1. The method of measurement of electron density The measurement of the electron density distribution in the topside ionosphere is made by a radio-frequency electron probe which was developed for this satellite. This probe measures the local electric permittivity of the medium in the vicinity of the satellite using a probing frequency of 10 Mc/s and from a knowledge of the permittivity the electron density is readily calculated. The electrodes consist of a pair of flat disk-shaped grids, 4 in. in diameter and spaced 3 1|2 in. apart. These grids are supported on the ends of two short tubes which, in turn, are mounted on a small junction box. This complete unit, which forms the measuring head, is fixed on the end of a retractable boom which extends about 3 ft. from the hull of the satellite. The permittivity is measured in terms of the current that flows between the two electrodes in response to a constant applied signal of 3 V r.m.s. This signal is provided by a 10 Mc/s crystal controlled oscillator, the amplitude being electronically stabilized at the above value.