±2 mm ±7.5 mm

0Z 0 0Z ±11 mm

tZ ±40 mm ±40 mm

tZ ±4 mm ±15 mm

Requirements

Hzsrad51034.1 Hzsrad 281076.1

Vibrations


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Vibrational random noise on board the MPO inside the frequency band

Frequency Hz 5103 34 1010 110

Acceleration values ( Hzsm // 2 ) 9103 910 810

Micro-vibration random noise on board the MPO outside the frequency band

ISA thermal issues


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ISA operative temperature: -20/+30 °CISA non operative temperature: -40/+40 °C

FEE electronic stability: 5∙10-8 m/s2/ HzACC. mechanical stability: 5∙10-7 m/s2/ 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

ISA thermal issues


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ISA thermal control system performance

ISA Error Budget


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Data reduction procedure

dd/mm/yyyyGruppo di Gravitazione Sperimentale

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28/09/2009

25

Gruppo di Gravitazione Sperimentale

Performance and calibration


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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

Calibration on ground


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Measurement of rotation matrix between ISA axes and optical cubeMeasurement of rotation matrix between ISA axes and optical cubeMeasurement of transduction factors for ISA sensing elementsMeasurement of transduction factors for ISA sensing elementsCheck of alignment constancy after vibrational testsMeasurement of sensing masses position at zero gravityCheck of alignment constancy in time and possible measurement of agingCheck of alignment constancy after thermal stress

ISA operations in cruise


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• RSE support• In-cruise calibration• Periodical checkouts• Long-term stability tests• ISA “cruise science”



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Superior Conjunction Experiment (SCE)

•Quiet dynamical environment (no thrust)•High-precision tracking•ISA on

ISA support of POD

5000 5500 6000 6500 7000 75000.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2BepiColombo MCS Cruise - 2013 trajectory

t [MJD2000]

Dis

tanc

e fr

om t

he S

un [

AU

]

Distance from the Sun



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The direct solar radiation pressure signal is above ISA sensitivity (possibly inside ISA band if MCS is rotating)

5000 5500 6000 6500 7000 75000

0.2

0.4

0.6

0.8

1

1.2

1.4x 10

-7 BepiColombo MCS Cruise - 2013 trajectory

t [MJD2000]

Acc

eler

atio

n [m

s-2

]


Possibility of instrument calibration by comparison with tracking



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Required information for in-cruise calibration

• Estimation of tracking accuracy during SCEtracking accuracy during SCE, and therefore of the accuracy in recovering the MCS acceleration

• Estimation of the expected acceleration signal acting on the expected acceleration signal acting on the MCSMCS, to be confronted with the tracking accuracy and accelerometer sensitivity

• Re-assessment of expected signals acting on the sensing expected signals acting on the sensing elementselements, taking into account the different positioning of ISA with respect to MCS COM (instead of MPO COM)

• Estimation of tracking accuracy during SCEtracking accuracy during SCE, and therefore of the accuracy in recovering the MCS acceleration

• Estimation of the expected acceleration signal acting on the expected acceleration signal acting on the MCSMCS, to be confronted with the tracking accuracy and accelerometer sensitivity

• Re-assessment of expected signals acting on the sensing expected signals acting on the sensing elementselements, taking into account the different positioning of ISA with respect to MCS COM (instead of MPO COM)

Transducer factor



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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



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“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

Calibration in orbit


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Nominal procedureNominal procedure: calibration using the internal actuators before every measurement arcBackup procedureBackup 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

Current status


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Feasibility study and proposal to ESAFeasibility study and proposal to ESA (May 2004) ISA selected for MPO payloadISA selected for MPO payload (November 2004) Phase A/B1 Kick OffPhase 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 ModelDemonstration Model ongoing (developed technologies) ISA Team laboratoriesISA Team laboratories renewed for performance and

calibration tests on the various models

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