Magnetic Field Instrument for the BepiColombo Planetary Orbiter Magnetic Cleanliness and Data Processing Methods Chris Carr & André Balogh U. Auster (IGeP),

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?


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








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


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




3. Dual Magnetometer MethodExamples: The Double Star Mission




Team Experience

InstituteImperial

College

London

IGEP

TU-Braunschweig

IWF

Graz

ISAS

JAXA

ExpertiseSpecification

Coordination

DC & AC Magnetic Analysis

‘MAGNET’ Software



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


Magnetically ‘Clean’ Spacecraft

• Ulysses

• Cassini

• Cluster


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: ???








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


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


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


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


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


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


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


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


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








MERMAG Support• Expertise, Experience & Modelling s/w

– Available to the BepiColombo mission

InstituteImperial

College

London

IGEP

TU-Braunschweig

IWF

Graz

ISAS

JAXA

ExpertiseSpecification

Coordination


‘MAGNET’ Software



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


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


Concluding Remarks


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

Documents

Magnetic Field Instrument for the BepiColombo Planetary Orbiter Magnetic Cleanliness and Data Processing Methods Chris Carr & André Balogh U. Auster (IGeP),