The SEM System consists of a three-axis vector magnetometer, an Energetic Particle Sensor (EPS) and associated High-Energy Proton and Alpha Detector (HEPAD), and an X-Ray Sensor (XRS). This set of instruments is designed to provide real-time measurement of solar activity, the charged particle environment, and the Earth's magnetic field at synchronous orbit.

The Magnetometer measures the magnitude and direction of the Earth's ambient magnetic field with three orthogonal sensors, located in a sensor assembly and attached to a boom that places the sensor three meters away from the body of the spacecraft.

The EPS and HEPAD monitor solar protons and alpha particles, produced during large flares, which are a radiation hazard to manned and unmanned operations in space and the ionosphere at high latitudes. The HEPAD instrument covers the very high-energy protons and alpha particles that are produced in large solar flares.

The XRS performs real-time measurements of the solar X-ray emissions in two channels covering the spectral ranges of 0.5 to 3 Angstroms (shortwave) and 1 to 8 Angstroms (longwave). The sensitivity of the sensor was chosen to permit quiet sun background measurements at as low a level of solar activity as possible while detecting events at the lowest practicable threshold for early event warning.