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Accueil > Recherche > Plasmas Spatiaux > Instrumentation spatiale > Contribution aux missions spatiales > MMS/SCM

MMS/SCM

The MMS (Magnetospheric Multi-Scale mission) consists of four identical satellites with an equatorial orbit around the Earth. LPP provides five tri-axial magnetic antennas or search-coil magnetometers (SCM) including four flight models and a spare. These instruments belong to the Fields consortium driven by the university of New Hampshire.

You can browse SCM quicklooks for the MMS mission on our dedicated webpage.

 LPP team

Scientists  : Olivier Le Contel
Sensor and electronics engineers : M. Mansour, D. Alison
Software engineer : L. Mirioni

 Objectives

Following the european mission named Cluster, MMS will provide tri-dimensional measurements in the Earth magnetosphere as the four spacecraft will evolve in a tetrahedron configuration. However, MMS will investigate the magnetospheric physics at the electron scale thanks to smaller inter-satellite distances which will vary from 10 to 100 km. Particle measurements will have a better time resolution (30 ms for electrons and 150 ms for ions) and the three components of the electric field will be measured thanks to a pair of electric antennas deployed along the spin axis of the satellite in addition to the classical spin-plane antennas already existing on Cluster mission.

 SCM overview

The tri-axial search-coil magnetometer (SCM) provides the three components of the magnetic field fluctuations from 1 Hz to 6 kHz. The SCM consists of a tri-axial set of magnetic sensors with its associated preamplifier box. The SCM sensor is mounted on the same five meter boom as the analog flux-gate magnetometer (AFG), 4 meter from the spacecraft and 1 meter from AFG. The SCM preamplifier box is mounted on the spacecraft deck (outside of the central electronics box) near the base of the AFG/SCM magnetometer boom. The SCM preamplifier is connected to the digital signal processor (DSP) which digitizes and processes the three analog waveforms delivered by SCM. Once per orbit, the DSP injects to SCM a calibration signal. A thermistance mounted on the SCM structure provides the temperature of the SCM sensor (housekeeping).

SCM sensor

The SCM sensor is constituted by three magnetic sensors mounted in a tri-axial structure (Fig. 1). This structure is designed to ensure a precise alignment of the sensors with respect to the satellite axis. The orthogonality of the three mechanical axis of this structure is better or equal to 0.05 degree in order to satisfy the final +/- 1° required between SCM and spacecraft axis.

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Figure 1. Fluxmètre tri-axial pour MMS

The magnetic sensor consists of an optimized machined ferrite (ferromagnetic material with a high magnetic permeability) core of 10 cm length and 4 mm of diameter in order to amplify the external magnetic field (Fig. 1).

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Figure 2. Noyau de ferrite pour le capteur MMS

Then a primary winding with a large number of turns (more than ten thousands) is added to collect the voltage induced by the time variation of the magnetic flux (Fig. 3).

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Figure 3. Prototype du capteur pour MMS : longueur 100 mm, diamètre10mm, masse 15g, 16000 tours de bobinage

Finally a secondary winding with a smaller number of turns provides a flux feedback to flatten the frequency response of the antenna gain. This feedback circuit allows removing the resonance behavior associated with the primary winding and makes the response of the antenna smoother in phase and independent of temperature variations.

Internal electrostatic shielding is implemented around each antenna to minimize their sensitivity to electric fields (Fig. 3).

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Figure 4. Blindage électrostatique autour du capteur

This shielding is connected to the shielding of the harness and finally to the analog ground of the electronic device in order that the whole electrostatic shielding be connected to a reference potential. This electrostatic shielding is also reinforced by the multi-layer insulation (MLI) or thermal blanket added to ensure the thermal isolation of the sensors. Note that the three analog signals are routed to the SCM preamplifier via the SCM harness equipped by a silver-plated copper conductive shield braid.

SCM preamplifier

SCM preamplifier designed at LPP has been realized in multi-chip vertical technology (hydrid) by the French 3D+ company (Fig. 5). It has two stages of amplification. The first stage has a low-noise input and a gain of 46 dB. The second stage has a gain of 31.5 dB and ensures low and high-pass filterings. A power supply regulation is also implemented as well as a calibration buffer in order to receive the onboard calibration signal provided by DSP.

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Figure 5. Préamplificateur SCM

Onboard calibration signal

The calibration signal is digitally synthesized by the DSP FPGA and filtered using a low pass filter to smooth the signal. It is applied to the feedback winding of the search coil. It consists of a sweeping sine wave with a frequency that doubles every 4 cycles from 0.125 Hz to 4096 Hz and then stops. This signal is analyzed to estimate any possible modifications of the SCM transfer function during the mission.

  Noise equivalent magnetic induction (NEMI) or sensitivity of the SCM for MMS

The NEMI is obtained by measuring the output noise of the instrument in a very quiet facility (National magnetic Observatory of Chambon-la-forêt). Then this output noise is divided by the SCM frequency response to provide the NEMI or sensitivity of the SCM (in a frequency band normalized to 1 Hz).

SCM requirements

The SCM provides the three components of the magnetic fluctuations in the 1 Hz – 6 kHz nominal frequency range. The noise equivalent magnetic induction (NEMI or sensitivity) of the search-coil antenna is less than or equal to : 2 pT/sqrt(Hz) at 10 Hz, 0.3 pT/sqrt(Hz) at 100 Hz and 0.05 pT/sqrt(Hz) at 1 kHz (Fig. 6). The SCM resolution at 1k Hz is 0.15pT.

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Figure 6. Courbe de bruit en champ magnétique (« NEMI ») de chaque axe du fluxmètre triaxe

Main properties of the instrument

SCM characteristics for the MMS mission
Sensitivity at 10Hz 2pT\sqrt{Hz}
Sensitivity at 100Hz 0.3pT\sqrt{Hz}
Sensitivity at 1kHz 0.05pT\sqrt{Hz}
Sensor length 10cm
Sensor Mass (before potting) 15gr (/sensor)
Mass of a tri-axial antenna (3 sensors+support) 214gr
Mass of electronic device (pcb+connectors+box) 206gr (37g for 3Dplus cube)
Power consumption 130mW

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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Pascal Chabert (Directeur)