The US Air Force has launched the 21st Defense Support Program reconnaissance satellite aboard a Titan IV rocket. The satellites, which were first put into service in the early 1970s, are equipped with infrared sensors to detect missile and space launches and nuclear detonations from an altitude of more than 22,000 miles. The launch had been delayed because of concerns about the Titan's gui-dance system. The rockets, which are produced by Lockheed Martin Space Systems Co. of Denver, have been plagued by reliability problems. Between August 1998 and April 1999, more than $2 billion in payloads and launch vehicles was lost to mission failures. The Air Force plans to replace the Defense Support Program satellites over the next decade with the Space-Based Infrared System, a constellation of approximately 30 satellites in various orbits that will feature higher resolution.