The Defense Support Program is a series of Earth-orbiting satellites launched and operated by the United States Air Force. The first was launched in the early 1970s under heavy secrecy. The DSP satellites are the critical component of the Space-Based Early Warning system; using infrared detectors and other sensors, they watch the Earth looking for the distinctive energetic plumes of ballistic missile launches. They relay information back to the CONUS using both ground-based relay stations and by relaying data to each other. As the program has evolved, the satellites have changed with it - early birds provided 400 watts of power via solar panel and used gas thrusters to maintain attitude. They were intended to last for 1.25-2 years. However, it was found that the satellites were lasting longer than expected - three years in the case of early models, and longer for later ones. They are maintained in a constellation of five satellites - three of these satellites watch the Earth from coordinated geosynchronous orbit 22,300 miles up, and the two next oldest satellites are kept active as backups.
Originally, the DSP data was sent directly to NORAD in its famous Cheyenne Mountain facility, code named CRYSTAL PALACE. However, as Cheyenne Mountain is now closed, the data is sent to Air Force minders at Buckley AFB in Colorado and can also be disseminated directly to secure facilities.
Newer versions of the DSP satellites generate up to 1275 watts of power for additional, more powerful sensors - the number of IR detectors aboard each has gone from 2,000 to over 6,000. In addition to IR sensors, DSP satellites contain bhangmeter-based NUDETS to watch for nuclear explosions. Several years into the program, the satellites were given reaction control wheels in order to allow them to change their attitude using electrical power rather than scarce thruster fuel - this contributed to the increase in their lifespans. The satellites scan the view 'beneath' them in a cone, and this cone is rotated by spinning the satellite at six RPM around an axis drawn directly between it and the earth's surface to provide a 'scanning' motion for the sensors.
Nearly all of the 25 DSP satellites lofted were sent up using Titan IV boosters. One was deployed from the Space Shuttle, and the last one was launched in 2007 aboard the Air Force's new Evolved Expendable Launch Vehicle the Delta IV Heavy. After that, the DSP program will be replaced by the Space Based Infrared System, known as SBIRS.
The DSP satellites, originally designed to watch solely for high-energy missile launch plumes, have ended up being used for all manner of threats not originally planned. The SLOW WALKER and FAST WALKER programs were implemented using the DSP IR sensors. During the Gulf War and after, DSP was used to not only watch for strategic weapon launches but also used to track small tactical ballistic missile launches, warning of SCUD shots against U.S. Forces and allies.
Later DSP satellites weight 5,250 lbs, generate 1,485 watts at maximum using solar panels, and are 32.8 feet long by 22 feet in diameter when they are on orbit and their systems are fully deployed. They were built with Northrop-Grumman as their prime contractor.