istituto di fisica dello spazio interplanetario new concept for the bepicolombo radio science...

Istituto di Fisica dello Spazio Interplanetario

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

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

XCIV Congresso Nazionale SIFGenova, 22-27 Settembre 2008

V. Iafolla, E. Fiorenza, C. Lefevre, S. Nozzoli,R. Peron, M. Persichini, A. Reale, F. SantoliIstituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Roma, ItalyIstituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Roma, Italy


BepiColombo Radio Science Experiment (RSE)BepiColombo Radio Science Experiment (RSE)

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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 ObjectivesRSE 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, 082001Milani et al., Plan. Space Sci. 49, 1579Milani et al., Phys. Rev. D 66, 082001


RSE measurementsRSE 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 accelerometer

• Determination of the MPO absolute attitudeabsolute attitude by means of a Star–Tracker• Determination 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 goalsRSE science goals

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• 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


RSE science goalsRSE 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 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


RSE total noiseRSE total noise

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Instrument bandwidth


ISA accuracy requirement inside ISA accuracy requirement inside the frequency bandthe frequency band

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Frequency Hz 5103 34 1010 110

Acceleration values Hzsm // 2 8103 810 710


Direct solar radiation pressure: solar irradianceDirect solar radiation pressure: solar irradiance

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


Direct solar radiation pressure: the Direct solar radiation pressure: the transversal acceleration and its spectrumtransversal acceleration and its spectrum

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ISA descriptionISA description

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ISA sensing element

ISA pick-up


Differential AccelerometerDifferential Accelerometer

08/06/2006 Emiliano Fiorenza12

Mechanical arrangement

Seismic noise rejection


Accelerations acting on ISAAccelerations acting on ISA

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

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ISA PositioningISA Positioning

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

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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


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

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VibrationsVibrations


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

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

ISA thermal systemISA thermal system

• 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

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Type Due to Spectral content requirement on Error %

A0

Residuals of the

gravity gradients

and apparent forces

after the process of

removal

MPO nominal

motion. ( n ; 00 ; )

Orbital period P and

½P

MPO CoM

position

knowledge

( 0R

; tRt

)

80% A0

Thermal effects ISA thermal

sensitivity. Orbital period P

MPO thermal

variations ( T ) 15% A0

Components

coupling

Axes

misalignment

and crosstalk

Orbital period P

Sensing axes

misalignment

wrt inertial

reference

system ( )

5% A0

Total 100% A0

ISA Error Budget: Deterministic ContributionsISA Error Budget: Deterministic Contributions

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ISA EID-B, BC-EST-RS-02520ISA EID-B, BC-EST-RS-02520

It needs to be revised following the new RSE concept (see later)


Type Due to Spectral content

requirement on Error percent

Apparent forces ( ; ) Random

MPO AOCS performance

( ; ) and

MPO CoM position

( tRR t

;0 )

60% S0

Thermal effects ISA thermal sensitivity

Random MPO thermal noise

HzCTnoise /4 30% S0

MPO mechanical vibrations

Reaction wheels, MPO mechanisms, HGA movements, fuel sloshing etc.

Random 10% S0

MPO rigid body motion removal residual

COM displacement due to the HGA movements ,fuel consumption etc.

Random

MPO CoM displacements knowledge

( tRt

)

70% S0

Components coupling Axes misalignment and crosstalk

Random random component of misalignment angle ( )

10% S0

ISA intrinsic noise Random 10% S0

Total (not correlated noise) <100% S0

ISA Error Budget: Random ContributionsISA Error Budget: Random Contributions

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ISA EID-B, BC-EST-RS-02520ISA EID-B, BC-EST-RS-02520

It needs to be revised following the new RSE concept (see later)


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Performance and calibrationPerformance and calibration

For “calibration” we mean a characterization of the instrument and its response, in all the phases and operative conditions

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


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•Functional TestsFunctional Tests•Stability TestsStability Tests•Calibration by internal actuatorsCalibration by internal actuators•Calibration by external accel.Calibration by external accel.

• On groundOn ground• After S/C integrationAfter S/C integration• Near Earth commissioningNear Earth commissioning• In cruiseIn cruise• Mercury commissioningMercury commissioning• Nominal operationsNominal operations

PHASESPHASES

TESTS (instrument):TESTS (instrument):

PRE-CALIBRATION AND PRE-CALIBRATION AND PERFORMANCE TESTSPERFORMANCE TESTS(single sensor-instrument)(single sensor-instrument)

Performance and calibrationPerformance and calibration


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New RSE conceptNew 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.

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 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 …)

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


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Current statusCurrent status

• Instrument Science Requirement Review (ISRR)Instrument Science Requirement Review (ISRR) completed (scientific requirements frozen, apart from small changes due to the new RSE concept)

• Instrument Preliminary Design Review (IPDR)Instrument Preliminary Design Review (IPDR) foreseen before the end of the year

• Demonstration ModelDemonstration Model ongoing (developed technologies)• ISA Team laboratoriesISA Team laboratories renewed for performance and

calibration tests on the various models

• Instrument Science Requirement Review (ISRR)Instrument Science Requirement Review (ISRR) completed (scientific requirements frozen, apart from small changes due to the new RSE concept)

• Instrument Preliminary Design Review (IPDR)Instrument Preliminary Design Review (IPDR) foreseen before the end of the year

• Demonstration ModelDemonstration Model ongoing (developed technologies)• ISA Team laboratoriesISA Team laboratories renewed for performance and

calibration tests on the various models

istituto di fisica dello spazio interplanetario new concept for the bepicolombo radio science...

Documents

better slide

santoli istituto

fotocamera slide

mpo angular acceleration

planet orbit

mpo frame

instrument bandwidth

position of mercury