The S-300 search coil magnetometer measures the  AC magnetic fields up to 30 kHz, the  DC/AC electric fields and plasma resonances up to 80 kHz, the mutual and self-impedance. The instrument consists of:
- 2 vitreous carbon spheres mounted on radial booms (42 m tip-to-tip)
- 2 sets of three-axis search coil magnetometers, one for the ULF/ELF range and one for the VLF range (each is mounted on the locking 3 m booms at a distance of 2 m from the spacecraft), 4 meshed spheres mounted at the end of the 2.5 m axial booms.