FGM (Fluxgate Magnetometer) consists of two tri-axial fluxgate magnetometers mounted on one of the two 5 m radial booms, plus a data processing unit (DPU) on the main equipment platform. The magnetometer boom lies in the spacecraft spin plane, with the outboard sensor at the boom tip and the inboard sensor mounted 1.5 m inboard from the tip, to minimize the importance of the spacecraft background magnetic field and its variations. The sensors are mounted such that the X axes are aligned with the spacecraft spin axis, and the other axes are in the spin plane. There are five sensitivity ranges, from +/- 256 nT to +/- 65,500 nT (used only to facilitate ground testing). Range switching can be automatic or by command. Calibration is done by introducing a known current step into the feedback loop of the sensors. Telemetry allocation limits the normal output to about 20 vectors/s.