The Ionosphere Sounding Satellite (ISS) was part of Japan's contribution to the International Magnetospheric Study (IMS). Its objectives were to accumulate data for study of the topside ionosphere and to survey radio noise at four frequencies, from both earth and cosmic sources. It prepared world-wide maps of F2 critical frequency from the ionosphere sounding data. The ISS 2 was a small observatory with four experiments on board. The spacecraft, a right cylinder, 82 cm long and 93.5 cm in diameter, was spin stabilized at about 13 rpm with the spin axis normal to the ecliptic plane. Two pairs of crossed dipole antennas extended from the central part of the satellite and lay perpendicular to the spin axis. These antennas, 36.8 and 11.4 m long, were unfurled in orbit and were shared by ionospheric sounding and radio noise experiments. A spherical retarding-potential trap sensor was mounted on a boom perpendicular to the spin axis. A magnetic attitude sensor was mounted on a similar boom on the opposite side of the spacecraft. The remaining experiment involved a Bennett-type mass spectrometer with two sensors flush-mounted on opposite ends of the spacecraft. Spacecraft attitude was determined by means of a magnetometer, a solar sensor, and an earth horizon sensor. Small telemetry and command antennas extended from the spacecraft. The spacecraft was powered from a battery solar-cell system with solar cells covering most of the cylindrical surface. One recorder on board permitted spacecraft operation in either a recorded (for up to 112 min) or real-time mode. Readout and real-time operation were done from Kashima, Japan, and Ottawa, Canada.
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