magnetic field instrument for the bepicolombo planetary orbiter magnetic cleanliness and data...
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Magnetic Field Instrument for the
BepiColombo Planetary Orbiter
Magnetic Cleanliness and Data Processing Methods
Chris Carr & André Balogh
U. Auster (IGeP), M. Delva (IWF)
February 2005
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 2
The Problem
1. Short boom– Minimum 1.5m– Maximum 3m– Due to mass and thermal/mechanical stability considerations
2. Magnetically ‘dirty’ spacecraft– Magnetics shall not be a design or cost driver for the spacecraft
3. Planetary magnetic field determination requires high accuracy magnetometer measurements
Q: How do we meet the science goals?
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 3
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets
2. MERMAG Consortium – Previous Experience
3. Dual Magnetometer MethodsExamples: The Double Star Mission
The Venus Express Mission
4. MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 4
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets
2. MERMAG Consortium – Previous Experience
3. Dual Magnetometer MethodsExamples: The Double Star Mission
The Venus Express Mission
4. MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 5
Instrument Performance
• The DC part of the spacecraft field shall be low enough to allow operation of the magnetometer in its most sensitive operating range
• The stability of the spacecraft magnetic field is the most critical parameter
Meeting the science goals: The magnetometer shall have an accuracy of 1nT
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 6
Magnetic Cleanliness• Magnetic cleanliness objective:
To provide an acceptable magnetic environment without major cost / schedule / mass impact at system level
Parameter Performance Goal
Residual DC Magnetic Field measured at MAG OB sensor
< 100nT (TBC)
Stability of Residual DC Magnetic Field measured at MAG OB sensor
< 2nT variation (TBC)
Determination of the variable part of the spacecraft field by in-flight (dual-magnetometer) measurements
5%
MERMAG Proposed Accuracy(including all instrument & spacecraft error sources)
0.5 nT
• MERMAG accuracy includes ALL error sources:– Sensor calibration (knowledge), including stability w.r.t. temperature
– Determination of spacecraft contributions, both DC and AC
– Sensor position / attitude knowledge, and timing accuracy
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 7
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets
2. MERMAG Consortium – Previous Experience
3. Dual Magnetometer MethodExamples: The Double Star Mission
The Venus Express Mission
4. MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 8
Team Experience
InstituteImperial
College
London
IGEP
TU-Braunschweig
IWF
Graz
ISAS
JAXA
ExpertiseSpecification
Coordination
DC & AC Magnetic Analysis
‘MAGNET’ Software
DC & AC Magnetic Analysis
DC & AC Magnetic Analysis
Missions
(PI)
Ulysses, Cassini,
Cluster, Double Star
Cassini, Cluster, Double Star,
Rosetta, Venus Express, Themis
Cassini, Cluster, Double Star,
Rosetta, Venus Express, Themis
Nozomi, Selene
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 9
Magnetically ‘Clean’ Spacecraft
• Ulysses
• Cassini
• Cluster
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 10
Magnetically ‘Dirty’ Spacecraft
• Rosetta– No magnetic control– Units measured (DC)– System model performed– Result: BAD
• Double Star– Supposed to be clean– Solar Panels not tested before launch– Result: BAD
• Venus Express– NO magnetic control– NO measurement– NO System model– Result: ???
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 11
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets
2. MERMAG Consortium – Previous Experience
3. Dual Magnetometer MethodsExamples: The Double Star Mission
The Venus Express Mission
4. MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 12
The Dual Magnetometer Method
…for determination of spacecraft fields
Principle
• Two radially separated magnetometers
plus
• Knowledge of location on the spacecraft of the disturbing source
allows
• Estimate strength of the disturbing field
• Original technique Ness et al. (1971)
• Successful application to Double Star
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 13
Dual Magnetometer:Application to Double Star
• Magnetic disturbances:– Signals at the spin frequency and harmonics
• Source: solar panels
– Sudden shifts in the DC ‘background’ field from the spacecraft• Source: current loops – power distribution
U. Auster, K.-H. Fornacon, E. Georgescu
IGeP TU-BS
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 14
Eclipse
Field generated by current loop
Field generated by solar arrays
Dual Magnetometer:Application to Double Star
• Approach
1. De-spin the data2. Average data over spin-period
– Result: interference signals are reduced to ‘offsets’
– These offsets are unknown, and change with the spacecraft power modes
3. Remove remaining offsets using weighted differences between sensors– Modified dual-magnetometer method
4. Evaluate any residual offsets using traditional calibration techniques
– Result: Accuracy of this spin-averaged data is comparable to the equivalent Cluster magnetometer data
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 15
Application to Venus Express
– NO magnetic control– NO measurement– NO System model
• Highly applicable to BepiColombo
– 3-axis stabilised– Short (1m) boom
M. Delva et al. IWF GrazIGeP TU-BSUniv Kosice
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 16
VEX MC – MeasurementsExample at Alenia – Aug. 2004
S1S2
S3
S4
Solar Array Dynamic Motor (on SC +y side) switched on resp. modes
Idea: learn to know the SC magnetically
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 17
Automatic Correction with Neural Network
Cooperation with Univ. of Kosice (Slovakia)• Basic idea: Event-pattern recognition & correction by neural network• Two sensors are needed -> use difference of change as indicator for event of SC-origin
Neural network “learns” characteristic pattern of event from measurements at two sensors
e.g. from MC - measurements on Earth from magnetometer measurements during commissioning
phase
bscx1, bobsx2- bobsx1
bscz1, bobsz2- bobsz1
bscy1, bobsy2- bobsy1
bscx1, bobsx2- bobsx1
bscz1, bobsz2- bobsz1
bscy1, bobsy2- bobsy1
time t1 time t2
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 18
Test of method with real in-flight data: Double Star data (TC-1)
Difference in Btotal at 2 sensors
Recognize jumps > 1 nTCorrect data -> difference
disappears
before correction
after correction: diff < 1 nT
Neural Network Tested with Double Star data
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 19
Double Star / Venus Express: Lessons for BepiColombo
• Magnetic cleanliness programme should give equal effort to– DC magnetic– Stray fields from current loops (Double Star experience)– Moving parts (Venus Express)
• Characterise the spacecraft before launch– Sufficient mode information in the housekeeping
• Magnetometer Instrument Design– Optimised dual-sensor modes of operation– Programmable anti-aliasing filters
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 20
Venus Express / Mars Express Experience:Problem sub-systems
SC subsystem Mean dipole mom. [mA m2]
Expected field at VEX-MAG IS
[nT]
Expected field at VEX-MAG OS( 1m) [nT]
GYROS 74 +/- 39 35 - 152 2 - 9
SADM 708 +/- 234 54 - 213 8 - 39
Reaction Wheels 1,2,3 908 +/- 552 19 – 34 6 - 9
Reaction Wheel 4
963 +/- 459 21 – 32 / - 11
Thrusters, different modes
602 +/- 161 16 - 30 5-10
• Similar for Rosetta
• Can identify problem sub-systems early for Bepi
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 21
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets
2. MERMAG Consortium – Previous Experience
3. Dual Magnetometer MethodsExamples: The Double Star Mission
The Venus Express Mission
4. MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 22
MERMAG Support• Expertise, Experience & Modelling s/w
– Available to the BepiColombo mission
InstituteImperial
College
London
IGEP
TU-Braunschweig
IWF
Graz
ISAS
JAXA
ExpertiseSpecification
Coordination
DC & AC Magnetic Analysis
‘MAGNET’ Software
DC & AC Magnetic Analysis
DC & AC Magnetic Analysis
Missions
(PI)
Ulysses, Cassini, Cluster,
Double Star
Cassini, Cluster,
Double Star, Rosetta, Venus
Express, Themis
Cassini, Cluster,
Double Star, Rosetta, Venus
Express, Themis
Nozomi, Selene
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 23
Magnetic Control for BepiColombo
• Specification for each unit: DC and AC at 1m– Gradiometer– MCF– Extend EMC test specification to LF Magnetic– Critical Unit Identification
• Gyros, SADM, Reaction Wheels etc.
• First Steps– Spacecraft design, boom length– Knowledge of magnetic contamination sources– Establish a ‘Magnetic Control Group’ – ESA, MERMAG and industry– Design Guidelines
• For payload
• For industry / system
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 24
Concluding Remarks
BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 25
Outline for ESTEC Magnetometer Workshop
• Team experience– ‘Clean’ spacecraft such as Ulysses, Cassini, Cluster– ‘Dirty’ spacecraft such as Rosetta, Double Star, Venus Express
• The Double Star Experience– Why it is magnetic– Basic principles of the dual-magnetometer technique (Ness et al.)– Application to Double Star (using input from Uli, Edita and Karl-Heinz and others)
• Venus Express (using inputs from Magda)– Background– Techniques– Applicability to BepiColombo– Re-use of techniques
• Selene (using inputs from Masaki)– Applicability of the Selene magnetic cleanliness programme to BepiColombo
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