In 1963, the KH-4 system was introduced with dual cameras and the program made completely secret, by the then president, Kennedy. The Discoverer label was dropped and all launches became classified. Because of the increased satellite mass, the basic Thor-Agena vehicle was enhanced by the addition of three Castor solid-fueled strap-on motors. On 28 February 1963, the first Thrust Augmented Thor lifted from Vandenberg Air Force Base at Launch Complex 75 carrying the first KH-4 satellite. The launch of the new and unproven booster went awry as one SRB failed to ignite. Eventually the dead weight of the strap-on motor dragged the Thor off its flight path, leading to a Range Safety destruct. It was suspected that a technician had not attached an umbilical on the SRB properly. Although some failures continued to occur during the next few years, the reliability rate of the program significantly improved with KH-4. Maneuvering rockets were also added to the satellite beginning in 1963. These were different from the attitude stabilizing thrusters which had been incorporated from the beginning of the program. Corona orbited in very low orbits to enhance resolution of its camera system. But at perigee (the lowest point in the orbit), Corona endured drag from the atmosphere of Earth. In time, this could cause its orbit to decay and force the satellite to re-enter the atmosphere prematurely. The new maneuvering rockets were designed to boost Corona into a higher orbit, and lengthen the mission time even if low perigees were used. For use during unexpected crises, the National Reconnaissance Office (NRO) kept a Corona in "R-7" status, meaning ready for launch in seven days. By the summer of 1965, NRO was able to maintain Corona for launch within one day.