A new era in near-earth magnetic field measurements will begin with NASA's launch of Magsat in September of this year. Magsat will provide the first truly global geomagnetic survey since the Pogo (Polar Orbiting Geophysical Observatories) satellites, Ogo 2, 4, and 6 of 1965–1971; it will also provide the very first global survey of all vector components of the geomagnetic field. Therefore this seems an appropriate time to review previous magnetic field experiments on nearearth spacecraft. These were designed to map the main geopotential field originating in the earth's core; to determine the long-term temporal, or secular, variations in that field; and to investigate those short-term field perturbations that are due to ionospheric and magnetospheric currents. While analyzing data from the Pogo satellites, Regan et al.  discovered that the loweraltitude data contained separable fields which were caused by crustal anomalies, thus opening the door to a new class of investigations.
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