Development of the fixed-frequency topside-sounder satellite

A fixed-frequency ionosphere sounder satellite, Explorer XX, was developed for the purpose of obtaining pulsed soundings of the topside of the ionosphere. The basic sounder instrumentation comprised a two-frequency transceiver coupled to an electrically short dipole antenna. Three transceivers were used for sounding at six fixed frequencies between 1.50 and 7.22 MHz. A two-frequency sounder payload was developed for initial rocket testing to demonstrate the feasibility of topside sounding and to provide design data for both United States and Canadian topside-soundersatellites. In 1961, two rocket experiments were successfully conducted, providing the first topside soundings: the first during quiet, and the other during disturbed, ionospheric conditions. Explorer XX, launched in 1964 into an 800-km polar orbit, provided soundings, generally on three or more of the frequencies, for more than one year. The development of sounder instrumentation, including a 100-watt peak power transmitter and antenna system, is discussed and the payload system design is summarized.