An optimized low-frequency three-axis search coil magnetometer for space research

The design of a three-axis search coil magnetometer in the 1 Hz–20 kHz range developed for the scientific satellite DEMETER is described. The sensitivity is a critical parameter as the search coil must be able to detect very weak signals in the Earth’s magnetosphere. The sensitivity is mainly constrained by the size and the mass allocated onboard the spacecraft. Significant improvements are made in terms of mass and sensitivity over previous space-borne search coils. We achieve a noise level of 4 fT Hz − 1 ∕ 2 at 6 kHz and a mass of 430 g for the three search coils and the bracket. A good agreement is observed on the estimation by finite element analysis of the sensor impedance and the measurements. The gain and noise model of the search coil and its preamplifier also agree quite well with measurements.