modes of operation were used: one instrument cycle of 156 intensity measurements every 46-seconds, or one cycle of 312 intensity measurements every 92-seconds. Triaxial Fluxgate Magnetometer A four-range, triaxial fluxgate magnetometer mounted on a boom, was used to measure the ambient magnetic field. The three axes were sampled sequentially three times each 5.72-seconds. Sensitivities and accuracies of the four ranges were ± 150 and 1.2, 450 and 3.5, 1500 and 11.7, and 25,000 and 195.3-nT, respectively. The sensitivity was switched by ground command. Frequency response was DC to 1-Hz (flat); down 3-dB at 10-Hz; then falling at 6-dB per octave at higher frequencies. Satellite stray fields were constrained to be less than 0.1 nT, which was also the rms instrument noise level. Inflight calibration was performed once every 98-minutes. Spacecraft The spacecraft was spin-stabilized satellite with a nominal rotational period of 11-seconds. In celestial coordinates, the positive spin axis coordinates were right ascension 299.4° (± 1.1°) and declination 8.6° (± 1.5°). There was no onboard orientation or spin rate control, but the orientation of the spin axis was stable. An optical aspect system operated from launch until 3 September 1974 at which time the optical aspect system was turned off and failed to turn back on. After this period, aspect had to be determined by observing the effect of optical illumination from the Sun on a plasma measurement system. Using the sharp peak observed in this data, corrected orientation information was obtained and rewritten to the data records. The complete spacecraft with instruments had a mass of . Power of 36 watts, depending on solar aspect, was obtained from solar cells. Explorer 52 participated in the International Magnetospheric Study (IMS) and during the first half of 1977 data acquisition was confined to IMS special intervals. Data were obtained in real time only, at frequencies of 136 and 400-MHz at 100 bit/s (or 200 bit/s with convolutional coding) plus wideband Very low frequency (VLF) data. It was designed, built, and tracked by personnel at the Department of Physics and Astronomy, University of Iowa whose sports teams are the Iowa Hawkeyes. The spacecraft was launched on 3 June 1974 into a polar orbit with an apogee over the North Pole and re-entered on 28 April 1978 after 667 orbits or nearly four years of continuous operation. The spacecraft apogee was of with perigee. The orbital period was 51.3 hours. During its lifetime,