11 MAGIC (Magnetometer Imperial College) Patrick Brown & Tim Horbury, Blackett Laboratory, Imperial College

22 Heritage: Vector DC Magnetometers (0 - 10Hz) –Highly accurate three axis fluxgate design –Mass optimised two core implementation –Dual sense and feedback windings –Offset stability < 0.05 nT/°C –Noise density < 8pT/root –Accuracy – Better than 0.1nT –Missions: Cluster, Cassini, Double Star Limitations –Accuracy dependent on core size, power, –Resource overhead Mass 200g, Power ~0.4W, Magnetic Cleanliness –Current space plasma S/C use two fluxgates on long boom –Alternative sensing technology needed for very small sats Double Star Magnetometer

33 Fluxgate Analogue Control Electronics Benefits include improved linearity and temperature stability. Scale factor depends only on feedback resistor/gain stage and coil constant. Considerable effort spent minimising even harmonics in drive signal. Some odd harmonics due to transformer effect. Includes anti-aliasing filter

444 Fluxgate: Offset and Noise Density Carr et al Noise density stable <5pT/Rt(Hz) across -60 o C to +30 o C Offset stability <0.05nT/ o C Sensor offset recently tested down to -150 o C with noise floor of <20pT/Rt(Hz)

55 Fluxgate: Digital Implementation Digital control loop at TRL4 Baseline design for Solar Orbiter Currently studying migrated to rad-hard mixed signal ASIC Current design: Noise level <10pT/√Hz at 1Hz (O’Brien et al 2007) <30pT/√Hz at 1Hz

6 Anisotropic Magnetoresistance Magneto Resistance Effect –Change of resistance in magnetic field –AMR single layer permalloy, –AMR ΔR/Rmin of order 1- 2% Barber Poles –Max, sensitivity & linearity at M v H 45 o –Conductive strips for linear operation AMR Sensors –Thin film solid state devices –Mass <1g –Implemented as Wheatstone bridge –Sensitivity varies with bridge voltage, V B Picture Courtesy Philips

7 Integrated coils Set - Reset Coils –On chip coil parallel to easy axis –Used to re-align the anisotropic direction –Large current spike needed –Clocked pulsing known as ‘flipping’ –Can be used to extract sensor offset –Requires de-modulation to DC –Improves sensor noise floor Offset coils –On chip coil parallel to field sensitive axis –Used in closed loop back off measured field –Improves linearity of device Both Offset and Set – Reset used in low field ( < 1000nT) operation

888 Stutzke et alZimmermann et al AMR Noise Floor 100pT/Rt(Hz)AMR Noise Floor 24pT/Rt(Hz) Some recent results

99 Magnetoresistive magnetometer Control loop will be compatible with Digimag ASIC Hybrid 3 axis sensor as a single chip planned for late 2010 MAGIC Breadboard currently single axis 3axis Fluxgate Single axis MR CINEMA OB Triaxial sensor

10 Stimulus measurement – Fluxgate vs AMR FGM and AMR in 3Hz 6nT ptp stim field AMR Fluxgate

11 Magnetometer time series Magnetometer PSD overlaid with simulated ULF wave on ground MAGIC Noise

12 Noise Comparison with Fluxgate

13 Effect of flipping DSP FGM MR MR flipping at 1kHz. Drift much worse without flipping

14 Test results on most recent design Very encouraging result. Next - Test again over LEO field ranges and across temperature range Full Three axis calibration before end of Motivation: To get reduced drift and science mode power saving of up to 0.4W.

15 Imperial Test & Calibration Facilities –CALIBRATION Facility (Staffordshire UK) Three axis Helmholtz design Dynamic Field Compensation Will determine Gains, Misalignments, Linearity –MOBILE Coil Facility (Currently at Astrium, Friedrichshafen) Two axis Helmholtz design Magnetometer Array and rotation table Under PC Control with Multi-Dipole Model fitting Use: Measurement of magnetic moment Potential use on a CubeSat 3axis Helmholtz calibration coil facility Mobile coil facility to measure magnetic moments

16 Moonlite Impact Trial – May 2008 Location –MOD Rocket Track at Pendine, Wales Purpose of trial –Demonstrate survivability of shell & payload –Determine internal acceleration environment –Assess alternate packing regimes (potting, glass spheres) Test Set up –Accelerated to 40g to 300ms -1 for 0.9sec

17 Magnetometer Flow holes Impact Data –Only sensors flown in axial and vertical orientations –Compartment potted after assembly –Magnetometer tested before and after firing and no change in performance

18 MoonLITE (Light weight Interior and Tele-coms Experiment) UK Mission to the moon – Phase A 2010 Mission Goal –Instrumentation on the moon by 2014 Two mission elements An orbiter deployment and relay spacecraft –100kmx40km elliptical polar orbit Four instrumented penetrators –Target Regions Near side, far side and at the poles Shadowed craters a priority Mission lifetime –Orbiter – 1.5 years –Penetrators – 1 year –Releases over 2 months

19 Impact Trial Photos

20 Radiation sensor Magnetometers Batteries Mass spectrometer Micro-seismometers Drill assembly Accelerometers Power Interconnection Processing Accelerometers, Thermometer Batteries,Data logger Trial Payload

21 Trial Test Results –Max deceleration load 11kG max on-axis –~10kG on vertical axis –Magnetometer survived OK –Further penetrators trials targeted at Ganymede very likely to be funded by ESA –Next trial plan to include magnetometer electronics and be powered during firing Smith et al, 2008