The coplanar, polar orbits for Dynamics Explorers 1 and 2 drawn approximately to scale. The NASA/GSFC spacecraft were placed in orbit on 3 August 1981 from the Western Test Range at Vandenberg, California with a McDonnel-Douglas Thor/Delta 3914 launch vehicle. Initial parameters of the DE1 orbit include perigee and apogee altitudes of 570 km and 3.65 earth radii, respectively, an orbital period of 6.83 hours and a 0.328°/day precession rate for the line of apsides. The orbital inclination is 90°. Initial latitude of apogee was 78.2° N at a geographic local time of 2 hours. The direction of precession increased the latitude of apogee to 90° N thirtysix days after launch and to the equatorial plane by early June 1982. Apogee was located over the South Pole in early March 1983 and returned to northern polar latitudes in the fall of 1984. The viewing geometry for a particular latitude and altitude is repeated every three years. With apogee at polar latitudes, The University of Iowa's auroral imaging instrumentation provides continuous imaging of the aurora for more than five hours during a single orbit.