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The Bennett spectrometer to be flown on Atmosphere Explorer-C and -E (AE-C and AE-E) is designed to measure, throughout the 120 to 4000-km orbit, the concentrations of all thermal positive ions in the mass range 1 to 72 amu and number density range 5 to 5 × 106 ions cm−3. To reduce the buildup of ram pressure and facilitate measurements at low altitude, the analyzer is vented, and a multigrid ion-current collector is employed. An extensive command capability permits optimization of instrument parameters for particular measurement objectives; commandable functions include mass-scan range and period, the sensitivity-resolution characteristic of the analyzer, orifice potential, and in-flight calibration. Any combination of three mass ranges (1 to 4, 2 to 18, 8 to 72 amu) may be selected as the mass-scan mode; each range is normally scanned in 1.6 sec, corresponding to a distance of 12 km along the orbit. Ion spectra will be simultaneously telemetered in both analog and digital form; the digital data result from on-board processing of the analog spectra and consist only of ion-peak coordinates. The three overlapping mass ranges will enable in-flight evaluation of mass discrimination within the ion analyzer. Verification of absolute instrument sensitivity will require correlation of the spectrometer data with results from the companion electrostatic probe and retarding-potential analyzer. Such correlations should permit individual ion concentrations to be determined with an accuracy of ±10%.