Photo - DSP mounted on IUS (169K)
Photo - DSP mounted on IUS (169K)
Originally designed (with first publicaly acknowledged launches in 1970s) to detect the flair of a ballistic missile launch, the system is thought to have also been designed to detect nuclear detonation, however that system (NUDET) may also have been embarked on other DoD space platforms as well.
In the nuclear launch detection role, the system is merely a system used to detect the bright signature of a missile launch and thus really is not a nuclear launch detector at all, but simply a MRBM or ICBM missile launch device. In actuallity, the system may also be able to detect any bright flair associated with any missile launch. Thus, if the system's filters aren't precisely tuned to a particular light frequency, even large explosions will be detected.
The actual number of satellites is not public knowledge, however, it is clear that in order to detect high energy flairs, the system must be fairly close to the Earth, and therefore the visible horizon effects the effective placement of the satellites. The conclusion is that there are probably in excess of 30 in order to cover some 12 degrees of global territory. This IS a real SWAG however, as the orbit is also not known.
In terms of nuclear detection (NUDET), that package is embarked on the DSP satellites (according to Air Force Magazine's Air Force Almanac, 1997 1 and it is highly probable that it uses spectrum analysis (even of a crude form) to detect both the high energy flash, but also the particular spectrum of a nuclear blast.
For instance, the actual composition of the nuclear weapon may be detectable by looking at the chief spectra components which would identify the casing and trigger elements in the weapon. Speculation by fictional writers citing the systems usage (for instance Tom Clancy just to name one...Sum of All Fears), the system could tell us if the weapon exploded were of a particular manufacturer class...high precision nuclear materials would perhaps indicate the U.S. or Soviet production lines versus a weapon made in India or Iraq.
The DSP system is supported by a network of ground based systems, and may now also take advantage of the spaceborne DoD communications relay satellites that can relay both ground based and space based data signals.
1 Air Force Magazine also reports that the DSP program will end with unit number 23 (MILNET's guess is that there may have been at least one failure in the series so 23 might be number 22 orbited...who knows if the first few are still in service). The replacement units are part of a program called the SBIR (SpaceBased Infrared) system. Presumably these new satellites (total planned is unknown) will embark more capable MRBM capabilities to further enhance the U.S. military's multi-tier defense program for battlefield defense.
Air Force Magazine also reports the dimensions as 32ft 8 in high, with a diameter of 22 ft (solar panels extended). Weight is estimated at 5,000 lbs. The orbit is estimated by Air Force Magazine at 22,000 miles altitude and are in geosynchronous orbits