Magnetic Field Instrument for the Bepi Colombo Planetary
Magnetic Field Instrument for the Bepi. Colombo Planetary Orbiter Magnetic Cleanliness and Data Processing Methods Chris Carr & André Balogh U. Auster (IGe. P), M. Delva (IWF) February 2005 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005
The Problem 1. Short boom – Minimum 1. 5 m – Maximum 3 m – 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? Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 2
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets 2. MERMAG Consortium – Previous Experience 3. Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission 4. MERMAG Support to the Bepi. Colombo Project & Outline Magnetic Control Plan Bepi. Colombo 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 Methods Examples: The Double Star Mission The Venus Express Mission 4. MERMAG Support to the Bepi. Colombo Project & Outline Magnetic Control Plan Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 4
Instrument Performance Meeting the science goals: The magnetometer shall have an accuracy of 1 n. T • 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 5
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 < 100 n. T (TBC) Stability of Residual DC Magnetic Field measured at MAG OB sensor < 2 n. T 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 n. T • 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 6
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets 2. MERMAG Consortium – Previous Experience 3. Dual Magnetometer Method Examples: The Double Star Mission The Venus Express Mission 4. MERMAG Support to the Bepi. Colombo Project & Outline Magnetic Control Plan Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 7
Team Experience Imperial College London IGEP TUBraunschweig IWF Graz Expertise Specification Coordination DC & AC Magnetic Analysis ‘MAGNET’ Software DC & AC Magnetic Analysis Missions (PI) Ulysses, Cassini, Cluster, Double Star Institute ISAS JAXA DC & AC Magnetic Analysis Cassini, Cluster, Double Star, Rosetta, Venus Nozomi, Selene Express, Themis Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 8
Magnetically ‘Clean’ Spacecraft • Ulysses • Cassini • Cluster Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 9
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: ? ? ? Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 10
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets 2. MERMAG Consortium – Previous Experience 3. Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission 4. MERMAG Support to the Bepi. Colombo Project & Outline Magnetic Control Plan Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 11
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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 12
Dual Magnetometer: Application to Double Star U. Auster, K. -H. Fornacon, E. Georgescu IGe. P TU-BS • 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 13
Dual Magnetometer: Application to Double Star • Field generated by current loop Approach Eclipse 1. De-spin the data 2. Average data over spin-period – Result: interference signals are reduced to ‘offsets’ – These offsets are unknown, and change with the spacecraft power modes Field generated by solar arrays 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 14
M. Delva et al. IWF Graz IGe. P TU-BS Univ Kosice Application to Venus Express – NO magnetic control – NO measurement – NO System model • Highly applicable to Bepi. Colombo – 3 -axis stabilised – Short (1 m) boom Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 15
VEX MC – Measurements Example at Alenia – Aug. 2004 Idea: learn to know the SC magnetically S 4 S 3 S 2 S 1 Solar Array Dynamic Motor (on SC +y side) switched on resp. modes Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 16
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 bscz 1, bobsz 2 - bobsz 1 bscx 1, bobsx 2 - bobsx 1 bscy 1, bobsy 2 - bobsy 1 time t 2 § 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 17
Neural Network Tested with Double Star data Test of method with real in-flight data: Double Star data (TC-1) before correction Recognize jumps > 1 n. T Correct data -> difference disappears after correction: diff < 1 n. T Difference in Btotal at 2 sensors Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 18
Double Star / Venus Express: Lessons for Bepi. Colombo • 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 19
Venus Express / Mars Express Experience: Problem sub-systems SC subsystem Mean dipole mom. [m. A m 2] Expected field at VEXMAG IS [n. T] Expected field at VEXMAG OS ( 1 m) [n. T] 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. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 20
1. Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets 2. MERMAG Consortium – Previous Experience 3. Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission 4. MERMAG Support to the Bepi. Colombo Project & Outline Magnetic Control Plan Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 21
MERMAG Support • Expertise, Experience & Modelling s/w – Available to the Bepi. Colombo mission Institute Expertise Missions (PI) Imperial College London IGEP TUBraunschweig IWF Graz Specification Coordination DC & AC Magnetic Analysis ‘MAGNET’ Software DC & AC Magnetic Analysis Ulysses, Cassini, Cluster, Double Star, Rosetta, Venus Express, Themis Nozomi, Selene Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 22 ISAS JAXA
Magnetic Control for Bepi. Colombo • Specification for each unit: DC and AC at 1 m – – 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 Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 23
Concluding Remarks Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 24
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 Bepi. Colombo Re-use of techniques • Selene (using inputs from Masaki) – Applicability of the Selene magnetic cleanliness programme to Bepi. Colombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005 25
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