Istituto Nazionale Di Astrofisica Istituto 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. Santoli Istituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Istituto di Fisica dello Spazio Interplanetario (IFSI/INAF), Roma, Italy Roma, Italy The BepiColombo mission to planet Mercury and the fundamental role of ISA accelerometer for the Radio Science Experiments 13 September 2007 1 Iafolla et al. - ERE07 uentros Relativistas Españoles - Spanish Relativity Meeting E Puerto de la Cruz, Tenerife, Spain
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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
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
13 September 2007 Iafolla et al. - ERE07 2
MercuryMercury
From Andreas Cellarius, Harmonia Macrocosmica, plate 8
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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)
BepiColombo space mission objectivesBepiColombo space mission objectives
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
BepiColombo space mission objectivesBepiColombo space mission objectives
13 September 2007 5Iafolla et al. - ERE07
a = 3389 km
e = 0.162
I = 90°
009.009.4
P = 8355 sdegrees/day
degrees/day
degrees/day
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
RSE total noiseRSE total noise
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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)
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
Direct solar radiation pressure: solar irradianceDirect solar radiation pressure: solar irradiance
13 September 2007 14Iafolla et al. - ERE07
From http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstanthttp://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
Direct solar radiation pressure: the transversal Direct solar radiation pressure: the transversal acceleration and its spectrumacceleration and its spectrum
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
• 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
ISA General DescriptionISA General Description
13 September 2007 17Iafolla et al. - ERE07
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
Electromechanical actuator
ISA calibration
ISA General DescriptionISA General Description
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
13 September 2007 19Iafolla et al. - ERE07
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
ISA mechanical configurationISA mechanical configuration
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
ISA mechanical configurationISA mechanical configuration
13 September 2007 26Iafolla et al. - ERE07
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
13 September 2007 27Iafolla et al. - ERE07
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
(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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Requirements on ISA PositioningRequirements on ISA Positioning
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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VibrationsVibrations
Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
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
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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario
ISA OPERATIVE TEMPERATURE: -20/+30 [°C]ISA NON OPERATIVE TEMPERATURE: -40/+40 [°C]