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

Q: How do we meet the science goals? The Problem Short boom Minimum 1.5m Maximum 3m Due to mass and thermal/mechanical stability considerations Magnetically ‘dirty’ spacecraft Magnetics shall not be a design or cost driver for the spacecraft 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

MERMAG Consortium – Previous Experience Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets MERMAG Consortium – Previous Experience Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

MERMAG Consortium – Previous Experience Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets MERMAG Consortium – Previous Experience Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

Instrument Performance Meeting the science goals: The magnetometer shall have an accuracy of 1nT 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 BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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 < 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

MERMAG Consortium – Previous Experience Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets MERMAG Consortium – Previous Experience Dual Magnetometer Method Examples: The Double Star Mission The Venus Express Mission MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

Ulysses, Cassini, Cluster, Double Star Team Experience Institute Imperial College London IGEP TU-Braunschweig IWF Graz ISAS JAXA Expertise Specification Coordination DC & AC Magnetic Analysis ‘MAGNET’ Software Missions (PI) Ulysses, Cassini, Cluster, Double Star Cassini, Cluster, Double Star, Rosetta, Venus Express, Themis Nozomi, Selene BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

Magnetically ‘Clean’ Spacecraft Ulysses Cassini Cluster BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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 Venus Express NO magnetic control NO measurement NO System model Result: ??? BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

MERMAG Consortium – Previous Experience Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets MERMAG Consortium – Previous Experience Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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

Dual Magnetometer: Application to Double Star U. Auster, K.-H. Fornacon, E. Georgescu IGeP 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 BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

Dual Magnetometer: Application to Double Star Eclipse Field generated by current loop Field generated by solar arrays Dual Magnetometer: Application to Double Star Approach De-spin the data Average data over spin-period Result: interference signals are reduced to ‘offsets’ These offsets are unknown, and change with the spacecraft power modes Remove remaining offsets using weighted differences between sensors Modified dual-magnetometer method 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

Application to Venus Express M. Delva et al. IWF Graz IGeP TU-BS Univ Kosice Application to Venus Express NO magnetic control NO measurement NO System model Highly applicable to BepiColombo 3-axis stabilised Short (1m) boom BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

S4 S3 S2 S1 VEX MC – Measurements Example at Alenia – Aug. 2004 Idea: learn to know the SC magnetically S4 S3 S2 S1 Solar Array Dynamic Motor (on SC +y side) switched on resp. modes BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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 bscx1, bobsx2- bobsx1 bscz1, bobsz2- bobsz1 bscy1, bobsy2- bobsy1 time t1 time t2 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 BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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 nT Correct data -> difference disappears after correction: diff < 1 nT Difference in Btotal at 2 sensors BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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

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

MERMAG Consortium – Previous Experience Magnetometer Performance Requirements vs. Spacecraft Magnetic Cleanliness Targets MERMAG Consortium – Previous Experience Dual Magnetometer Methods Examples: The Double Star Mission The Venus Express Mission MERMAG Support to the BepiColombo Project & Outline Magnetic Control Plan BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

Ulysses, Cassini, Cluster, Double Star MERMAG Support Expertise, Experience & Modelling s/w Available to the BepiColombo mission Institute Imperial College London IGEP TU-Braunschweig IWF Graz ISAS JAXA Expertise Specification Coordination DC & AC Magnetic Analysis ‘MAGNET’ Software Missions (PI) Ulysses, Cassini, Cluster, Double Star Cassini, Cluster, Double Star, Rosetta, Venus Express, Themis Nozomi, Selene BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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

Concluding Remarks BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005

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 BepiColombo MPO Magnetic Cleanliness C. Carr & A. Balogh. February 2005