The objectives of satellites OV1-16 (Cannon Ball I) and OV1-15 (SPADES) were to measure atmospheric density and related properties in the lower thermosphere with particular emphasis on the altitude region 120 to 150 km. This region is where least data are available and where data are urgently required for Air Force Systems vehicles and for accurate calculations of the re- entry locations of satellites. To achieve an orbit at the lowest possible altitude the mass-to-area ratio of Cannon Ball I was maximized and a spherical shape chosen to optimize the accuracy of the drag density measurements. Both satellites were launched into a polar orbit on 11 July 1968. The initial perigee of Cannon Ball I was 148 km and the apogee 575 km. The initial perigee for SPADES was 158 km and the apogee 1850 km. A considerable amount of density data was obtained from orbital drag and from an onboard triaxial accelerometer on Cannon Ball I and from orbital drag, accelerometer, and ionization gauges on SPADES.