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FGM Overview and Specifications Hans-Ulrich Auster Institut für Geophysik und Meteorologie

FGM Overview and Specifications Hans-Ulrich Auster Institut für Geophysik und Meteorologie TU Braunschweig. FGM Overview. Digital Ringcore Fluxgate Magnetometer Working principle of fluxgate magnetometer Digital versus analogue fluxgate Ringcore versus double rod core Themis Design

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FGM Overview and Specifications Hans-Ulrich Auster Institut für Geophysik und Meteorologie

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  1. FGM Overview and Specifications Hans-Ulrich Auster Institut für Geophysik und Meteorologie TU Braunschweig

  2. FGM Overview • Digital Ringcore Fluxgate Magnetometer • Working principle of fluxgate magnetometer • Digital versus analogue fluxgate • Ringcore versus double rod core • Themis Design • Blockdiagram • Requirements & instrument parameters

  3. feedback pick up Bexcitation Bambient + Bexcitation Bambient excitation Bambiant – Bexcitation FGM Principle

  4. Uf0 = 0 U2f0 =  2 t t t 2  t FGM Principle Theory:

  5. FGM Principle In Practice: f0 = -12dBV 2f0 = -50dBV (20nT) 3f0 = -15dBV

  6. DAC Filter sensor signal sensor feedback DC signal Controller sensor signal sensor feedback ADC 40kHz ADC 100Hz phase sensitive detector AC signal FPGA U/I U/I FPGA Controller Digital vs. Analogue FGE Analog Magnetometer Digital Magnetometer

  7. Advantages DigMag • Less electronics, more simple and more robust • Combination of ADC and DAC values provides a large dynamic range: No data loss during range changes • Less analogue e-parts: less temperature dependency (software is not sensitive to temperature) • Direct control of signal acquisition, excitation and feedback makes parameters like bandwidth flexible • Auto-compensation, auto-calibration and health check can be easily implemented

  8. Amplifiers DAC’s MAG FB control SPM HV control Interface FPGA SPM signal input ADC’s Pressure signal input Amplifiers MAG signal input Board size: 120mm x 100mm Example ROMAP

  9. Ringcore vs. Double rod core Excitation: ringcore has a closed excitation loop, no demagnetisation losses  Sensitivity: demagnetisation factor for external field is very high for ringcores  Noise: signal / noise ratio shall be better for ringcores  Stability: better for double rods, because magnetic axis is well defined   • German ringcores: • developed by Karl-Heinz Fornacon since 1982 • Soft magnetic material: • similar to that flown by Mario Acuna (Permalloy) • Technology: • temper, cutting, polishing … • Flight proven ? • Yes: Phobos (1988) ….

  10. FGM Blockdiagram

  11. FGM Parameter • The absolute stability of the FGM shall be less than 1nT • Determination of 3 offsets and 9 elements of the calibration matrix (scale values, non-orthogonality, sensor orientation) through in-flight calibration once per orbit • Spinning S/C provides 8 of 12 calibration numbers • Scale values are known accurately enough from pre-flight calibration • The determination of the spin axis offset can be done to 1nT accuracy by known physics during a standard orbit and to +/-0.1nT accuracy when being in solar wind

  12. FGM Parameter • The relative stability of the FGM shall be less than 0.2nT/12hrs • Offset / Time: < 0.1nT/h; < 0.2nT/12hrs; < 1nT/year; • Offset / Temperature: < 0.1nT/°C • Scale value / Temp.: 24ppm (0.8nT/°C @ 32000nT) • Orthogonality / Temp.: can be neglected • The FGM noise level @ 1Hz shall be less than 0.03nT/sqrt(Hz) • Sensor noise: < 10pT/sqrt(Hz) @ 1 Hz

  13. FGM Parameter • The FGM digital resolution shall be less than 0.1nT • The FGM science range shall exceed 0-1000nT • FGM provides 0.01nT digital resolution independent of the external field due to digital magnetometer principle • The maximum feedback field (range) is about 32000nT • 24 bits per field component will be sent to the IDPU • 16 bits will be selected for transmission • 10pT digital resolution if B < 320nT • 160pT digital resolution if B > 2500nT • 1.2nT digital resolution if B > 20000nT

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