As we move towards proliferated space and smaller, more cost-effective satellites, miniaturization of quality science instrumentation is becoming necessary and possible. This paper describes the Relativistic Electron Magnetic Spectrometer (REMS), a miniaturization of NASA’s Van Allen Probes’ MagEIS-Medium instrument, which was designed to measure radiation belt electrons. REMS was designed, built, and delivered for NASA’s GTOSat mission (Blum et al. in Proc. SPIE 11389, micro- and nanotechnology sensors, systems, and applications XII, 113892E, 2020), and was integrated onto the spacecraft bus in 2021. REMS’s electron energy range is one decade and can be set between ∼30–1500 keV. The GTOSat REMS main rate electron energy centroids range from 136–1088 keV. REMS has 9 electron main rate energy channels and 32 electron histogram channels per detector pixel, allowing for more precise energy measurements that enable background subtraction as was accomplished with MagEIS. GTOSat REMS measures ∼400–9000 keV protons. The angular resolution ranges from ∼3°–20° depending on the REMS orientation and the detector pixel in question. On the spinning GTOSat, REMS provides a pitch angle resolution down to 5.63° (32 sectors per spin) for the main rate electron data product, 30° for the proton data product, and 22.5° for the histogram data product. A different sectoring choice would alter the pitch angle resolution. At the time of writing, the mission is awaiting a launch. In the meantime, REMS has been included in additional mission concepts and proposals. The REMS instrument provides high quality energy and pitch angle resolved measurements of electrons and protons, enabling future exploration and monitoring of Earth’s radiation belts with small satellites.
