Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario V. Iafolla, E....

Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario

V. Iafolla, E. Fiorenza, C.Lefevre, D.M. Lucchesi, A. Morbidini, 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

The BepiColombo mission to planet Mercury and the fundamental role of ISA accelerometer for the Radio

Science Experiments

13 September 2007 1Iafolla et al. - ERE07

Encuentros Relativistas Españoles - Spanish Relativity Meeting ERE07Puerto de la Cruz, Tenerife, Spain


13 September 2007 Iafolla et al. - ERE07 2

MercuryMercury

From Andreas Cellarius, Harmonia Macrocosmica, plate 8

Photo © NASA


BepiColombo space mission objectivesBepiColombo space mission objectives

The space mission BepiColombo (BC), one of the Cornerstones of the European Space Agency (ESA), aims to perform (Grard & Balogh, 2001;

Iess & Boscagli 2001; Milani et al., 2001, 2002):

1. a detailed study of the planet Mercury and its environment

2. and to test Einstein’s General Relativity (GR) to an unprecedented

level of accuracy

Launch epoch August 2013

+ 1 year of nominal duration

Arrival epoch August 2019

Launch on Soyuz 2-1B Fregat-M



The accurate knowledge of the Mercury Planetary Orbiter (MPO) orbit represents the way to achieve a large part of these targets:

• The MPO will be characterized by a polar orbit around

Mercury, with a semi–major axis of about 3389 km and a

relatively high eccentricity of 0.162

• 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)




400

1500

MPO

MPO 400 1500 km polar orbit

Nadir pointingParameter Nominal

ValueTolerance

Apocentre height (hA) 1,500 km ±20 kmPericentre height (hP) 400 km ±20 kmInclination (i) 90° ±2°Argument ofpericentre

180° ±20°

Period 2.32hours

Right ascension ofascending node

248° ±20°

Eclipse duration <42 min



a = 3389 km

e = 0.162

I = 90°

009.009.4

P = 8355 sdegrees/day

degrees/day

degrees/day


The 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 experimentBasically, on–board the MPO, the instruments used for these experiments are:

• Ka–band TransponderKa–band Transponder

• Star–TrackerStar–Tracker

• High Resolution CameraHigh Resolution Camera

• AccelerometerAccelerometer

BepiColombo Radio Science ExperimentsBepiColombo Radio Science Experiments



MORE: Science GoalsMORE: Science Goals

Spherical harmonic coefficients of the gravity field of the Spherical harmonic coefficients of the gravity field of the planet up to degree and order 25planet up to degree and order 25

Degree 2 (CDegree 2 (C2020 and C and C2222) with 10) with 10-9 -9 accuracy (Signal/Noise Ratio accuracy (Signal/Noise Ratio 10 1044))

Degree 10 with SNR Degree 10 with SNR 300 300 Degree 20 with SNR Degree 20 with SNR 10 10 Love number kLove number k22 with SNR with SNR 50 50 Obliquity of the planet to an accuracy of 4 arcsec (40 m on Obliquity of the planet to an accuracy of 4 arcsec (40 m on

surface – needs also SYMBIO-SYS)surface – needs also SYMBIO-SYS) Amplitude of physical librations in longitude to 4 arcsec (40 m Amplitude of physical librations in longitude to 4 arcsec (40 m

on surface – needs SYMBIO-SYS).on surface – needs SYMBIO-SYS). CCmm/C (ratio between mantle and planet moment of inertia) to /C (ratio between mantle and planet moment of inertia) to

0.05 or better 0.05 or better C/MRC/MR2 2 to 0.003 or better to 0.003 or better



MORE: Science GoalsMORE: Science Goals

Spacecraft position Spacecraft position in a in a MMercuryercury--centric frame to 10 cm – 1m (depending on centric frame to 10 cm – 1m (depending on the tracking geometry)the tracking geometry)

Planetary figure, including mean radius, polar radius and equatorial radius Planetary figure, including mean radius, polar radius and equatorial radius to 1 part in 107 (by combining MORE and BELA laser altimeter data )to 1 part in 107 (by combining MORE and BELA laser altimeter data )

Geoid surface to 10 cm over spatial scales of 300 kmGeoid surface to 10 cm over spatial scales of 300 km Position of Mercury in a solar system baricentric frame to better than 10 cmPosition of Mercury in a solar system baricentric frame to better than 10 cm

PN parameter PN parameter , controlling the deflection of light and the time delay of , controlling the deflection of light and the time delay of ranging signals to 2.5∙10ranging signals to 2.5∙10-6-6

PN parameter PN parameter , controlling the relativistic advance of Mercury’s perihelion, , controlling the relativistic advance of Mercury’s perihelion, to to 5∙105∙10-6-6

PN parameter PN parameter (controlling the gravitational self-energy contribution to the (controlling the gravitational self-energy contribution to the gravitational mass to gravitational mass to 2∙102∙10-5-5

The gravitational oblateness of the Sun (JThe gravitational oblateness of the Sun (J22) to ) to 2∙102∙10-9-9

The time variation of the gravitational constant (The time variation of the gravitational constant (d(lnG)/dtd(lnG)/dt) to ) to 3∙103∙10-13-13 years years-1-1



RSE total noiseRSE total noise



ISA accuracy requirement inside the frequency bandISA accuracy requirement inside the frequency band

Frequency Hz 5103 34 1010 110

Acceleration values Hzsm // 2 8103 810 710



The ISA (Italian Spring Accelerometer) accelerometer — developed at IFSI/INAF — has been selected to fly on–board the MPO. It is a three axes accelerometer.

• The key role of the accelerometer is to remove from the list of

unknowns the non–gravitational perturbations, i.e., the strong

non–conservative accelerations on the MPO produced by the Sun

visible radiation and by Mercury infrared radiation

• To fulfil the RSE the non–gravitational accelerations must be

limited to a level of 10-9g, which has been taken as the

accelerometer accuracy, in the frequency band 3∙10–5—10–1 Hz

BepiColombo Radio Science ExperimentsBepiColombo Radio Science ExperimentsISA (Italian Spring Accelerometer)ISA (Italian Spring Accelerometer)



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


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



mr

K

HzI

kf t 5.3

2

10

Al 5056

ISA mechanical oscillator and its equivalence with a linear harmonic oscillator:

MkI tt Fkxxxmr

Hzm

kf

r

5.32

10

The accelerometer works at frequencies lower than the resonance frequency of the mechanical oscillator, where the transfer function between the acceleration of the sensitive mass and its displacement is:

20 a

x

ISA General DescriptionISA General Description



• The displacements of a proof–mass due to a perturbing acceleration are

detected by means of capacitive transducers in a bridge configuration, followed

by a low noise amplifier

• The capacitive bridge (CB) is biased at high frequency ( fp = 10 kHz ) so that

accelerations at frequencies fs produce their unbalance

• At the output of the CB the signals are seen as a modulation of the bias voltage

at the two side–bands f = fp fs in the frequency domain

2fp




Electromechanical actuator

ISA calibration




12.74 12.75 12.76 12.77 12.78 12.79 12.8

-8.246

-8.244

-8.242

-8.24

-8.238

-8.236

x 10-3

Ac

ce

lera

zio

ne

[g

]

Tempo giorni dal 2001

Misure GEOSTAR

Earthquakes 13 gen 2001 17:33:29:22Off Coast of Central AmericaRegistrazione Gravimetro Geostar Ustica

Solid tide of Earth

Teleseismic (free oscillation of Earth)Seismic Noise

Geophysical Measurements at theGeophysical Measurements at theIstituto Nazionale di Fisica Nucleare (INFN) Gran Sasso Laboratory Istituto Nazionale di Fisica Nucleare (INFN) Gran Sasso Laboratory



Differential Accelerometer: Mechanical Arrangement

08/06/2006 Emiliano Fiorenza13 September 2007 20Iafolla et al. - ERE07


Rejection: Electrical scheme Rejection: Electrical scheme

08/06/2006 Emiliano Fiorenza13 September 2007 21Iafolla et al. - ERE07


13 September 2007 Iafolla et al. - ERE07 22

Pick Up Pick Up SystemSystem


ISA experimental calibration: Seismic Noise RejectionISA experimental calibration: Seismic Noise Rejection



Acceleration acting on ISA installed on a satelliteAcceleration acting on ISA installed on a satellite

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



ISA mechanical configurationISA mechanical configuration



ISA PositioningISA PositioningThis result suggest for the best configuration of the accelerometer a location with the three sensitive masses aligned along the rotation axis of the MPO, and with 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



(XYZ) represents the MPO frame with origin in COM(xyz) defines the MPO position with respect to Mercury

instantaneous orbital planeX–axis, along the radial

direction (Mercury–MPO radius vector)

Z–axis, along the out–of–plane direction (perpendicular to the orbital plane)

Y–axis, along the transversal direction in the along–track direction ( )

ZYX ˆˆˆ

ISA PositioningISA Positioning

X

Y

Z

R

ISA com

COM

Rotation axis

YB

Z B

X B

0R

tR

)()(00 tRtRRRR tt

ISA Gauss frame

X Y Z

ISA LVLH frame

ZLVLH XLVLH YLVLH



2890 /1010 smgA

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

)()(00 tRtRRRR tt

com ≡ COM

Requirements on ISA PositioningRequirements on ISA Positioning



Requirements on the angular rate and angular acceleration for the following fixed values of the position of the spacecraft COM: Xt = 30 mm, Yt = 20 mm and Zt = 40 mm

Case A) Hzsrad51034.1 Hzsrad 281076.1

Case A: com ≡ COM


Requirements on ISA PositioningRequirements on ISA Positioning


Vibrational random noise on board the MPO inside the Vibrational random noise on board the MPO inside the frequency bandfrequency band

Frequency Hz 5103 34 1010 110

Acceleration values ( Hzsm // 2 ) 9103 910 810

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


VibrationsVibrations


ISA Block ISA Block DiagramDiagram

Hzf 5.30

10Q 610

Dynamic

decdB

Hzf

FiltericAna

cat

/40

1

log

65536

Dynamic

HzatdB

Hzf

FilterDigital

cat

180

1.0



ISA OPERATIVE TEMPERATURE: -20/+30 [°C]ISA NON OPERATIVE TEMPERATURE: -40/+40 [°C]

FEE ELECTRONIC STABILITY: 5*10^-8 [m/s^2/sqr(Hz)]ACC. MECHANICAL STABILITY:5*10^-7 [m/s^2/sqr(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



ISA thermal system overviewISA thermal system overview

QI=0.078W

COVER_BOX

BOTTOM_BOX

PCB QI=0.88WOVEN_PCB

THERMAL INSULATOR

CONTROL ELECTRONIC

SPACECRAFT

GR=0,0005

GL=0,003

GL=0,5 GR=0,014

GR=0,008

ENVIROMENT

GR=0,0067

GL=0,3

GL=0,8

GL=0.03

GR=0,0028

GL=5

GR=0,05GL=3

OVEN BOX

ACCELEROMETERPAKAGES

QI = 3.7 W

ISA Thermal Mathematical model



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: Pseudo Sinusoidal ContributionsISA Error Budget: Pseudo Sinusoidal Contributions



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 ContributionISA Error Budget: Random Contribution



ISA Control Electronics

ISA Detector Assy

ISA PAYLOAD OVERVIEWISA PAYLOAD OVERVIEW



ISA Detector Assy

External Shield

Internal Thermal shieldAccelerometer ThermalInsulation

Electrical I/F

ISA DETECTOR ASSY OVERVIEWISA DETECTOR ASSY OVERVIEW

Accelerometer Package



ISA Power and Mass Budget ISA Power and Mass Budget OverviewOverview

EID_B EID_ATOTAL MASS [kg] 6.6*** 5.03

POWER * [W] 8.3 - 9 6DATA RATE** [bps] 1500 800

* Without heaters 3,67 W** After digital filtering, 80 kbps without filtering*** Whit margin 10%


Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario V. Iafolla, E....

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