A compact channel-multiplier instrument has been designed and developed for spectral analysis of low-energy (0.5-500 keV) electrons and protons at synchronous altitude on the ATS-E satellite. The instrument package, which weighs 1.3 kg, contains 11 sensors and features a completely automatic in-flight calibration system for the interrogation of sensor and electronic performance. A detailed discussion of experimental objectives and design and calibration data for the instrument is presented. The results of studies of the long-term behavior of channel-multipliers is emphasized. Copyright © 1968 by The Institute of Electrical and Electronics Engineers, Inc.